Transcript
EIGHTH EDITION CLINICAL BIOCHEMISTRY & METABOLIC MEDICINE This page intentionally left blank EIGHTH EDITION CLINICAL BIOCHEMISTRY & METABOLIC MEDICINE Professor Martin Andrew Crook BSc MB BS MA P...
EIGHTH EDITION CLINICAL BIOCHEMISTRY & METABOLIC MEDICINE This page intentionally left blank EIGHTH EDITION CLINICAL BIOCHEMISTRY & METABOLIC MEDICINE Professor Martin Andrew Crook BSc MB BS MA PhD FRCPath FRCPI FRCP Consultant in Chemical Pathology and Metabolic Medicine Guy’s, St Thomas’ and University Hospital Lewisham, London, UK, and Visiting Professor, School of Science, University of Greenwich, UK First published in Great Britain in 1971 by Lloyd-Luke (Medical Books) Ltd Second edition 1975 Third edition 1979 Fourth edition 1984 Fifth edition published in 1988 by Edward Arnold (Publishers) Ltd Sixth edition published in 1994 Seventh edition published in 2006 by Hodder Arnold This eighth edition published in 2012 by Hodder Arnold, an imprint of Hodder Education, Hodder and Stoughton Ltd, a division of Hachette UK,338 Euston Road, London, NW1 3BH http://www.hodderarnold.com © 2012 Hodder & Stoughton Ltd All rights reserved. Apart from any use permitted under UK copyright law, this publication may only be reproduced, stored or transmitted in any form or by any means with prior permission in writing of the publishers or in the case of reprographic production in accordance with the terms of licences issued by the Copyright Licensing Agency. In the United Kingdom such licences are issued by the Copyright Licensing Agency: Saffron House, 6–10 Kirby Street, London EC1N 8TS. Whilst the advice and information in this book are believed to be true and accurate at the date of going to press, neither the author nor the publisher can accept any legal responsibility or liability for any errors or omissions that may be made. In particular (but without limiting the generality of the preceding disclaimer), every effort has been made to check drug dosages; however, it is still possible that errors have been missed. Furthermore, dosage schedules are constantly being revised and new side effects recognized. For these reasons the reader is strongly urged to consult the drug companies’ printed instructions, and their websites, before administering any of the drugs recommended in this book. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN-13 978-1-4441-4414-7 ISBN-13 [ISE] 978-1-4441-4416-1 1 2 3 4 5 6 7 8 9 10 In order that we can ensure that students continue to benefit from the availability of our special editions, if you have purchased your copy in an un-authorized country, please email exportenquiries@ hodder.co.uk letting us know where, when and from which organization or individual you made the purchase. Commissioning Editor: Joanna Koster Project Editor: Jenny Wright Production Controller: Francesca Wardell Cover Designer: Amina Dudhia Indexer: Laurence Errington Cover: © PASIEKA/SCIENCE PHOTO LIBRARY Typeset in 11/13 pt Minion by Phoenix Photosetting, Chatham, Kent Printed and bound in India What do you think about this book? Or any other Hodder Arnold title? Please send your comments to www.hoddereducation.com Contents Preface vii List of abbreviations viii 1 Requesting laboratory tests and interpreting the results 1 2 Water and sodium 6 3 The kidneys 36 4 Acid–base disturbances 59 5 Potassium 83 6 Calcium, phosphate and magnesium 95 7 The hypothalamus and pituitary gland 116 8 The adrenal cortex 129 9 The reproductive system 146 10 Pregnancy and infertility 157 11 Thyroid function 164 12 Carbohydrate metabolism 176 13 Plasma lipids and lipoproteins 200 14 Nutrition 216 15 Vitamins, trace elements and metals 224 16 The gastrointestinal tract 235 17 Liver disorders and gallstones 252 18 Plasma enzymes in diagnosis (clinical enzymology) 270 19 Proteins in plasma and urine 282 20 Purine and urate metabolism 303 21 Disorders of haem metabolism: iron and the porphyrias 310 22 Cardiovascular disease 325 vi Contents 23 Cerebrospinal, pleural and ascitic fluids 332 24 Metabolic effects of tumours 338 25 Therapeutic drug monitoring and poisoning 347 26 Clinical biochemistry at the extremes of age 358 27 Inborn errors of metabolism 371 28 Genetics and deoxyribonucleic acid-based technology in clinical biochemistry 384 29 Patient sample collection and use of the laboratory 391 30 Point-of-care testing 397 Appendix 1: Units in clinical chemistry 401 Index 403 COMPANION WEBSITE This book has a companion website available at: www.hodderplus.com/clinicalbiochemistry The website contains downloadable images from the book as well as self assessment questions. To access these resources please register on the website using the following access details: Serial number: 145jw76mc8fw Once you have registered you will not need the serial number but can log in using the username and password that you will create during your registration. Preface Were it not for the textbook Clinical Chemistry in Diagnosis and Treatment by Joan Zilva and Peter Pannall, I would not be a chemical pathologist. As a medical student, I was so struck by its clarity, depth and clinical relevance that I decided that theirs was the medical field I wished to work in. Over the years, the field of clinical biochemistry has changed radically. Confusingly, there is no consensus on the name for this field of medicine, which is known variously as clinical chemistry, chemical pathology or clinical biochemistry, to name but a few. Additionally, the field now overlaps with that of metabolic medicine, a clinical specialty involved with the management and treatment of patients with disorders of metabolism. Clinical biochemistry laboratories have become further automated, molecular biology technologies have entered the diagnostic arena, and chemical pathologists have become more clinically orientated towards running out-patient clinics for a variety of biochemical disturbances. This book aims to address these new changes. Indeed, it is difficult to imagine a branch of medicine that does not at some time require clinical biochemistry tests, which may not be too surprising, given the fact that every body cell is composed of chemicals! Unfortunately, there have been some difficulties in recent times, with a relative shortage of graduates entering the specialty, which has not been helped by some people’s attitude that clinical biochemistry is merely a laboratory factory churning out results that anyone can interpret. There are also concerns that medical student clinical biochemistry teaching may become ‘diluted’ as part of an expanding curriculum. It is hoped that this book will excite a new generation to enter this fascinating and essential field, as well as benefit patients as their doctors learn more about their biochemical and metabolic problems. I am most grateful to Dr Sethsiri Wijeratne, Dr Alam Garrib (particularly for molecular biology expertise) and Dr Paul Eldridge for constructive criticism of the text. I am also grateful to Professor Philip Mayne for his earlier contributions and the anonymous medical student reviewer(s) who commented on the text. The book has also greatly benefited from the wise, helpful and experienced input of Dr Andrew Day – many thanks. Although every effort has been made to avoid inaccuracies and errors, it is almost inevitable that some may still be present, and feedback from readers is therefore welcome. Martin Crook London, 2012 Disclaimer The publishers and author accept no responsibility for errors in the text or misuse of the material presented. Drugs and their doses should be checked with a pharmacy, and the investigation protocols with an appropriate clinical laboratory. Dynamic test protocols should be checked with an accredited clinical investigation unit and may require different instructions in the elderly, children and the obese. List of abbreviations ABC1 adenosine triphosphate-binding cassette CA carbohydrate antigen protein 1 CaE calcium excreted per litre of glomerular ACE angiotensin-converting enzyme filtrate ACP acid phosphatase CAH congenital adrenal hyperplasia ACR albumin to creatinine ratio cAMP cyclic adenosine monophosphate ACTH adrenocorticotrophic hormone CaSR calcium-sensing receptor (corticotrophin) CAT computerized axial tomography ADH antidiuretic hormone (arginine CBG cortisol-binding globulin (transcortin) vasopressin) CD carbonate dehydratase (carbonic A&E accident and emergency (department) anhydrase) AFP a-fetoprotein CEA carcinoembryonic antigen AIDS acquired immunodeficiency syndrome CETP cholesterol ester transfer protein AIS autoimmune insulin syndrome CK creatine