Why is inulin used to measure GFR specifically?

Inulin is freely filtered at the glomerulus but neither reabsorbed nor secreted by the tubules afterward — so the amount that appears in urine per minute exactly equals the amount filtered per minute, making its clearance a direct measure of GFR. Creatinine approximates this in clinical practice (it's endogenous, no infusion needed) but is slightly secreted, which makes measured creatinine clearance run a bit higher than true GFR.

Why does PAH clearance estimate renal plasma flow instead of GFR?

Para-aminohippurate (PAH) is both filtered at the glomerulus and actively secreted by the tubules, so nearly all of it is removed from the plasma in a single pass through the kidney. That means its clearance approximates the total plasma flow through the kidney (RPF), not just the filtered fraction.

If clearance is higher than GFR, what does that mean?

A substance with clearance greater than GFR (like PAH) is being net secreted by the tubules in addition to being filtered — the kidney is actively adding it to the urine beyond what filtration alone would deliver. Clearance below GFR means net reabsorption is occurring.

Renal Clearance Calculator — GFR & Filtration Fraction, Free

Clearance: Cx = (Ux × V) / Px

Ux — urine concentration (mg/mL)

V — urine flow rate (mL/min)

Px — plasma concentration (mg/mL)

Clearance

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Filtration fraction: FF = GFR / RPF

GFR (mL/min, e.g. from inulin)

RPF (mL/min, e.g. from PAH)

How to calculate renal clearance

Clearance is the volume of plasma the kidneys completely clear of a substance per minute: Cx = (Ux × V) / Px, where Ux is the substance's urine concentration, V is urine flow rate, and Px is its plasma concentration. Which substance you use determines what the number means. Inulin is freely filtered but never reabsorbed or secreted, so its clearance directly equals GFR (glomerular filtration rate). PAH is filtered and almost completely secreted, so its clearance approximates RPF (renal plasma flow). The ratio GFR/RPF is the filtration fraction — normally about 20%, the share of plasma flowing through the glomerulus that actually becomes filtrate.

Normal values: GFR ≈ 125 mL/min (~180 L/day), RPF ≈ 600-700 mL/min, filtration fraction ≈ 20%.

Related tools: Osmotic pressure & tonicity · all biochem tools.

Worked example: inulin clearance (GFR) and filtration fraction

Inulin: urine concentration 125 mg/mL, flow rate 1 mL/min, plasma concentration 1 mg/mL (the tool's defaults).

Cx = (Ux × V) / Px = (125 × 1) / 1 = 125 mL/min This is a normal GFR — inulin's clearance is used as the reference standard because it's neither reabsorbed nor secreted.

With RPF measured (via PAH clearance) at 625 mL/min:

FF = GFR / RPF = 125 / 625 = 0.20 = 20% Right at the normal filtration fraction — about a fifth of the plasma flowing through the glomerulus becomes filtrate.

Practice problems

1. PAH: urine concentration 15 mg/mL, flow rate 1.2 mL/min, plasma concentration 0.03 mg/mL. Find the clearance (this estimates RPF).

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Cx = (15 × 1.2) / 0.03 = 18/0.03 = 600 mL/min A textbook-normal renal plasma flow — consistent with PAH being almost completely cleared from plasma in one pass through the kidney.

2. Creatinine: urine concentration 60 mg/mL, flow rate 1.5 mL/min, plasma concentration 1.2 mg/mL. Find the clearance. Is this GFR normal, low, or high?

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Cx = (60 × 1.5) / 1.2 = 90/1.2 = 75 mL/min 75 mL/min is well below the normal ~125 mL/min GFR — consistent with reduced kidney function (a plasma creatinine of 1.2 mg/mL is itself already a mild-to-moderate elevation above the typical ~0.6-1.0 mg/mL range).

FAQ

Why inulin for GFR specifically?
It's freely filtered but never reabsorbed or secreted, so urine output per minute exactly equals the filtered amount — a direct GFR measure. Creatinine approximates this clinically (endogenous, no infusion needed) but is slightly secreted, running measured clearance a bit above true GFR.

Why does PAH estimate renal plasma flow instead?
PAH is filtered AND actively secreted, so it's almost completely removed from plasma in one pass — its clearance approximates total plasma flow through the kidney, not just the filtered fraction.

What does filtration fraction tell you?
FF = GFR/RPF, normally ~20% — the share of renal plasma flow that actually gets filtered into the tubule, versus the ~80% that continues to the peritubular capillaries.

What does clearance above or below GFR mean?
Above GFR (like PAH) means net tubular secretion is adding to what filtration alone delivers. Below GFR means net reabsorption is occurring.