A patient receiving intravenous fluids has a urine flow rate of 2 mL/min, urine osmolality of 600 mOsm/kg, and plasma osmolality of 300 mOsm/kg. What is her free water clearance?

• A +2 mL/min, indicating production of dilute urine
• B Zero, indicating the kidney is neither concentrating nor diluting urine
• C +4 mL/min, indicating solute-free water generation
• D -2 mL/min, indicating active water conservation by ADH ✓

Explanation

Free water clearance equals urine flow rate minus osmolar clearance. Osmolar clearance here is (600 x 2)/300, which equals 4 mL/min. So free water clearance is 2 minus 4, giving -2 mL/min. A negative value means the kidney is excreting concentrated urine, conserving free water under ADH action, as in volume depletion or SIADH. A positive value would require urine osmolality below plasma, which is not present here.

Reference: Vander's Renal Physiology, 9th ed.

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