kinase AKI acute kidney injury CKD chronic kidney disease ALA 5-aminolaevulinic acid CNP C-type natriuretic peptide ALP alkaline phosphatase CNS central nervous system ALT alanine aminotransferase (also known CoA coenzyme A as glutamate pyruvate aminotransferase, COPD chronic obstructive pulmonary disease GPT) CRH corticotrophin-releasing hormone AMC arm muscle circumference CRP C-reactive protein ANA antinuclear antibody CSF cerebrospinal fluid ANCA antineutrophil cytoplasmic antibody CT computerized tomography ANP atrial natriuretic peptide CV coefficient of variation APA aldosterone-producing adenoma Cys C cystatin C apo apolipoprotein APRT adenine phosphoribosyl transferase 2,3-DPG 2,3-diphosphoglycerate APUD amine precursor uptake and DDAVP 1-desamino-8-D-arginine vasopressin decarboxylation (desmopressin acetate) ARA angiotensin II receptor antagonist DHEA dehydroepiandrosterone ARB angiotensin II receptor blocker DHEAS dehydroepiandrosterone sulphate ARMS amplification refractory mutation DIT di-iodotyrosine system DNA deoxyribonucleic acid AST aspartate aminotransferase (also DPP-4 dipeptidyl peptidase-4 known as glutamate oxaloacetate DVT deep vein thrombosis aminotransferase, GOT) ATPase adenosine triphosphatase ECF extracellular fluid ATP adenosine triphosphate ECG electrocardiogram EDTA ethylenediamine tetra-acetic acid BJP Bence Jones protein eGFR estimated glomerular filtration rate BMD bone mineral density ENA extractable nuclear antigen BMI body mass index ENT ear, nose and throat (department) BMR basal metabolic rate ERCP endoscopic retrograde BNP brain natriuretic peptide cholangiopancreatography BPH benign prostatic hyperplasia ESR erythrocyte sedimentation rate List of abbreviations ix EUS endoscopic ultrasonography 5-HT hydroxytryptamine (serotonin) 5-HTP hydroxytryptophan FAD flavine adenine dinucleotide HVA homovanillic acid FCH familial combined hyperlipidaemia FDH familial dysalbuminaemic IAH idiopathic adrenal hyperplasia hyperthyroxinaemia IDL intermediate-density lipoprotein FENa% fractional excretion of sodium IDMS isotope dilution mass spectrometry FEPi% fractional excretion of phosphate IEM inborn errors of metabolism FH familial hypercholesterolaemia IFG impaired fasting glucose FMN flavine mononucleotide IFN interferon FSH follicle-stimulating hormone Ig immunoglobulin fT4 free T4 IGF insulin-like growth factor fT3 free T3 IGT impaired glucose tolerance IL interleukin GAD glutamic decarboxylase INR international normalized ratio GDM gestational diabetes mellitus GFR glomerular filtration rate LADA latent autoimmune diabetes of adults GGT g-glutamyl transferase LCAT lecithin–cholesterol acyltransferase GH growth hormone LDH lactate dehydrogenase GHRH growth hormone-releasing hormone LDL low-density lipoprotein GIP gastric inhibitory peptide LH luteinizing hormone GLP-1 glucagon-like peptide 1 LR likelihood ratio GnRH gonadotrophin-releasing hormone G6P glucose-6-phosphate MCADD medium-chain acyl coenzyme A G6PD glucose-6-phosphate dehydrogenase dehydrogenase deficiency GRA glucocorticoid remediable aldosteronism MCH mean corpuscular haemoglobin MCV mean corpuscular volume HAV hepatitis A virus MDRD modification of diet in renal disease Hb haemoglobin (formula) HbA1c glycated haemoglobin MEGX monoethylglycinexylidide HBsAg viral surface antigen MEN multiple endocrine neoplasia HBD hydroxybutyrate dehydrogenase MGUS monoclonal gammopathy of HBV hepatitis B virus undetermined significance hCG human chorionic gonadotrophin MIBG metaiodobenzylguanidine HCV hepatitis C virus MIT mono-iodotyrosine HDL high-density lipoprotein MODY maturity-onset diabetes of the young HELP heparin extracorporeal low-density MPS mucopolysaccharidosis lipoprotein precipitation MRCP magnetic resonance HFE human haemochromatosis protein cholangiopancreatography HGPRT hypoxanthine–guanine phosphoribosyl MRI magnetic resonance imaging transferase mRNA messenger ribonucleic acid 5-HIAA 5-hydroxyindole acetic acid MSH melanocyte-stimulating hormone HIV human immunodeficiency virus mtDNA mitochondrial DNA HLA human leucocyte antigen MTHFR methylenetetrahydrofolate reductase HMG-CoA 3-hydroxy-3-methyl glutaryl coenzyme A HMMA 4-hydroxy-3-methoxymandelic acid NAD nicotinamide adenine dinucleotide HNF hepatocyte nuclear factor NADP nicotinamide adenine dinucleotide HONK hyperosmolal non-ketotic (coma) phosphate HRT hormone replacement therapy NAFLD non-alcoholic fatty liver disease hs-CRP high-sensitivity C-reactive protein NAG N-acetyl-b-D-glucosaminidase x List of abbreviations NASH non-alcoholic steatotic hepatitis SCID severe combined immunodeficiency NEFA non-esterified fatty acid SD standard deviation NGAL neutrophil gelatinase-associated lipocalin SHBG sex-hormone-binding globulin NHS National Health Service SIADH syndrome of inappropriate antidiuretic NICTH non-islet cell tumour hypoglycaemia hormone secretion NP natriuretic peptide SLE systemic lupus erythematosus NSAID non-steroidal anti-inflammatory drug STEMI ST-segment elevation myocardial NSTEMI non-ST segment elevation myocardial infarction infarction T3 tri-iodothyronine OGTT oral glucose tolerance test T4 thyroxine OTC ornithine transcarbamylase TBG thyroxine-binding globulin TBW total body water PABA para-amino benzoic acid TCA tricarboxylic acid PBG porphobilinogen TfR transferrin receptor PCR polymerase chain reaction TIBC total iron-binding capacity PEG polyethylene glycol TNF tumour necrosis factor PH primary hyperaldosteronism TPO thyroid peroxidase PI protease inhibitor TPMT thiopurine methyltransferase PIVKA proteins induced by vitamin K absence TRH thyrotrophin-releasing hormone PKU phenylketonuria TSH thyroid-stimulating hormone PNI prognostic nutritional index TSI thyroid-stimulating immunoglobulin POCT point-of-care testing TTKG transtubular potassium gradient PPAR peroxisome proliferator-activated receptor PRPP phosphoribosyl pyrophosphate UGT uridine glucuronyl transferase PSA prostate-specific antigen UIBC unsaturated iron-binding capacity PTH parathyroid hormone URL upper reference limit PTHRP parathyroid hormone-related protein VIP vasoactive intestinal polypeptide RBP retinol-binding protein VLCFA very long-chain fatty acid RDS respiratory distress syndrome VLDL very low-density lipoprotein RFLP restriction fragment length VDBP vitamin D-binding protein polymorphism VDR vitamin D receptor RNA ribonucleic acid ROC receiver operating characteristic (curve) WHO World Health Organization RRT renal replacement therapy 1 Requesting laboratory tests and interpreting the results Requesting laboratory tests 1 Interpreting results 2 How often should I investigate the patient? 1 Is the abnormality of diagnostic value? 3 When is a laboratory investigation ‘urgent’? 1 Diagnostic performance 4 REQUESTING LABORATORY TESTS rapidly in patients given large doses of diuretics and There are many laboratory tests available to the clinician. these alterations may indicate the need to instigate or Correctly used, these may provide useful information, change treatment (see Chapter 5). but, if used inappropriately, they are at best useless and Laboratory investigations are very rarely needed at worst misleading and dangerous. more than once daily, except in some patients receiving In general, laboratory investigations are used: intensive therapy. If they are, only those that are to help diagnosis or, when indicated, to screen for essential should be repeated. metabolic disease, WHEN IS A LABORATORY to monitor treatment or detect complications, INVESTIGATION ‘URGENT’? occasionally for medicolegal reasons or, with due The main reason for asking for an investigation to be permission from the patient, for research. performed ‘urgently’ is that an early answer will alter Overinvestigation of the patient may be harmful, the patient’s clinical management. This is rarely the causing unnecessary discomfort or inconvenience, case and laboratory staff should be consulted and the delaying treatment or using resources that might be sample ‘flagged’ as clearly urgent if the test is required more usefully spent on other aspects of patient care. immediately. Laboratories often use large analysers Before requesting an investigation, clinicians should capable of assaying hundreds of samples per day consider whether its result would influence their clinical (Fig. 1.1). Point-of-care testing can shorten result management of the patient. turnaround time and is discussed in Chapter 30. Close liaison with laboratory staff is essential; they may be able to help determine the best and quickest procedure for investigation, interpret results and discover reasons for anomalous findings. HOW OFTEN SHOULD I INVESTIGATE THE PATIENT? This depends on the following: How quickly numerically significant changes are likely to occur: for example, concentrations of the main plasma protein fractions are unlikely to change significantly in less than a week (see Chapter 19), similarly for plasma thyroid-stimulating hormone (TSH; see Chapter 11). See also Chapter 3. Whether a change, even if numerically significant, will alter treatment: for example, plasma transaminase activities may alter within 24 h in the course of acute hepatitis, but, once the diagnosis has been made, this Figure 1.1 A laboratory analyser used to assay is unlikely to affect treatment (see Chapter 17). By hundreds of blood samples in a day. Reproduced with contrast, plasma potassium concentrations may alter kind permission of Radiometer Limited. 