Which measurement provides an estimate of glomerular filtration rate (GFR)?

• A. Creatinine clearance
• B. BUN
• C. Uric acid
• D. Inulin clearance

Answer: (A)

Estimating GFR relies on a substance whose renal handling mirrors filtration. Creatinine is produced at a relatively constant rate by muscle and is freely filtered at the glomerulus. Most of it is excreted in urine without being reabsorbed, and only a small amount is secreted by the proximal tubules. Because of this, the clearance of creatinine closely matches the rate at which the kidneys filter plasma, giving a practical estimate of GFR. In clinical practice, this is often done by measuring creatinine clearance from a urine sample over 24 hours or by estimating GFR from serum creatinine using formulas.

BUN, by contrast, is influenced by hydration status, protein intake, and liver function, so it does not reliably reflect GFR. Uric acid clearance isn’t a standard measure of GFR either. Inulin clearance would determine GFR more accurately since inulin is freely filtered and neither reabsorbed nor secreted, but it’s impractical for routine use, so creatinine clearance remains the standard clinical estimate.