Clues Pointing TO This Diagnosis

NSAID use for two weeks — reduces prostaglandin-mediated afferent arteriolar dilation — reduces GFR
Rising creatinine — acute onset — not chronic
Decreased urine output — oliguria
Lower extremity edema — fluid retention
Fatigue — uremia developing
Recent significant physical exertion — rhabdomyolysis as additional consideration
Responds to stopping offending agent and IV fluids
Creatinine peaks then trends down — reversible cause

Why It's NOT the Similar Diseases

NOT chronic kidney disease — CKD develops over months to years — small kidneys on ultrasound — anemia of chronic disease. Robert's kidneys were normal before — this is ACUTE — creatinine rose over days to weeks.
NOT nephrotic syndrome — nephrotic syndrome causes massive proteinuria above 3.5 grams per day — hypoalbuminemia — edema — hyperlipidemia. Robert has minimal proteinuria — the problem is reduced filtration not protein leakage.
NOT urinary obstruction causing post-renal AKI — obstruction causes bilateral hydronephrosis on ultrasound — difficulty voiding — distended bladder. Robert has no voiding symptoms.
NOT glomerulonephritis — GN causes hematuria — proteinuria — red cell casts — hypertension — often post-streptococcal or autoimmune context. Robert has a clear medication trigger.
NOT rhabdomyolysis alone — rhabdomyolysis from exercise causes AKI through myoglobin toxicity — tea-colored urine — markedly elevated CK. CK should be checked — may coexist with NSAID-induced AKI.

Differential Diagnosis List

Chronic kidney disease — ruled out by acute onset — normal baseline creatinine — reversibility
Nephrotic syndrome — ruled out by minimal proteinuria — no hypoalbuminemia
Post-renal obstruction — ruled out by ultrasound showing no hydronephrosis
Glomerulonephritis — ruled out by NSAID trigger — no hematuria — no casts
Rhabdomyolysis — check CK — may coexist with NSAID-induced AKI after marathon

Key Distinguishing Test

Serum creatinine and BUN — rising confirms AKI. Urinalysis — assess for casts — proteinuria — hematuria. Renal ultrasound — rules out obstruction. Fractional excretion of sodium — FENa — below 1% in prerenal — above 1% in intrinsic renal disease. Urine sodium — low in prerenal — high in intrinsic. CK — rules out rhabdomyolysis.

Board Note

📝 AKI CLASSIFICATION: Prerenal — reduced blood flow to kidneys — volume depletion — NSAIDs — ACE inhibitors — responds to fluids — FENa below 1%. Intrinsic — damage to kidney itself — ATN — glomerulonephritis — FENa above 1%. Post-renal — obstruction — BPH — stones — bilateral hydronephrosis on ultrasound. KDIGO criteria define AKI as creatinine rise of 0.3 mg/dL within 48 hours or 1.5 times baseline within 7 days or urine output below 0.5 mL/kg/hour for 6 hours. The dangerous NSAID plus ACE inhibitor combination blocks both afferent dilation and efferent constriction — dramatically reducing filtration pressure.

“Robert’s Story”