2 Requesting laboratory tests and interpreting the results Laboratories usually have ‘panic limits’, when Test reference ranges highly abnormal test results indicate a potentially life- By convention, a reference (‘normal’) range (or interval) threatening condition that necessitates contacting the usually includes 95 per cent of the test results obtained relevant medical staff immediately. To do so, laboratory from a healthy and sometimes age- and sex-defined staff must have accurate information about the location population. For the majority of tests, the individual’s of the patient and the person to notify. results for any constituent are distributed around INTERPRETING RESULTS this mean in a ‘normal’ (Gaussian) distribution, the 95 per cent limits being about two standard deviations When interpreting laboratory results, the clinician from the mean. For other tests, the reference distribution should ask the following questions: may be skewed, either to the right or to the left, around Is the result the correct one for the patient? the population median. Remember that 2.5 per cent of Does the result fit with the clinical findings? the results at either end will be outside the reference Remember to treat the patient and not the range; such results are not necessarily abnormal for that ‘laboratory numbers’. individual. All that can be said with certainty is that If it is the first time the test has been performed the probability that a result is abnormal increases the on this patient, is the result normal when the further it is from the mean or median until, eventually, appropriate reference range is taken into account? this probability approaches 100 per cent. Furthermore, If the result is abnormal, is the abnormality of a normal result does not necessarily exclude the disease diagnostic significance or is it a non-specific that is being sought; a test result within the population finding? reference range may be abnormal for that individual. If it is one of a series of results, has there been Very few biochemical tests clearly separate a ‘normal’ a change and, if so, is this change clinically population from an ‘abnormal’ population. For significant? most there is a range of values in which ‘normal’ and ‘abnormal’ overlap (Fig. 1.2), the extent of the overlap Abnormal results, particularly if unexpected and differing for individual tests. There is a 5 per cent chance indicating the need for clinical intervention, are best that one result will fall outside the reference range, and repeated. with 20 tests a 64 per cent chance, i.e. the more tests done, the more likely it is that one will be statistically CASE 1 abnormal. No result of any investigation should be interpreted A blood sample from a 4-year-old boy with without consulting the reference range issued by the abdominal pain was sent to the laboratory from an accident and emergency department. Some of the results were as follows: 160 Plasma 140 Bilirubin 14 µmol/L (< 20) 120 Alanine transaminase 14 U/L (< 42) Number of subjects Alkaline phosphatase 326 U/L (< 250) 100 Albumin 40 g/L (35–45) 80 g-Glutamyl transferase 14 U/L (< 55) ‘Normal’ Albumin-adjusted calcium 2.34 mmol/L (2.15–2.55) 60 subjects Overlap between DISCUSSION 40 ‘normal’ and ‘ill’ The patient’s age was not given on the request form 20 and the laboratory computer system ‘automatically’ ‘Ill subjects’ used the reference ranges for adults. The plasma 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 alkaline phosphatase activity is raised if compared Arbitrary units with the adult reference range, but in fact is within ‘normal limits’ for a child of 4 years (60–425). See Figure 1.2 Theoretical distribution of values for ‘normal’ and ‘abnormal’ subjects, showing overlap at also Chapters 6 and 18. the upper end of the reference range. Is the abnormality of diagnostic value? 3 laboratory carrying out the assay. Some analytes have the patient fasting. This is not usually possible, and risk limits for treatment, such as plasma glucose (see these variations should be taken into account when Chapter 12), or target or therapeutic limits, such as serial results are interpreted. plasma cholesterol (see Chapter 13). Some constituents vary monthly, especially in Various non-pathological factors may affect the women during the menstrual cycle. These variations results of investigations, the following being some of can be very marked, as in the results of sex hormone the more important ones. assays, for example plasma oestradiol, which can only be interpreted if the stage of the menstrual cycle is known; Between-individual differences plasma iron may fall to very low concentrations just Physiological factors such as the following affect the before the onset of menstruation. Other constituents interpretation of results. may also vary seasonally. For example, vitamin D concentrations may be highest in the summer months. Age-related differences Some of these changes, such as the relation between These include, for example, bilirubin in the neonate plasma glucose and meals, have obvious causes. (see Chapter 26) and plasma alkaline phosphatase activity, which is higher in children and the elderly (see Random Chapter 18). Day-to-day variations, for example in plasma iron concentrations, can be very large and may swamp regular Sex-related differences cycles. The causes of these are not clear, but they should be Examples of sex-related differences include plasma allowed for when serial results are interpreted – for example urate, which is higher in males, and high-density the effect of ‘stress’ on plasma cortisol concentrations. lipoprotein cholesterol, which is higher in pre- The time of meals affects plasma glucose menopausal women than in men (see Chapters 13 and concentrations, and therefore correct interpretation 20). Obviously, sex hormone concentrations also differ is often only possible if the blood is taken when the between the sexes (see Chapter 9). patient is fasting or at a set time after a standard dose of glucose (see Chapter 12). Ethnic differences These may occur because of either racial or environmental Methodological differences between laboratories factors, for example plasma creatine kinase may be higher in black than in white people (see Chapter 18). It has been pointed out that, even if the same method is used throughout a particular laboratory, it is Within-individual variations difficult to define normality clearly. Interpretation There are biological variations of both plasma may sometimes be even more difficult if the results concentrations and urinary excretion rates of many obtained in different laboratories, using different constituents, and test results may be incorrectly analytical methods, are compared. Agreement between interpreted if this is not taken into consideration. laboratories is close for many constituents partly due Biological variations may be regular or random. to improved standardization procedures and because many laboratories belong to external quality control Regular schemes. However, for others, such as plasma enzymes, Such changes occur throughout the 24-h period different methods may give different results. For various (circadian or diurnal rhythms, like those of body technical reasons, the results would still vary unless the temperature) or throughout the month. The daily substrate, pH and all the other variables were the same. (circadian) variation of plasma cortisol is of diagnostic value, but, superimposed on this regular variation, IS THE ABNORMALITY OF DIAGNOSTIC ‘stress’ will cause an acute rise (see Chapter 8). Plasma VALUE? iron concentrations may fall by 50 per cent between Relation between plasma and cellular morning and evening (see Chapter 21). To eliminate concentrations