A 65-year-old woman develops AKI (creatinine rises from 0.8 to 3.2 mg/dL in 3 days) after starting ibuprofen for knee pain. FENa is 0.8%. Urine microscopy shows granular casts and renal tubular epithelial cells. Urinary sodium is 45 mEq/L. The most likely diagnosis is:

• A Pre-renal AKI (volume depletion from NSAID-reduced prostaglandins)
• B Acute interstitial nephritis (AIN) due to NSAIDs
• C Minimal change disease with nephrotic syndrome
• D NSAID-induced acute tubular necrosis (ATN) ✓

Explanation

Granular (muddy brown) casts and renal tubular epithelial (RTE) cells in urinary sediment are the hallmark of acute tubular necrosis. FENa >1% (here 0.8% — but with NSAID use, FENa can be borderline due to vasoconstriction) and urinary sodium >40 mEq/L with tubular cast formation indicates intrinsic renal damage from ATN. NSAID-induced pre-renal AKI has FENa <1% and bland urine. Acute interstitial nephritis shows WBC casts and eosinophiluria. NSAID-induced minimal change disease presents with heavy proteinuria and nephrotic syndrome.

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

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Written and medically reviewed by the StethoPrep medical team.