A patient has urine osmolality 900 mOsm/kg, urine flow 0.5 mL/min, and plasma osmolality 290 mOsm/kg. What is the free water clearance (CH2O)?
- A +0.5 mL/min (positive free water clearance)
- B −1.05 mL/min (negative free water clearance, i.e., TcH2O = 1.05 mL/min) ✓
- C −0.5 mL/min
- D Zero — urine is isotonic to plasma
Correct answer: B. −1.05 mL/min (negative free water clearance, i.e., TcH2O = 1.05 mL/min)
Explanation
Free water clearance: CH2O = V − Cosm, where Cosm (osmolar clearance) = (Uosm × V) / Posm = (900 × 0.5) / 290 = 450/290 = 1.55 mL/min. Therefore CH2O = 0.5 − 1.55 = −1.05 mL/min. A negative value (TcH2O = 1.05 mL/min) means the kidney is producing concentrated urine and reabsorbing free water from the collecting duct (ADH present). Positive free water clearance (CH2O > 0) indicates dilute urine formation (ADH absent). Urine osmolality 900 >> plasma 290, confirming concentrated urine and ADH activity.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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