the unwanted effect of circadian variations, blood Intracellular constituents are not easily sampled, should ideally always be taken at the same time of day, and plasma concentrations do not always reflect the preferably in the early morning and, if indicated, with situation in the cells; this is particularly true for those 4 Requesting laboratory tests and interpreting the results constituents, such as potassium and phosphate, that DIAGNOSTIC PERFORMANCE are at much higher concentrations intracellularly Before one can interpret day-to-day changes in results than extracellularly. A normal, or even high, plasma and decide whether the patient’s biochemical state potassium concentration may be associated with has altered, one must know the degree of variation cellular depletion if equilibrium across cell membranes to be expected in the results derived from a normal is abnormal, such as in diabetic ketoacidosis. Analyte population. We have already discussed intraindividual concentrations may differ between plasma (the (same person) analyte variation. However, there is also aqueous phase of anticoagulated blood) and serum (the unavoidable analytical variation. aqueous phase of clotted blood). The concentration of potassium, for example, is higher in serum than in CASE 3 plasma samples because of leakage from cells during clotting, and the total protein concentration is lower in One hundred patients with chest pain were screened serum than in plasma because the protein fibrinogen is with a new biochemical test that showed 80 to be removed during the clotting process. positive for chest pain. What is the test’s sensitivity? Answer: 80/100 ¥ 100% = 80% Non-specific abnormalities The same test was used on 100 patients without chest The concentrations of all protein fractions, including pain, and 95 had a negative screening result. What is immunoglobulins, and of protein-bound substances the test’s specificity? may fall by as much as 15 per cent after as little as 30 min Answer: 95/100 ¥ 100% = 95% recumbency, owing to fluid redistribution in the body. DISCUSSION This may account, at least in part, for the low plasma Sensitivity is true-positive rate per total affected. albumin concentrations found in even quite minor Specificity is true-negative rate per total unaffected. illnesses. In-patients often have blood taken early in the morning, while recumbent, and plasma concentrations of protein and protein-bound substances tend to be Reproducibility of laboratory estimations lower than in out-patients (see Chapter 19). Most laboratory estimations should give results that are reproducible to well within 5 per cent; some, such as those for sodium and calcium, should be even more CASE 2 precise, but the variability of some hormone assays, for example, may be greater. Small changes in results A 54-year-old Nigerian man was seen in an accident produced by relatively imprecise methods are not likely and emergency department because of chest pain. to be clinically significant. His electrocardiogram (ECG) was normal. The Imprecision is the term used to describe the random following results were returned from the laboratory, changes that reduce the agreement between replicate 6 h after his chest pain started: assay measurements. This can be considered in terms of Plasma the within-assay precision, which is the assay variability Creatine kinase 498 U/L (< 250) when the same material is assayed repeatedly within Troponin T 10 pg/L (< 20) the same assay batch, or day-to-day precision, which DISCUSSION is the variability when the same material is assayed on The raised plasma creatine kinase activity suggested an different days. acute myocardial infarction (see also Chapters 18 and The assay coefficient of variation (CV) is used 22). The patient was, however, subsequently found not to express imprecision and can be calculated by the to have had a myocardial infarction (confirmed by a following equation: normal troponin T result) and the raised plasma creatine standard deviation of the assay kinase activity was thought to be due to his racial origin. CV% = ¥ 100% mean of the assay results (The reference range of < 250 U/L was based on that (1.1) of the predominantly white UK population; normal This should be as small as possible for each assay, and plasma creatine kinase activity may be two to three can be expressed as the intra-assay CV when describing times higher in black than in white people.) the imprecision within a single run or batch. Diagnostic performance 5 Test sensitivity and specificity 1 Diagnostic sensitivity is a measure of the frequency of a A B test being positive when a particular disease is present, C that is, the percentage of true-positive (TP) results. Diagnostic specificity is a measure of the frequency of Sensitivity a test being negative when a certain disease is absent, that is, the percentage of true-negative (TN) results. Ideally, a test would have 100 per cent specificity and 100 per cent sensitivity. The usefulness of tests can be expressed visually as receiver operating characteristic (ROC) curves (Fig. 1.3). Unfortunately, in population screening, some 0 subjects with a disorder may have a negative test (false- 0 1 negative, FN); conversely, some subjects without the 1 – Specificity condition in question will show an abnormal or positive Figure 1.3 Receiver operating characteristic (ROC) result (false-positive, FP). curve. The greater the area under the curve, the more The predictive value of a negative result is the percentage useful the diagnostic test. Test B is less useful than of all negative results that are true negatives, that is, the test A, which has greater sensitivity and specificity. C frequency of subjects without the disorder in all subjects depicts chance performance (area under the curve 0.5). with negative test results. A high negative predictive value is important in screening programmes if affected specificity decline. Conversely, if a diagnostic test has its individuals are not to be missed. This can be expressed as: cut-off or action limit set too high, fewer falsely positive TN individuals will be encompassed, but more individuals ¥ 100% (1.2) TN + FN will be falsely defined as negative, that is, its sensitivity The predictive value of a positive result is the will decrease and its specificity will increase. percentage of all positive results that are true positives: Likelihood ratios of laboratory tests in other words, the proportion of screening tests that Some may find predictive values confusing, and the are correct. A high positive predictive value is important likelihood ratio (LR) may be preferable. This can be to minimize the number of false-positive individuals defined as the statistical odds of a factor occurring being treated unnecessarily. This can be expressed as: in one individual with a disorder compared with it TP occurring in an individual without that disorder. ¥ 100% (1.3) TP + FP The LR for a negative test is expressed as: The overall efficiency of a test is the percentage of 1 – sensitivity patients correctly classified by the test. This should be (1.5) specificity as high as possible and can be expressed as: The LR for a positive test is expressed as: TP + TN ¥ 100% (1.4) sensitivity (1.6) TP + FP + TN + FN If the cut-off, or action, limit of a diagnostic test 1 – specificity is set too low, more falsely positive individuals will The greater the LR, the more clinically useful is the be included, and its sensitivity will increase and its test in question. SUMMARY Careful thought is required when it comes to The laboratory reference range should be consulted requesting and interpreting clinical biochemistry when interpreting biochemical results, and results tests. should be interpreted in the light of the clinical findings. Communication with the laboratory is essential to Just because a result is ‘abnormal’ does not mean ensure optimal interpretation of results and patient that the patient has an illness; conversely, a ‘normal’ management. result does not exclude a disease process. 