Renal Function Tests PDF

Renal Function Tests Dr Sheromna Sewpersad University of Pretoria NHLS – Tshwane Academic Division Department of Chemical Pathology  Kidney function Excretion of waste Maintenance of extracellular fluid volume and composition, including acid– base balance Hormone...

Renal Function Tests Dr Sheromna Sewpersad University of Pretoria NHLS – Tshwane Academic Division Department of Chemical Pathology  Kidney function Excretion of waste Maintenance of extracellular fluid volume and composition, including acid– base balance Hormone synthesis Kidney function tests available Urinalysis: Dipstick Microalbuminuria Blood: Electrolytes (Na, K, Cl) Urea Creatinine Cystatin C Beta 2 microglobulin Equations: Creatinine clearance eGFR Urinalysis Proteinuria Most positive dipsticks for protein are benign Microalbuminuria Sensitive test (lower limit of detection compared to urine dipstick) Detects low concentration of albumin present in the urine 30-300mg/day Indicates early nephropathy e.g. screen for diabetic nephropathy Site of damage Glomerular proteinuria: 2-3 g /day Heavy proteinuria (mainly albuminuria) Indication of CKD (DM, glomerular disease or HT) Tubular proteinuria (LMW: 60 population with proven screening for CKD mL/min renal impairment Takes into account the sex, age, Takes into account the Takes into account body weight serum creatinine value and racial sex, age, serum creatinine in addition to sex, age and origin value and racial origin serum creatinine concentration Mainly used for screening the Many drug dosing regimens, population for CKD such as some of those used in chemotherapy, are based on this GFR eGFR misleading in the following circumstances: AKI increased volume of distribution for creatinine, such as with the oedema of heart failure or nephrotic syndrome pregnancy decreased muscle mass, such as with paraplegia, amputations or muscle wasting disorders increased muscle mass in athletes and body builders extremes of age ethnic groups in whom the formula has not been validated malnutrition and obesity after a meat-rich meal treatment with drugs that interfere with creatinine secretion by the renal tubules. Cystatin C Cystatin C is a small protein that is produced by all nucleated cells. It is freely filtered by the glomerulus and completely reabsorbed and catabolized by the PCT cells thus its plasma concentration reflects the GFR more accurately than creatinine. It is not much influenced by sex or muscle mass but may be increased in malignancy, hyperthyroidism and by treatment with corticosteroids. Sensitive screening test for early assessment of changes in the GFR Special population groups Some eGFR equations include cystatin C in their calculations Beta 2 microglobulin Serum ẞ2M is freely filtered by the glomerulus and freely reabsorbed by the PCT of the nephron Good as a measure of GFR (similar to serum creatinine) Urine ẞ2M : very sensitive indicator of renal tubular disease Not specific but relatively sensitive marker of various neoplastic, inflammatory and infectious conditions Prognostic marker in multiple myeloma Tests for renal tubular function Performed less frequently than tests of glomerular function. Glycosuria with a normal blood glucose concentration implies proximal tubular malfunction that may be either isolated (renal glycosuria) or part of a generalized tubular defect (renal Fanconi syndrome). Amino aciduria can occur with tubular defects and may also be generalized or specific to a few amino acids. Tests of proximal tubular bicarbonate reabsorption may be required in the assessment of proximal renal tubular acidosis (RTA). The small amount of protein that is filtered by the glomeruli is normally almost completely reabsorbed by and catabolized in the proximal renal tubular cells. Tests for renal tubular function The increased excretion of low-molecular-weight proteins in urine can indicate renal tubular damage. β2-Microglobulin quantitation has been used for this purpose but it is unstable in acidic or infected urine. Retinol-binding protein or α1-microglobulin is more reliable but, in practice, specific evidence of tubular damage is rarely required clinically. The only tests of distal tubular function in widespread use are the water deprivation test, to assess renal concentrating ability, and tests of urinary acidification, to diagnose distal RTA. Conclusion Renal function tests aim to give us a glimpse of the state of the kidneys GFR is the most widely accepted measure of renal function however direct measurement is not routinely performed Plasma concentrations of analytes can be useful to identify dehydration as well as tubular and glomerular defects Equations for the estimation of GFR using creatinine + Cystatin C have made it easier to acquire an idea of the functioning of the kidneys in a quick manner, however, caution needs to be taken in certain circumstances. References Marshall WJ, Lapsley M, Day AP, Shipman K. Clinical Chemistry. Ninth. Elsevier; 2020. Predictive System Implementation to Improve the Accuracy of Urine Self-Diagnosis with Smartphones: Application of a Confusion Matrix- Based Learning Model through RGB Semiquantitative Analysis - Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Urine-dipstick-color- comparison-test-table_fig1_362192528 [accessed 4 Jan, 2024]