A 62-year-old man with diabetes mellitus type 2 presents with acute onset oliguria and serum creatinine rise from 1.2 to 3.8 mg/dL over 48 hours following contrast-enhanced CT angiography for suspected pulmonary embolism. Urine output is 200 mL/day. Which investigation is most appropriate to confirm the diagnosis of contrast-induced acute kidney injury (CI-AKI)?

Explanation

## Diagnosis of Contrast-Induced Acute Kidney Injury ### Clinical Context CI-AKI is a form of acute tubular necrosis (ATN) that occurs within 24–72 hours of contrast exposure in susceptible patients (diabetes, chronic kidney disease, volume depletion). The temporal relationship to contrast administration is the key diagnostic clue. ### Why Urine Osmolality & U/P Creatinine Ratio? **Key Point:** In ATN (including CI-AKI), the tubular epithelium is damaged and cannot concentrate urine or reabsorb filtered creatinine effectively. **High-Yield:** The diagnostic pattern in ATN is: - **Urine osmolality < 350 mOsm/kg** (dilute urine despite oliguria) - **Urine-to-plasma creatinine ratio < 20** (loss of tubular reabsorption) - **FENa > 2%** (sodium wasting due to tubular damage) This constellation of findings confirms **intrinsic renal (tubular) injury** and distinguishes CI-AKI from prerenal azotemia (where urine would be concentrated and U/P Cr ratio > 40). ### Why Not FENa Alone? **Clinical Pearl:** FENa is sensitive but not specific for ATN. It is elevated in ATN but also in early sepsis, rhabdomyolysis, and other causes of intrinsic AKI. FENa *alone* does not confirm CI-AKI; the combination of low urine osmolality + low U/P Cr ratio + high FENa is diagnostic. ### Differential Interpretation Table | Finding | Prerenal AKI | ATN/CI-AKI | Postrenal AKI | |---------|--------------|-----------|---------------| | Urine osmolality | > 500 | < 350 | Variable | | U/P Cr ratio | > 40 | < 20 | < 20 | | FENa | < 1% | > 2% | > 2% | | Urine appearance | Clear | Muddy brown casts | Clear/hematuria | ### Why Urine Microscopy Is Not the Answer **Warning:** While muddy brown casts (epithelial cell casts) support ATN, they are not specific to CI-AKI and may be absent in early or mild cases. Microscopy is supportive but not confirmatory for CI-AKI specifically. ### Why Renal Artery Duplex Is Wrong Duplex ultrasound evaluates for renal artery stenosis (prerenal cause), not intrinsic tubular injury. CI-AKI is a direct toxic injury to tubular epithelium, not a vascular occlusion.