2 Water and sodium Total sodium and water balance 6 Urinary sodium estimation 13 Control of water and sodium balance 7 Disturbances of water and sodium metabolism 15 Distribution of water and sodium in the body 9 It is essential to understand the linked homeostatic mainly in the ECF (Table 2.2). Water and electrolyte mechanisms controlling water and sodium balance intake usually balance output in urine, faeces, sweat when interpreting the plasma sodium concentration and expired air. and managing the clinically common disturbances of Water and sodium intake water and sodium balance. This is of major importance in deciding on the composition and amount, if any, of The daily water and sodium intakes are variable, but in intravenous fluid to give. It must also be remembered an adult amount to about 1.5–2 L and 60–150 mmol, that plasma results may be affected by such intravenous respectively. therapy, and can be dangerously misunderstood. Water and sodium output Water is an essential body constituent, and Kidneys and gastrointestinal tract homeostatic processes are important to ensure that the total water balance is maintained within narrow The kidneys and intestine deal with water and electrolytes limits, and the distribution of water among the in a similar way. Net loss through both organs depends vascular, interstitial and intracellular compartments is on the balance between the volume filtered proximally maintained. This depends on hydrostatic and osmotic Table 2.1 Approximate contributions of solutes to forces acting across cell membranes. plasma osmolality Sodium is the most abundant extracellular cation and, with its associated anions, accounts for most of Osmolality (mmol/kg) Total (%) the osmotic activity of the extracellular fluid (ECF); it Sodium and its anions 270 92 is important in determining water distribution across Potassium and its anions 7 cell membranes. Calcium (ionized) and its anions 3 Osmotic activity depends on concentration, and Magnesium and its anions 1 therefore on the relative amounts of sodium and water 8 Urea 5 in the ECF compartment, rather than on the absolute quantity of either constituent. An imbalance may cause Glucose 5 hyponatraemia (low plasma sodium concentration) or Protein 1 (approx.) hypernatraemia (high plasma sodium concentration), Total 292 (approx.) and therefore changes in osmolality. If water and sodium are lost or gained in equivalent amounts, the plasma Table 2.2 The approximate volumes in different body sodium, and therefore the osmolal concentration, is compartments through which water is distributed in a unchanged; symptoms are then due to extracellular 70-kg adult volume depletion or overloading (Table 2.1). As the metabolism of sodium is so inextricably related to that Volume (L) of water, the two are discussed together in this chapter. Intracellular fluid compartment 24 Extracellular fluid compartment 18 TOTAL SODIUM AND WATER BALANCE Interstitial (13) In a 70-kg man, the total body water (TBW) is about Intravascular (blood volume) (5) 42 L and contributes about 60 per cent of the total body weight; there are approximately 3000 mmol of sodium, Total body water 42 Control of water and sodium balance 7 and that reabsorbed more distally. Any factor affecting Control of antidiuretic hormone secretion either passive filtration or epithelial cellular function The secretion of ADH is stimulated by the flow of may disturb this balance. water out of cerebral cells caused by a relatively high Approximately 200 L of water and 30 000 mmol of extracellular osmolality. If intracellular osmolality sodium are filtered by the kidneys each day; a further is unchanged, an extracellular increase of only 10 L of water and 1500 mmol of sodium enter the 2 per cent quadruples ADH output; an equivalent intestinal lumen. The whole of the extracellular water fall almost completely inhibits it. This represents a and sodium could be lost by passive filtration in little change in plasma sodium concentration of only about more than an hour, but under normal circumstances 3 mmol/L. In more chronic changes, when the osmotic about 99 per cent is reabsorbed. Consequently, the gradient has been minimized by solute redistribution, net daily losses amount to about 1.5–2 L of water and there may be little or no effect. In addition, stretch 100 mmol of sodium in the urine, and 100 mL and receptors in the left atrium and baroreceptors in the 15 mmol, respectively, in the faeces. aortic arch and carotid sinus influence ADH secretion Fine adjustment of the relative amounts of water in response to the low intravascular pressure of severe and sodium excretion occurs in the distal nephron and hypovolaemia, stimulating ADH release. The stress the large intestine, often under hormonal control. The due to, for example, nausea, vomiting and pain may effects of antidiuretic hormone (ADH) or vasopressin also increase ADH secretion. Inhibition of ADH and the mineralocorticoid hormone aldosterone on secretion occurs if the extracellular osmolality falls, the kidney are the most important physiologically, for whatever reason. although natriuretic peptides are also important. Actions of antidiuretic hormone Sweat and expired air Antidiuretic hormone, by regulating aquaporin About 1 L of water is lost daily in sweat and expired 2, enhances water reabsorption in excess of solute air, and less than 30 mmol of sodium a day is lost in from the collecting ducts of the kidney and so sweat. The volume of sweat is primarily controlled dilutes the extracellular osmolality. Aquaporins are by skin temperature, although ADH and aldosterone cell membrane proteins acting as water channels have some effect on its composition. Water loss in that regulate water flow. When ADH secretion is a expired air depends on the respiratory rate. Normally, response to a high extracellular osmolality with the losses in sweat and expired air are rapidly corrected by danger of cell dehydration, this is an appropriate changes in renal and intestinal loss. However, neither response. However, if its secretion is in response to of these losses can be controlled to meet sodium and a low circulating volume alone, it is inappropriate to water requirements, and thus they may contribute the osmolality. The retained water is then distributed considerably to abnormal balance when homeostatic throughout the TBW space, entering cells along the mechanisms fail. osmotic gradient; the correction of extracellular depletion with water alone is thus relatively inefficient CONTROL OF WATER AND SODIUM in correcting hypovolaemia. Plasma osmolality BALANCE normally varies by less than 1–2 per cent, despite Control of water balance great variation in water intake, which is largely due to Both the intake and loss of water are controlled by the action of ADH. osmotic gradients across cell membranes in the brain’s In some circumstances, the action of ADH is hypothalamic osmoreceptor centres. These centres, opposed by other factors. For example, during an which are closely related anatomically, control thirst osmotic diuresis the urine, although not hypo-osmolal, and the secretion of ADH. contains more water than sodium. Patients with severe hyperglycaemia, as in poorly controlled diabetes Antidiuretic hormone (arginine vasopressin) mellitus, may show an osmotic diuresis. Antidiuretic hormone is a polypeptide with a half-life of about 20 min that is synthesized in the supraoptic Control of sodium balance and paraventricular nuclei of the hypothalamus and, The major factors controlling sodium balance are renal after transport down the pituitary stalk, is secreted blood flow and aldosterone. This hormone controls from the posterior pituitary gland (see Chapter 7). loss of sodium from the distal tubule and colon. 8 Water and sodium Aldosterone and the distal convoluted tubule. Renin is derived from Aldosterone, a mineralocorticoid hormone, is secreted prorenin by proteolytic action, and secretion increases by the zona glomerulosa of the adrenal cortex (see in response to a reduction in renal artery blood flow, Chapter 8). It affects sodium–potassium and sodium– possibly mediated by changes in the mean pressure in the hydrogen ion exchange across all cell membranes. Its afferent arterioles, and b-adrenergic stimulation. Renin main effect is on renal tubular cells, but it also affects splits a decapeptide (angiotensin I) from a circulating a2- loss in faeces, sweat and saliva. Aldosterone stimulates globulin known as renin substrate. Another proteolytic sodium reabsorption from the lumen of the distal enzyme, angiotensin-converting enzyme (ACE), which renal tubule in exchange for either potassium or is located predominantly in the lungs but is also present hydrogen ions (Fig. 2.1). The net result is the retention in other tissues such as the kidneys, splits off a further of more sodium than water, and the loss of potassium two amino acid residues. This is the enzyme that ACE and hydrogen ions. If the circulating aldosterone inhibitors (used to treat hypertension and congestive concentration is high and tubular function is normal, cardiac failure) act on. The remaining octapeptide, the