Physiology · Renal, fluids and acid–base

Renal clearance !

Measurement
Exam priority: high

Clearance tells how much plasma the kidney cleans of a substance per minute. It measures filtration and shows whether the tubule reabsorbs or secretes.

Updated 2026-11-01

Also called: Clearance

See it in 3D: Kidneys

What it is

The clearance of a substance is the volume of plasma completely freed of it per unit time. It is a virtual volume: it compares what leaves in the urine with what is in the plasma. Compared with GFR, it reveals what the tubule does with that substance.

The nephron: segments, transporters and diuretics

GFR ≈ 125 mL/minFiltrate ≈ 180 L/dayUrine ≈ 1–2 L/day

Tap a nephron segment: what it reabsorbs, its transporters, the hormones, the diuretics that act there and the classic disorders.

Glomerulus and arterioles
Proximal tubule
Thin descending limb
Thin ascending limb
Thick ascending limb
Distal convoluted tubule
Collecting tubule and duct
Juxtaglomerular apparatus (macula densa)
Cortex
Medulla
Interstitial osmolality (≈ 300 → 1200 mOsm/kg)

Illustrative schematic drawn by Durgical (a long-loop juxtamedullary nephron); proportions and percentages are approximate and not for clinical decisions.

Equations

ClearanceC_x = (U_x × V) / P_x

U_x: urine concentration
V: urine flow (mL/min)
P_x: plasma concentration

Units of flow (mL/min). Use the same concentration unit above and below.

Fractional excretionFE_x = C_x / GFR = (U_x × P_Cr) / (P_x × U_Cr)

P_Cr, U_Cr: plasma and urine creatinine

FENa < 1% suggests a sodium-retaining (prerenal) kidney, if no diuretic was given.

Tubular balanceExcretion = Filtration − Reabsorption + Secretion

Filtration: GFR × P_x (filtered load)

Comparing excretion with the filtered load shows whether reabsorption or secretion dominates.

Typical clearances

Parameter	Value	Note
Inulin	≈ 125 mL/min	= GFR
Creatinine	≈ 110–140 mL/min	slightly above GFR
PAH	≈ 585–625 mL/min	≈ effective renal plasma flow
Urea	≈ 50–70 mL/min	≈ 40–50% reabsorbed
Glucose	≈ 0 mL/min	fully reabsorbed below threshold

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

Reading clearance against GFR

Result	What the tubule does	Example
C = GFR	Neither reabsorbs nor secretes	Inulin
C < GFR	Net reabsorption	Na⁺, urea, glucose
C > GFR	Net secretion	PAH, penicillin, creatinine (a little)
C ≈ 0	Reabsorbs all that is filtered	Glucose, amino acids

Clinical notes

Estimated GFR (eGFR) from serum creatinine stages chronic kidney disease; muscle mass (elderly, malnourished, amputees) biases it.
Some drugs (trimethoprim, cimetidine) block creatinine secretion: creatinine rises without a fall in true GFR.
In acute kidney injury, FENa helps separate a prerenal cause (< 1%) from tubular necrosis (> 2%).

Test yourself

Think of the answer, then tap the question to see it.

A substance has a clearance of 300 mL/min with a GFR of 120 mL/min: what does the tubule do with it?

It secretes it: more is excreted than filtered, so there is net tubular secretion (like PAH or penicillin).

Why does creatinine clearance slightly overestimate GFR?

Besides being filtered, some creatinine is secreted in the proximal tubule, and that extra amount in urine raises the calculated clearance.

Why can PAH clearance estimate renal plasma flow?

PAH is filtered and the rest is almost completely secreted in a single pass, so nearly all plasma reaching the kidney is cleared of PAH.

Why does glucose appear in urine in uncontrolled diabetes when glucose clearance is normally zero?

When blood glucose rises, the filtered load exceeds the transport maximum of proximal tubule SGLTs; the excess is not reabsorbed and is excreted.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

Kidneys

In Biochemistry

In Pharmacology

Benzylpenicillin (penicillin G)

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 25.5 Physiology of Urine Formation (OpenStax (Rice University), 2022); 25.6 Tubular Reabsorption (OpenStax (Rice University), 2022); Chronic Kidney Disease (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 28). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.