Creatinine clearance is used routinely instead of inulin clearance to estimate GFR. The systematic error introduced is that creatinine clearance:

• A Overestimates GFR because creatinine is secreted by the proximal tubule ✓
• B Underestimates GFR because creatinine is partly reabsorbed
• C Overestimates GFR because creatinine binds plasma proteins
• D Underestimates GFR because creatinine raises tubular backpressure

Explanation

Creatinine is freely filtered but also enters the tubular fluid by proximal tubular secretion via organic cation transporters, so the amount appearing in urine exceeds the filtered load and the calculated clearance runs about 10 to 20 percent above true GFR. This error is small enough for routine use because rising serum creatinine offsets it clinically. Reabsorption would cause underestimation, which does not occur with creatinine.

Reference: Ganong Review of Medical Physiology, 26th ed.

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