urinary sodium concentration is low. angiotensin II, has a number of important actions: Many factors are involved in the feedback control of It acts directly on capillary walls, causing aldosterone secretion. These include local electrolyte vasoconstriction, and so probably helps to maintain concentrations, such as that of potassium in the adrenal blood pressure and alter the glomerular filtration rate gland, but they are probably of less physiological (GFR). Vasoconstriction may raise the blood pressure and clinical importance than the effect of the renin– before the circulating volume can be restored. angiotensin system. It stimulates the cells of the zona glomerulosa to synthesize and secrete aldosterone. The renin–angiotensin system It stimulates the thirst centre and so promotes oral Renin is an aspartyl protease secreted by the fluid intake. juxtaglomerular apparatus, a cellular complex adjacent to the renal glomeruli, lying between the afferent arteriole Poor renal blood flow is often associated with an inadequate systemic blood pressure. The release of Glomerulus renin results in the production of angiotensin II, which B– Na+ tends to correct this by causing aldosterone release, which stimulates sodium and water retention and hence restores the circulating volume. Thus, aldosterone secretion responds, via renin, to a reduction in renal blood flow. Sodium excretion is not directly related to total body sodium content or to plasma sodium concentration. B– Na+ Na+ Na+ Aldosterone Natriuretic peptides H+ HCO3– H+ A peptide hormone (or hormones) secreted from K+ Renal the right atrial or ventricular wall in response to the tubular stimulation of stretch receptors may cause high sodium lumen H2CO3 HB excretion (natriuresis) by increasing the GFR and by inhibiting renin and aldosterone secretion. However, CD the importance of this hormone (or hormones) in CO2 the physiological control of sodium excretion and in H2O pathological states has not yet been fully elucidated, although it is important in the pathophysiology of Renal tubular cell congestive cardiac failure (see Chapter 22). Figure 2.1 The action of aldosterone on the Monitoring fluid balance reabsorption of Na+ in exchange for either K+ or H+ from the distal renal tubules. See text for details. CD, The most important factor in assessing changes in carbonate dehydratase; B–, associated anion. day-to-day fluid balance is accurate records of fluid Distribution of water and sodium in the body 9 intake and output; this is particularly pertinent for may be affected by pre-existing abnormalities of unconscious patients. ‘Insensible loss’ is usually protein or red cell concentrations. assumed to be about 1 L/day, but there is endogenous Haemoconcentration ECF is usually lost from water production of about 500 mL/day as a result the vascular compartment first and, unless the of metabolic processes. Therefore the net daily fluid is whole blood, depletion of water and small ‘insensible loss’ is about 500 mL. The required daily molecules results in a rise in the concentration of intake may be calculated from the output during the large molecules, such as proteins and blood cells, previous day plus 500 mL to allow for ‘insensible with a rise in blood haemoglobin concentration and loss’; this method is satisfactory if the patient is haematocrit, raised plasma urea concentration and normally hydrated before day-to-day monitoring reduced urine sodium concentration. is started. Serial patient body weight determination Table 2.4 shows various intravenous fluid regimens can also be useful in the assessment of changes in that can be used clinically. A summary of the British fluid balance. Consensus Guidelines on Intravenous Fluid Therapy Pyrexial patients may lose 1 L or more of fluid in sweat for Adult Surgical Patients (GIFTASUP) can be found and, if they are also hyperventilating, respiratory water at www.bapen.org.uk/pdfs/bapen_pubs/giftasup.pdf. loss may be considerable. In such cases an allowance of about 500 mL for ‘insensible loss’ may be totally DISTRIBUTION OF WATER AND SODIUM inadequate. In addition, patients may be incontinent IN THE BODY of urine, and having abnormal gastrointestinal losses In mild disturbances of the balance of water and makes the accurate assessment of fluid losses very electrolytes, their total amounts in the body may be of difficult. less importance than their distribution between body Inaccurate measurement and charting are useless and compartments (see Table 2.2). may be dangerous. Water is distributed between the main body Keeping a cumulative fluid balance record is a useful fluid compartments, in which different electrolytes way of detecting a trend, which may then be corrected contribute to the osmolality. These compartments are: before serious abnormalities develop. In the example shown in Table 2.3, 500 mL has been intracellular, in which potassium is the predominant allowed for as net ‘insensible daily loss’; calculated losses cation, are therefore more likely to be underestimated than extracellular, in which sodium is the predominant overestimated. This shows how insidiously a serious cation, and which can be subdivided into: deficit can develop over a few days. – interstitial space, with very low protein The volume of fluid infused should be based on the concentration, and calculated cumulative balance and on clinical evidence – intravascular (plasma) space, with a relatively of the state of hydration, and its composition adjusted high protein concentration. to maintain normal plasma electrolyte concentrations. Electrolyte distribution between cells and Assessment of the state of hydration of a patient relies interstitial fluid on clinical examination and on laboratory evidence of Sodium is the predominant extracellular cation, its haemodilution or haemoconcentration. intracellular concentration being less than one-tenth Haemodilution Increasing plasma volume with of that within the ECF. The intracellular potassium protein-free fluid leads to a fall in the concentrations concentration is about 30 times that of the ECF. About of proteins and haemoglobin. However, these findings 95 per cent of the osmotically active sodium is outside Table 2.3 Hypothetical cumulative fluid balance chart assuming an insensible daily loss of 500 mL Measured intake (mL) Measured output (mL) Total output (minimum mL) Daily balance (mL) Cumulative balance (mL) Day 1 2000 1900 2400 –400 –400 Day 2 2000 2000 2500 –500 –900 Day 3 2100 1900 2400 –300 –1200 Day 4 2200 2000 2500 –300 –1500 10 Water and sodium Table 2.4 Some electrolyte-containing fluids for intravenous infusion Na+ K+ Cl– HCO3– Glucose Ca2+ Approximate (mmol/L) (mmol/L) (mmol/L) (mmol/L) (mmol/L) (mmol/L) osmolarity ¥ plasma Saline ‘Normal’ (physiological 0.9%) 154 – 154 – – – ¥1 Twice ‘normal’ (1.8%) 308 – 308 – – – ¥2 Half ‘normal’ (0.45%) 77 – 77 – – – ¥ 0.5 ‘Dextrose’ saline 5%, 0.45% 77 – 77 – 278 – ¥ 1.5 Sodium bicarbonate 1.4% 167 – – 167 – – ¥1 8.4%a 1000 – – 1000 – – ¥6 Complex solutions Ringer’s 147 4.2 156 – – 2.2 ¥1 Hartmann’s 131 5.4 112 29 b – 1.8 ¥1 a Most commonly used bicarbonate solution. Note marked hyperosmolarity. Only used if strongly indicated. b As lactate 29 mmol/L. cells, and about the same proportion of potassium is Osmotic pressure intracellular. Cell-surface energy-dependent sodium/ The net movement of water across a membrane that is potassium adenosine triphosphatase pumps maintain permeable only to water depends on the concentration these differential concentrations. gradient of particles – either ions or molecules – across Other ions tend to move across cell membranes that membrane, and is known as the osmotic gradient. in association with sodium and potassium. (The For any weight-to-volume ratio, the larger the particles, movement of hydrogen ions is discussed in Chapter 4.) the fewer there are per unit volume, and therefore the Magnesium and phosphate ions are predominantly lower the osmotic effect they exert. If the membranes intracellular, and chloride ions extracellular. were freely permeable to ions and smaller particles as well as to water, these diffusible particles would exert Distribution of electrolytes between plasma and interstitial fluid no osmotic effect across membranes, and therefore the larger ones would become more important in affecting The cell membranes of the capillary endothelium are water movement. This action gives rise to the effective more permeable to small ions than those of tissue cells. colloid osmotic (oncotic) gradient. Water distribution The plasma protein concentration is relatively high, in the body is thus dependent largely on three factors, but that of interstitial fluid is very low. The osmotic namely: effect of the intravascular proteins is balanced by very slightly higher interstitial electrolyte concentrations 1. the number of particles per unit volume, (Gibbs–Donnan effect); this difference is small and, for 2. particle size relative to membrane permeability, practical purposes, plasma electrolyte concentrations 3. concentration gradient across the membrane. can be assumed to be representative of those in the ECF as a whole. Units of measurement of osmotic pressure Osmolar concentration can be expressed as: Distribution of water the osmolarity (in mmol/L) of solution, Over half the body water is intracellular (see Table the osmolality (in mmol/kg) of solvent. 