Renal Clearance
Direct answer
Renal clearance of a substance is the volume of plasma completely cleared of that substance by the kidneys per unit time. It is calculated as clearance = (U × V) / P — urine concentration multiplied by urine flow rate, divided by plasma concentration — and expressed in millilitres per minute. Comparing a substance's clearance with the glomerular filtration rate shows how the kidney handles it: filtration alone, net reabsorption or net secretion.
What you must remember
- Inulin clearance equals the GFR (about 125 ml per minute): freely filtered, neither reabsorbed nor secreted — the gold standard.
- Creatinine clearance is the practical clinical estimate of GFR; slight tubular secretion makes it marginally higher than the true GFR, which is also why plasma creatinine rises only late in chronic kidney disease.
- Para-aminohippuric acid (PAH) clearance equals effective renal plasma flow (about 600–660 ml per minute), being filtered and almost completely secreted; renal blood flow is about 1100–1200 ml per minute, roughly a fifth of resting cardiac output.
- Filtration fraction = GFR divided by renal plasma flow — about 0.2, so around one-fifth of plasma perfusing the glomerulus is filtered.
- Interpretation rule: clearance above the GFR means net secretion (PAH); below the GFR, net reabsorption (urea); glucose at normal plasma levels has zero clearance because it is fully reabsorbed.
- Free water clearance = urine flow minus osmolar clearance: positive in dilute urine, negative in concentrated urine under antidiuretic hormone, zero when urine is isosmotic with plasma.
- Clinical use: estimated GFR from plasma creatinine stages chronic kidney disease and falls physiologically with age.
Common confusion
Clearance is often mistaken for the amount excreted in urine; it is a virtual volume of plasma, and the urine flow rate must be entered in millilitres per minute or the arithmetic fails. The second classic error is calling PAH clearance the true renal plasma flow — with extraction of about 90 per cent, it measures the effective renal plasma flow.
Exam-focused takeaway
In theory, define clearance, state the formula, then present the three standard examples — inulin for GFR, creatinine for clinical estimation, PAH for renal plasma flow — with the interpretation rule. In viva, expect why creatinine clearance overestimates GFR, the meaning of filtration fraction, and how clearance distinguishes reabsorption from secretion. In practicals, compute a creatinine clearance from given values, interpret it, and explain free water clearance with an example such as diabetes insipidus.
Practise MCQs and previous-year questions on renal clearance and GFR in the PrepElephant app. Free notes continue on this website.
Frequently asked questions
Which substance is used to measure GFR and why?
Inulin, because it is freely filtered and neither reabsorbed nor secreted, so its clearance — about 125 ml per minute — equals the GFR exactly.
Why does creatinine clearance overestimate GFR?
Creatinine is filtered and also secreted to a small extent by the tubules, so its clearance is slightly above the true GFR, though it remains the most convenient clinical estimate.
What is the filtration fraction and its normal value?
GFR divided by renal plasma flow: about 125/625, or approximately 0.2, meaning one-fifth of the plasma perfusing the kidney is filtered.
What does it mean if a substance's clearance exceeds the GFR?
Net tubular secretion, as with PAH; a clearance below the GFR indicates net reabsorption, as with urea.
What is free water clearance?
Urine flow minus osmolar clearance — positive in dilute urine, negative in concentrated urine under antidiuretic hormone, and zero when urine is isosmotic.
Practise this in the PrepElephant app
Question banks, previous-year questions, mock tests and revision tools — for Renal Clearance and Physiology. Free to start.