A 70-year-old woman on furosemide for heart failure develops a creatinine rise from 1.0 to 2.4 mg/dL over 2 days. Blood pressure is 96/60 mmHg lying, 82/55 mmHg standing. Urine sodium is 45 mEq/L and fractional excretion of sodium is 1.8%. Which additional value best distinguishes prerenal azotaemia from established acute tubular necrosis here?

• A Urinary urea nitrogen above 500 mmol/L confirms tubular necrosis
• B Fractional excretion of urea above 50% supports prerenal azotaemia
• C Fractional excretion of urea below 35% supports prerenal azotaemia despite loop diuretic use ✓
• D Fractional excretion of urea between 40% and 60% excludes intrinsic renal failure

Explanation

FENa loses its reliability once a loop diuretic is acting, because natriuresis pushes the value above 1% even in true hypovolaemia. Fractional excretion of urea is less affected by diuretics because urea reabsorption occurs mainly in the proximal tubule and medullary collecting duct, independent of the loop. C FEUrea below 35% indicates intact tubular handling and therefore prerenal physiology. C FEUrea above 50% favours intrinsic tubular injury, making option B the reverse of the truth.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.