A 45-year-old man has plasma creatinine of 2 mg/dL and urine creatinine of 160 mg/dL with a urine flow rate of 1 mL/min. His plasma urea is 60 mg/dL and urine urea is 1800 mg/dL. What is his GFR, and what does comparison with urea clearance indicate?

• A GFR 80 mL/min; urea clearance is 30 mL/min, indicating tubular reabsorption of urea ✓
• B GFR 80 mL/min; urea clearance equals GFR, confirming no tubular handling
• C GFR 160 mL/min; urea clearance is 30 mL/min, indicating tubular secretion of creatinine
• D GFR 40 mL/min; urea clearance exceeds GFR, indicating tubular secretion of urea

Explanation

Creatinine clearance = (Ucr × V) / Pcr = (160 × 1) / 2 = 80 mL/min, approximating GFR. Urea clearance = (Uurea × V) / Purea = (1800 × 1) / 60 = 30 mL/min. Because urea clearance (30) is less than GFR (80), net tubular reabsorption of urea is occurring—approximately 40–50% of filtered urea is reabsorbed in the proximal tubule and inner medullary collecting duct, which is physiologically normal.

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

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