A 45-year-old patient undergoing renal function assessment has a serum creatinine of 1.8 mg/dL. The creatinine clearance is 48 mL/min. Which biochemical principle makes creatinine a useful but imperfect glomerular filtration marker?
- A Creatinine is freely filtered and completely reabsorbed by tubules
- B Creatinine is freely filtered but also secreted by proximal tubules, causing GFR overestimation ✓
- C Creatinine is protein-bound and only partially filtered at the glomerulus
- D Creatinine is solely dependent on dietary protein intake and not endogenous production
Correct answer: B. Creatinine is freely filtered but also secreted by proximal tubules, causing GFR overestimation
Explanation
Creatinine is freely filtered at the glomerulus (not protein-bound) but is also secreted by proximal tubular cells via organic cation transporters; this secretory component becomes proportionally more significant as GFR falls, causing creatinine clearance to overestimate true GFR by 10–20% in normal individuals and more substantially in CKD. Additionally, non-creatinine chromogens in the Jaffé reaction cause slight overestimation. Cimetidine blocks tubular secretion of creatinine and can be used to measure true GFR.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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