CorpusHuman Anatomy
UrinaryPhysiological process

Renal clearance

Also known as: clearance, inulin clearance, creatinine clearance, PAH clearance, free water clearance

The volume of plasma from which a substance is completely removed by the kidneys per unit time, C = (U × V) / P; used to measure GFR (inulin, creatinine) and renal plasma flow (PAH).

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Overview

Inulin clearance≈125 mL/min (= GFR)
PAH clearance≈600–650 mL/min (≈ eRPF)
Glucose clearance0 (normal plasma glucose)

For any substance X: C_X = (U_X × V) / P_X, where U_X is its urine concentration, V the urine flow rate and P_X its plasma concentration.

Clearance = GFR for a substance that is freely filtered and neither reabsorbed nor secreted — inulin (gold standard; ≈125 mL/min). Creatinine clearance slightly overestimates GFR (≈10–20% is secreted) and is usually estimated rather than measured (eGFR). Clearance < GFR indicates net reabsorption (glucose 0; Na⁺ ≈1 mL/min; urea ≈60–70 mL/min). Clearance > GFR indicates net secretion; PAH is filtered and secreted so completely that its clearance (≈600–650 mL/min) estimates effective renal plasma flow. Renal blood flow = RPF / (1 − haematocrit) ≈1.1–1.2 L/min. Filtration fraction = GFR / RPF ≈0.2.

Osmolar clearance C_osm = (U_osm × V) / P_osm. Free-water clearance C_H₂O = V − C_osm: positive (dilute urine, low ADH, e.g. diabetes insipidus or water loading), negative (concentrated urine, high ADH, e.g. dehydration or SIADH).

Location

Whole-kidney concept.

Function

  • Quantify GFR, renal plasma flow and tubular handling of substances
  • Guide drug dosing in kidney disease
Physiology

Typical clearances (mL/min): glucose 0, Na⁺ ≈1, urea ≈70, inulin ≈125, creatinine ≈140, PAH ≈600–650.

Clinical significance

Drug dosing and eGFR

Renally cleared drugs (aminoglycosides, vancomycin, metformin, DOACs, lithium, digoxin) require dose adjustment by eGFR or Cockcroft–Gault creatinine clearance to avoid toxicity.

Test yourself

The clearance of which substance gives the best measure of glomerular filtration rate?
  1. Para-aminohippurate (PAH)
  2. Glucose
  3. Urea
  4. Inulin

Answer: Inulin

Inulin is freely filtered and neither reabsorbed, secreted nor metabolised, so its clearance equals GFR. Creatinine is used clinically but slightly overestimates GFR because some is secreted. PAH clearance estimates renal plasma flow.

PAH clearance approximates effective renal plasma flow because PAH is:
  1. Freely filtered and fully reabsorbed
  2. Filtered and almost completely secreted in a single pass
  3. Not filtered but completely reabsorbed
  4. Bound to albumin and excluded from the filtrate

Answer: Filtered and almost completely secreted in a single pass

PAH is filtered and then actively secreted by organic anion transporters of the PCT, so ≈90% of the plasma reaching the kidney is cleared of it in one pass. C_PAH ≈ effective RPF ≈600–650 mL/min.

More urinary system questions

Compiled from standard anatomy and physiology references — see the Urinary System sources. For education only; not medical advice.