2.2). About 15–20 per cent of the extracellular water is intravascular; the remainder constitutes the interstitial If solute is dissolved in pure water at concentrations fluid. The distribution of water across biological such as those in body fluids, osmolarity and osmolality membranes depends on the balance between the will hardly differ. However, as plasma is a complex hydrostatic pressure and the in vivo effective osmotic solution containing large molecules such as proteins, pressure differences on each side of the membrane. the total volume of solution (water + protein) is Distribution of water and sodium in the body 11 greater than the volume of solvent only (water) in contributes much more than 6 per cent to the measured which the small molecules are dissolved. At a protein plasma volume, the calculated osmolarity may be concentration of 70 g/L, the volume of water is about significantly lower than the true osmolality in the plasma 6 per cent less than the total volume of the solution (that water. A hypothetical example is shown in Figure 2.2. is, the molarity should theoretically be about 6 per cent Many formulae of varying complexity have been less than the molality). Most methods for measuring proposed to calculate plasma osmolarity. None of them individual ions assess them in molarity (mmol/L). If can predict the osmotic effect, but the following formula the concentration of proteins in plasma is markedly (in which square brackets indicate concentration) gives increased, the volume of solvent is significantly reduced a close approximation to plasma osmolality (although but the volume of solution remains unchanged. some equations omit the potassium, which may be Therefore the molarity (in mmol/L) of certain ions preferable): such as sodium will be reduced but the molality will be Plasma osmolality = 2[Na+] + 2[K+] + [urea] unaltered. This apparently low sodium concentration is + [glucose] in mmol/L (2.1) known as pseudohyponatraemia. The factor of 2, which is applied to the sodium and Measured plasma osmolality potassium concentrations, allows for the associated Osmometers measure changes in the properties of a anions and assumes complete ionization. This solution, such as freezing point depression or vapour calculation is not valid if gross hyperproteinaemia pressure, which depend on the total osmolality or hyperlipidaemia is present or an unmeasured of the solution – the osmotic effect that would be osmotically active solute, such as mannitol, methanol, exerted by the sum of all the dissolved molecules and ethanol or ethylene glycol, is circulating in plasma. ions across a membrane permeable only to water. A significant difference between measured These properties are known as colligative properties. and calculated osmolality in the absence of Sodium and its associated anions (mainly chloride) hyperproteinaemia or hyperlipidaemia may suggest contribute 90 per cent or more to this measured plasma alcohol or other poisoning. For example, a plasma osmolality, the effect of protein being negligible. As the alcohol concentration of 100 mg/dL contributes about only major difference in composition between plasma 20 mmol/kg to the osmolality. This osmotic difference is and interstitial fluid is in protein content, the plasma known as the osmolar gap and can be used to assess the osmolality is almost identical to the osmolality of the presence in plasma of unmeasured osmotically active interstitial fluid surrounding cells. particles. In such cases the plasma sodium concentration Calculated plasma osmolarity may be misleading as a measure of the osmotic effect. It It is the osmolam, rather than the osmolar, concentration is not possible to calculate urinary osmolarity because that exerts an effect across cell membranes and that of the considerable variation in the concentrations of is controlled by homeostatic mechanisms. However, different, sometimes unmeasured, solutes; the osmotic as discussed below, the calculated plasma osmolarity pressure of urine can be determined only by measuring is usually as informative as the measured plasma the osmolality. osmolality. Although, because of the space-occupying effect of Distribution of water across cell membranes protein, the measured osmolality of plasma should be Osmotic pressure gradient higher than the osmolarity, calculated from the sum of Because the hydrostatic pressure difference across the the molar concentrations of all the ions, there is usually cell membrane is negligible, cell hydration depends on little difference between the two figures. This is because the effective osmotic difference between intracellular there is incomplete ionization of, for example, NaCl to and extracellular fluids. The cell membranes are freely Na+ and Cl–; this reduces the osmotic effect by almost permeable to water and to some solutes, but different the same amount as the volume occupied by protein solutes diffuse (or are actively transported) across cell raises it. membranes at different rates, although always more Consequently, the calculated plasma osmolarity is a slowly than water. In a stable state, the total intracellular valid approximation to the true measured osmolality. osmolality, due mostly to potassium and associated However, if there is gross hyperproteinaemia or anions, equals that of the interstitial fluid, due mostly hyperlipidaemia such that either protein or lipid to sodium and associated anions; consequently, there is 12 Water and sodium Plasma [Na+] 144 mmol/kg H2O Plasma [Na+] Plasma [Na+] 135 mmol/L plasma 127 mmol/L plasma (144 ¥ 0.94) (144 ¥ 0.88) 0.94 L Plasma H2O 0.88 L Total plasma Total plasma volume 1.0 L volume 1.0 L Osmolarity Osmolality Osmolarity 270 mmol/L plasma 288 mmol/kg H2O 254 mmol/L plasma Raised protein Normal protein 0.12 L or lipid concentration concentration 0.06 L No lipid ‘Normal’ High content of large molecules Figure 2.2 The consequence of gross hyperproteinaemia or hyperlipidaemia on the plasma water volume and its effect on the calculated plasma osmolarity and the true plasma osmolality. no net movement of water into or out of cells. In some increased osmotic gradient alters cell hydration, but pathological states, rapid changes of extracellular solute in chronic uraemia, although the measured plasma concentration affect cell hydration; slower changes may osmolality is often increased, the osmotic effect of urea allow time for the redistribution of solute and have is reduced as the concentrations gradually equalize on little or no effect. the two sides of the membrane. Sodium In normal subjects sodium and its associated Glucose Like urea, the normally low extracellular anions account for at least 90 per cent of extracellular concentration of glucose does not contribute significantly osmolality. Rapid changes in their concentration therefore to the osmolality. However, unlike urea, glucose is actively affect cellular hydration. If there is no significant change transported into many cells, but once there it is rapidly in the other solutes, a rise causes cellular dehydration metabolized, even at high extracellular concentrations, and a fall causes cellular overhydration. and the intracellular concentration remains low. Severe Urea Normal extracellular concentrations are so hyperglycaemia, whether acute or chronic, causes a low as to contribute very little to the measured plasma marked osmotic effect across cell membranes, with osmolality. However, concentrations 15-fold or more movement of water from cells into the extracellular above normal can occur in severe uraemia and can compartment causing cellular dehydration. then make a significant contribution (see Chapter 3). Although hyperglycaemia and acute uraemia can However, urea does diffuse into cells very much more cause cellular dehydration, the contribution of normal