A 58-year-old man with diabetes mellitus type 2 and hypertension presents to the emergency department with acute onset oliguria (urine output 0.3 mL/kg/hr) for 8 hours. Serum creatinine has risen from baseline 1.2 mg/dL to 2.8 mg/dL. Urinalysis shows muddy brown casts and granular casts. On examination, he is euvolemic with normal JVP. Blood pressure is 148/92 mmHg. He reports no recent nephrotoxic drug exposure. What is the most appropriate immediate next step in management?

Explanation

## Clinical Context This patient presents with acute kidney injury (AKI) with clinical and laboratory features suggestive of acute tubular necrosis (ATN): muddy brown casts, granular casts, oliguria, and rapid rise in creatinine in a diabetic patient. The euvolemic status and normal JVP help exclude prerenal AKI from volume depletion. ## Why Measure FENa and Urine Osmolality? **Key Point:** The fractional excretion of sodium (FENa) and urine osmolality are the gold-standard bedside tests to differentiate prerenal AKI from intrinsic renal disease (ATN) in the acute phase. **High-Yield:** FENa calculation: $$FENa = \frac{(U_{Na} \times P_{Cr})}{(P_{Na} \times U_{Cr})} \times 100\%$$ - **FENa < 1%** → Prerenal (kidneys avidly reabsorbing sodium) - **FENa > 2%** → Intrinsic renal disease (ATN, glomerulonephritis) - **Urine osmolality > 500 mOsm/kg** → Prerenal - **Urine osmolality < 350 mOsm/kg** → ATN In this case, the muddy brown casts already suggest ATN, but FENa and urine osmolality confirm the diagnosis and guide fluid management strategy. ## Why Not the Other Options? | Step | Rationale | Timing | |------|-----------|--------| | Renal ultrasound | Indicated if obstruction suspected (anuria, flank pain, hydronephrosis risk); this patient is oligaric with casts → ATN more likely | Secondary (if FENa inconclusive) | | Antibiotics | No fever, no sepsis signs, no indication for empirical coverage | Not indicated | | IV hydration bolus | Contraindicated in euvolemic patient; risks pulmonary edema and worsening AKI if intrinsic renal disease | Harmful in this context | **Clinical Pearl:** In euvolemic or hypervolemic AKI with casts, aggressive fluid resuscitation can worsen outcomes. Establish the mechanism (prerenal vs. intrinsic) first via FENa before deciding on fluid strategy. **Mnemonic — AKI Workup Order: "FUSS"** - **F**ENa / Urine osmolality (differentiate mechanism) - **U**ltrasound (rule out obstruction if needed) - **S**pecific tests (biopsy, serology if glomerulonephritis suspected) - **S**upport (manage complications: hyperkalemia, acidosis, fluid balance) [cite:Harrison 21e Ch 279]