Q.20 Glomerular filtration rate is determined by creatinine clearance test and inulin clearance test. Which of the
following statements is true about GFR?

1. Inulin clearance gives the exact measurement of GFR.

2. Creatinine clearance gives the exact measurement of GFR.

3. Inulin clearance overestimates GFR.

4. Both inulin and creatinine clearance overestimate GFR.

GFR Measurement: Inulin vs Creatinine Clearance Explained

Inulin clearance provides the gold standard for exact glomerular filtration rate (GFR) measurement as it’s freely filtered and neither reabsorbed nor secreted by tubules.​

Correct Answer

1. Inulin clearance gives the exact measurement of GFR.
Inulin is an ideal marker: completely filtered at glomerulus, not reabsorbed, not secreted, and not metabolized, matching true GFR (~125 mL/min/1.73m² in healthy adults).

Option Explanations

1. Inulin Clearance (Correct)

Intravenous inulin infusion with timed urine/plasma collections yields precise GFR via clearance formula (U*V/P). It’s the reference standard despite being cumbersome.

2. Creatinine Clearance

Endogenous creatinine is filtered but slightly secreted by proximal tubules (~10-20%), causing 10-20% overestimation of GFR. Urine collection errors add variability.

3. Inulin Clearance Overestimates GFR

False; inulin defines true GFR without tubular handling, so it neither over- nor underestimates. Any deviation in practice stems from collection inaccuracies.​

4. Both Overestimate GFR

Incorrect; only creatinine overestimates due to secretion. Inulin remains exact, making this combination wrong.