Assessing Renal Function PDF

Assessing Renal Function Indicators of renal decline Analysis of urine and/or plasma Proteinuria / albuminuria Indicate Haematuria damage Estimated GFR / GFR Indicate Serum creatinine / urea function Calcium / phosphate Indicate homeostasis...

Assessing Renal Function Indicators of renal decline Analysis of urine and/or plasma Proteinuria / albuminuria Indicate Haematuria damage Estimated GFR / GFR Indicate Serum creatinine / urea function Calcium / phosphate Indicate homeostasis function but Electrolytes / pH not Fluid balance / urine volume quantitative Haemoglobin LO: 5 Measuring renal function Glomerular filtration rate (GFR) = volume of filtrate formed by all the nephrons in both kidneys per unit time GFR is directly related to the function of the nephrons & declines in all forms of progressive kidney diseases GFR is usually accepted as the best overall index of kidney function in health & disease Note: although it tells you about the function of the kidney it does not tell you the cause or location of the problem Linked to surface area of body ∴ typical young male GFR = 120 ml/min/1.73m2 GFR linked to age, sex and body size – declines with increasing age Difficult to measure directly Routine measures of GFR Rationale: If something that is normally filtered by the kidney builds up in the blood it indicates ↓GFR and therefore ↓renal function Measurement using exogenous markers is more accurate but can be time-consuming and expensive, eg inulin infusion Measurement via endogenous markers more cost-effective and used routinely in clinical practice Three tests used routinely to assess renal function:- 1. Serum urea Increasing 2. Serum creatinine accuracy 3. Estimated GFR (eGFR) All use a single serum (blood) measurement and are therefore more convenient for the routine assessment and monitoring of renal function Serum creatinine Serum creatinine more accurately reflects GFR than urea Creatinine is formed from the breakdown of creatine, a skeletal muscle component Usually produced at a more steady rate for a given individual Still relatively inaccurate as a point measure of GFR, though useful for monitoring trends Inaccuracies result from the way its production varies between individuals and relates to muscle mass Factors affecting muscle mass Age, sex, amputation, malnutrition, muscle wasting Diet also affects creatinine levels (vegetarian diet vs. meat rich diet) LO: 5 Clinicalkey: https://www.clinicalkey.com/student/content/book/3-s2.0-B978032359712800028X#hl00013 Estimated GFR (eGFR) Estimated glomerular filtration rate uses equations to calculate (estimate) the GFR based on a single serum measurement of a substance Incorporates simple clinical information along with a single serum measurement to generate an estimated GFR (eGFR) Most use serum creatinine, age, sex to calculate the eGFR CKD-EPI equation (CKD Epidemiology) – recommended by NICE https://www.kidney.org/professionals/kdoqi/gfr_calculator ‘Normal’ eGFR = >90 ml/min/1.73m2 Block 1 GFR & K idney Although most calculations use serum creatinine (eGFRcreatinine) Disease newer tests available use serum cystatin C (eGFRcystatinC) Worksh op LO: 5 Clinicalkey: https://www.clinicalkey.com/student/content/book/3-s2.0-B978032359712800028X#hl00013 Limitations of eGFR As no part of CKD-EPI equation includes a measure of body size, and is are based on serum creatinine, eGFR results can be influenced by factors that alter muscle mass E.g. a patient with an amputation / malnutrition would have a lower serum creatinine than may be expected for their age, sex and therefore this would lead to an incorrectly high eGFR Limitations on its use in children; acute kidney injury; drug dose calculations for highly toxic drugs However, eGFR more accurate than serum creatinine alone Useful in monitoring renal function & in detecting early decline LO: 5 Clinicalkey: https://www.clinicalkey.com/student/content/book/3-s2.0-B978032359712800028X#hl00013 Further reading Guyton & Hall Textbook of Medical Physiology; 13th edition (Hall) Medical Sciences, 2nd edition (Naish & Syndercombe Court) Physiology, 6th edition (Costanzo) Kumar & Clark’s Clinical Medicine, 8th edition (Kumar & Clark) Dr Rosemary Bland Dr Jamie Roebuck