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, Free

Clearance: Cx = (Ux × V) / Px

Ux, urine concentration (mg/mL)

V, urine flow rate (mL/min)

Px, plasma concentration (mg/mL)

Clearance

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%.

What actually controls GFR and RPF: arteriolar tone

The glomerulus sits between two arterioles, blood enters through the afferent arteriole and leaves through the efferent arteriole, and it's the relative tone of those two vessels, not just "how much blood is flowing," that sets GFR, RPF, and filtration fraction independently.

Constricting the afferent arteriole simply restricts inflow: less blood reaches the glomerulus, so both RPF and GFR fall together, and filtration fraction barely changes.

Constricting the efferent arteriole does something different: blood has a harder time leaving, so it backs up and glomerular pressure rises. RPF still falls (less blood is getting through overall), but GFR actually rises, more of whatever blood is there gets pushed through the filtration barrier. That combination (GFR up, RPF down) means filtration fraction rises whenever the efferent arteriole constricts.

This is exactly the mechanism angiotensin II uses to protect GFR when renal perfusion pressure drops (e.g. in hypotension): it preferentially constricts the efferent arteriole, propping GFR up even as RPF falls. It's also why ACE inhibitors and ARBs, which block angiotensin II, can precipitate a real drop in GFR in patients whose kidney function depends on that efferent tone (classically, bilateral renal artery stenosis): removing the efferent constriction lets that arteriole dilate, RPF rises, but GFR falls and filtration fraction drops with it.

One more caveat specific to PAH: this approximation (clearance ≈ RPF) only holds at low plasma PAH concentrations. The tubular secretion transporters that make PAH clearance work have a transport maximum (Tm), push plasma PAH high enough and they saturate, so a smaller fraction gets secreted per pass and clearance starts to underestimate true RPF. Real PAH clearance measurements are deliberately done at low, non-saturating concentrations for exactly this reason.

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).

Show answer

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?

Show answer

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.

How do the afferent and efferent arterioles differently affect GFR?
Constricting the afferent arteriole lowers both RPF and GFR together (less blood reaches the glomerulus at all). Constricting the efferent arteriole lowers RPF but raises GFR (blood backs up, glomerular pressure rises), which is why efferent constriction specifically raises filtration fraction while afferent constriction leaves it roughly unchanged.

Why can ACE inhibitors worsen kidney function in some patients?
Angiotensin II normally constricts the efferent arteriole preferentially, propping GFR up when renal perfusion is low. ACE inhibitors and ARBs remove that support, the efferent arteriole dilates, RPF rises, but GFR falls. In patients whose GFR depends heavily on that mechanism (classically bilateral renal artery stenosis), this can precipitate a real, dangerous drop in kidney function.