slowly than water. Consequently, in acute uraemia, the urea and glucose concentrations to plasma osmolality Urinary sodium estimation 13 is so small that reduced levels of these solutes, unlike is the most important protein contributing to the those of sodium, do not cause cellular overhydration. colloid osmotic pressure. It is present intravascularly Solutes such as potassium, calcium and magnesium at significant concentration but extravascularly only at are present in the ECF at very low concentrations. a very low concentration because it cannot pass freely Significant changes in these are lethal at much lower across the capillary wall. concentrations than those that would change osmolality. The osmotic gradient across vascular walls cannot be Mannitol is an example of an exogenous substance estimated by simple means. The total plasma osmolality that remains in the extracellular compartment because it gives no information about this. Moreover, the plasma is not transported into cells, and may be infused to reduce albumin concentration is a poor guide to the colloid cerebral oedema. Ethanol is only slowly metabolized, osmotic pressure. Although other proteins, such as and a high concentration in the ECF may lead to cerebral globulins, are present in the plasma at about the same cellular dehydration; this may account for some of the concentration as albumin, their estimation for this symptoms of a hangover. High glucose concentrations purpose is even less useful: their higher molecular weights account for the polyuria of severe diabetes mellitus. mean that they have even less effect than albumin. Large rises in the osmotic gradient across cell membranes may result in the movement of enough Relation between sodium and water homeostasis water from the intracellular compartment to dilute extracellular constituents. Consequently, if the change In normal subjects, the concentrations of sodium and in osmolality has not been caused by sodium and its its associated anions are the most important osmotic associated anions, a fall in plasma sodium concentration factors affecting ADH secretion. Plasma volume, by its is appropriate to the state of osmolality. If, under such effect on renal blood flow, controls aldosterone secretion circumstances, the plasma sodium concentration is not and therefore sodium balance. The homeostatic low, this indicates hyperosmolality. mechanisms controlling sodium and water excretion Generally, plasma osmolarity calculated from sodium, are interdependent. (A simplified scheme is shown potassium, urea and glucose concentrations is at least as in Fig. 2.4.) Thirst depends on a rise in extracellular clinically valuable as measured plasma osmolality. It has osmolality, whether due to water depletion or sodium the advantage that the solute responsible, and therefore excess, and also on a very large increase in the activity its likely osmotic effect, is often identified. of the renin–angiotensin system. A rise in extracellular osmolality reduces water Distribution of water across capillary membranes loss by stimulating ADH release and increases intake The maintenance of blood pressure depends on by stimulating thirst; both these actions dilute the the retention of fluid within the intravascular extracellular osmolality. Osmotic balance (and therefore compartment at a higher hydrostatic pressure than that cellular hydration) is rapidly corrected. of the interstitial space. Hydrostatic pressure in capillary Assessment of sodium status lumina tends to force fluid into the extravascular space. As already discussed, the plasma sodium concentration In the absence of any effective opposing force, fluid is important because of its osmotic effect on fluid would be lost rapidly from the vascular compartment. distribution. Plasma sodium concentrations should be Unlike other cell membranes, those of the capillaries are monitored while volume is being corrected to ensure permeable to small ions. Therefore sodium alone exerts that the distribution of fluid between the intracellular almost no osmotic effect and the distribution of water and extracellular compartments is optimal. The across capillary membranes is little affected by changes in presence of other osmotically active solutes should be electrolyte concentration. taken into account. Colloid osmotic pressure The very small osmotic effect of plasma protein URINARY SODIUM ESTIMATION molecules produces an effective osmotic gradient Urinary sodium excretion is not related to body content across capillary membranes; this is known as the but to renal blood flow. colloid osmotic, or oncotic, pressure. It is the most Estimation of the urinary sodium concentration in important factor opposing the net outward hydrostatic a random specimen may be of value in the diagnosis of pressure (Fig. 2.3). Albumin (molecular weight 65 kDa) the syndrome of inappropriate antidiuretic hormone 14 Water and sodium INTRACELLULAR COMPARTMENT CELL MEMBRANE Osmotic gradient INTERSTITIAL COMPARTMENT Hydrostatic Colloid osmotic gradient gradient INTRAVASCULAR COMPARTMENT Capillary membrane ARTERIOLE VENULE Figure 2.3 Osmotic factors that control the distribution of water between the fluid compartments of the body. (Posterior pituitary) ADH Plasma volume Thirst Renal blood flow Hypothalamic osmolality Renin release Angiotensin II Plasma [Na+] ALDOSTERONE (adrenal cortex) H2O reabsorption Na+ reabsorption Figure 2.4 Control of water and sodium homeostasis. ADH, antidiuretic hormone. secretion (SIADH) and may help to differentiate renal urine [sodium] plasma [creatinine] FENa% = ¥ ¥ 100% circulatory insufficiency (pre-renal) from intrinsic plasma [sodium] urine [creatinine] renal damage (see Chapter 3). (2.2) The fractional excretion of sodium (FENa%) may also be useful in helping to assess renal blood flow and A value of less than 1 per cent may be found in poor can be measured using a simultaneous blood sample renal perfusion, for example pre-renal failure, and of and spot urine sample: more than 1 per cent in intrinsic renal failure. Disturbances of water and sodium metabolism 15 DISTURBANCES OF WATER AND membranes, with redistribution of fluid between cells SODIUM METABOLISM and the ECF. However, gradual changes, which allow The initial clinical consequences of primary sodium time for redistribution of diffusible solute such as urea, disturbances depend on changes of extracellular and therefore for equalization of osmolality without osmolality and hence of cellular hydration, and those major shifts of water, may produce little effect on fluid of primary water disturbances depend on changes in distribution. extracellular volume. We now discuss in some detail conditions involving Plasma sodium concentration is usually a substitute water and sodium deficiency and excess. These are for measuring plasma osmolality. Plasma sodium discussed together, as the two are so closely inter-related concentrations per se are not important, but their effect in vivo and can result in abnormal plasma sodium on the osmotic gradient across cell membranes is, and concentrations. it should be understood that the one does not always reflect the other. Water and sodium deficiency (Figs 2.5–2.7) If the concentration of plasma sodium alters rapidly, Apart from the loss of solute-free water in expired and the concentrations of other extracellular solutes air, water and sodium are usually lost together from remain the same, most of the clinical features are due the body. An imbalance between the degrees of their to the consequence of the osmotic difference across cell deficiency is relatively common and may be due to + ADH ISOSMOTIC VOLUME – + + Thirst Renal blood flow + + Hypothalamic osmolality Renin release + + Angiotensin II + Plasma [Na+] ALDOSTERONE + Figure 2.5 Homeostatic correction of isosmotic volume depletion. The reduced intravascular volume impairs renal blood flow and stimulates renin and therefore aldosterone secretion. There is selective sodium reabsorption from the distal tubules and a low urinary sodium concentration. (Shading indicates primary change.) ADH, antidiuretic hormone. – ADH H2O VOLUME + – Renal blood flow – Hypothalamic – osmolality Renin release – – Angiotensin II – Plasma [Na+] ALDOSTERONE – Figure 2.6 Infusion of hypotonic fluid as a cause of predominant sodium depletion. Increased circulating volume with reduction in plasma osmolality inhibits aldosterone and antidiuretic hormone (ADH) secretion. (Shading indicates primary change.) 16 Water and sodium + + ADH H2O VOLUME – – Renal blood flow + Hypothalamic osmolality Renin release + Angiotensin II
