A patient with nephrotic syndrome has a GFR of 80 mL/min, serum creatinine 1.2 mg/dL, and 24-hour urine creatinine excretion of 1200 mg in 1500 mL urine. Using the urine:plasma creatinine ratio, which segment of the nephron is responsible for the observation that creatinine clearance consistently OVERESTIMATES GFR by approximately 10–20%?

• A Distal tubule reabsorbs creatinine by carrier-mediated transport, reducing apparent clearance
• B Glomerular back-filtration of creatinine occurs in disease states reducing its urinary concentration
• C Proximal tubule secretion of creatinine via organic cation transporters (OCT2/MATE1) adds creatinine to the filtrate, making urinary creatinine exceed filtered load ✓
• D Collecting duct secretion of creatinine via aquaporin channels elevates urine creatinine

Explanation

Creatinine is freely filtered at the glomerulus but is also actively secreted by organic cation transporter 2 (OCT2) on the basolateral membrane and MATE1/MATE2-K on the apical membrane of proximal tubule cells. This tubular secretion adds approximately 10–20% more creatinine to the urine beyond the filtered load, causing creatinine clearance to overestimate true GFR by 10–20%. This overestimation becomes clinically significant in kidney disease where both secretion and filtration fall, partially masking GFR decline initially. Cimetidine and trimethoprim block OCT2 and are sometimes used experimentally to block secretion for more accurate GFR estimation. Options B, C, and D describe incorrect or non-existent mechanisms.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.