Biol 131 L7.pdf

The Clinical Biochemistry lab • Also known as chemical pathology and clinical chemistry. • Blood Sciences – combined Biochemistry & Haematology • Chemical and biochemical methods are applied to the study of disease. • Discretionary or selective requesting by doctors is based on patient’s symptoms,...

The Clinical Biochemistry lab • Also known as chemical pathology and clinical chemistry. • Blood Sciences – combined Biochemistry & Haematology • Chemical and biochemical methods are applied to the study of disease. • Discretionary or selective requesting by doctors is based on patient’s symptoms, signs and previous history 1 Clinical Biochemistry Samples • Why are tests needed? • Diagnosis, monitoring of treatment, disease screening, prognosis • Samples are usually serum from venous blood or urine • Blood is collected into blood specimen tubes depending on the requested test • Blood • clot- centrifuge- top layer = SERUM • Blood • anticoagulant (e.g.heparin or EDTA )– centrifuge- top layer = PLASMA 2 Tubes used for blood collection Serum separating gel Gaw p 4 3 Biochemical analyses - other specimens • Urine specimen – preservative to inhibit bacterial growth, or acid to stabilise certain metabolites - container to hold 24 h sample (e.g. identification of adrenaline metabolites) •Arterial blood •Faeces •Cerebrospinal fluid (CSF) •Sweat •Saliva 4 Biochemical analyses – Processing a request •The laboratory must be provided with the correct specimen and a completed clinical biochemistry form •Appropriate information to ensure the correct test •Bar coding to ensure correct sample identification •Automated analysis • Result returned to the requesting clinician with minimum delay  correct patient identification 5 The clinical biochemistry process Gaw p5 6 Clinical biochemistry test results • Many diseases elicit significant changes in chemical composition of body fluids. Testing measures changes quantitatively. Examples: 1. A raised blood sugar in diabetes mellitus due to lack of insulin 2. Raised blood cardiac biomarkers due to their release from heart muscle after heart attack (e.g. Troponin T) 3. Raised blood excretion products in kidney disease ‘Analyte’ =substances being determined in an analysis 7 Precision and Accuracy • PRECISION = reproducibility • ACCURACY = how close to the measured value is the actual value? Gaw p6 8 Analytical Sensitivity and Specificity • Sensitivity – how little of the analyte can be detected by the assay? • Specificity – how good is the assay at discriminating between the requested analyte and other interfering substances. 9 Quality assurance. • How can the lab staff ensure that the results of their tests are reliable? • Internal quality assurance samples • Daily or every time a test used • Test results compared to monitor performance • External quality assurance samples • Identical samples distributed to several labs. • Comparison of several laboratories to maintain high standards. P6-7 Gaw 10 Reference intervals/ranges • Reference range (interval) chosen to include 95% of the values found in healthy volunteers. • So… 5% of the normal population will be outside the reference range! • 67% - mean ± 1 SD • 95% - mean ± 2 SD • 99% - mean ± 3 SD Note –reference ranges vary slightly according to source/lab. E.g. potassium might be listed as 3.4-4.9 mmol/l, 3.5-5.3 mmol/l, 3.8-5.0 mmol/l 11 Use the range you have been supplied with in each case. False positives and False negatives False positive: • A result outside the reference range, but person is healthy and within the 5% excluded in the reference range. False negative: •A patient has the disease, but gives a ‘normal’ result P7 Gaw 12 Some biological factors affecting interpretation of biochemical results • Different reference ranges for genetically male and female individuals - age, pregnancy. • Diet – (for example a fasting blood glucose sample) • Time of day e.g. cortisol (hormone) • Stress/anxiety/menstrual cycle can affect levels of hormones • Strenuous Exercise - release of enzymes interfere with other tests Gaw p 7 13 Point of care testing • Rapid reassurance or further investigations • Convenient • Clinically meaningful? Range of analytes • • • • • Glucose Protein Cholesterol Urea and creatinine Hormones Human chorionic gonadotropin (hCG) – pregnancy testing 14 Some problems with biochemical specimens • Difficult blood sampling  haemolysis  potassium (K+) release  pseudohyperkalaemia • Insufficient specimen - minimum volume required • Errors in timing e.g. 24-h urine specimen • Incorrect specimen container e.g. blood • Inappropriate sampling site e.g. ‘downstream’ from an i.v. drip • Incorrect specimen storage e.g. in a blood sample stored overnight blood cells become leaky e.g. K+ Gaw p 4 15 Some core biochemical tests • Us & Es (Urea and creatinine & Electrolytes): • sodium, potassium, chloride, bicarbonate, • calcium and phosphate • LFTs (Liver function tests): • Alkaline phosphatase; alanine amino transferase; • bilirubin; albumin • Thyroxine and thyroid stimulating hormone • H+, pCO2, pO2 (blood gases) • Glucose 16 Specialised tests – Specialised techniques (not offered by every hospital) • trace elements • hormones • specific proteins e.g. tumour markers • Drugs • DNA analysis 17