A 42-year-old woman with no prior renal disease develops acute kidney injury after receiving intravenous contrast for a coronary angiogram. Serum creatinine rises from 0.9 to 3.2 mg/dL within 72 hours. Urine osmolality is 350 mOsm/kg, and fractional excretion of sodium (FENa) is 2.8%. Which finding best distinguishes contrast-induced ATN from prerenal azotemia?

Explanation

## Contrast-Induced ATN vs. Prerenal Azotemia: Discriminating Features ### Clinical Context Both contrast-induced ATN and prerenal azotemia present with acute rise in creatinine post-angiography. However, their **urine findings and pathophysiology differ fundamentally**. ### Comparative Urinalysis | Feature | Prerenal Azotemia | Contrast-Induced ATN | |---------|-------------------|---------------------| | **FENa** | **< 1%** | **> 2%** (often 2–3%) | | **Urine osmolality** | **> 500 mOsm/kg** | **300–350 mOsm/kg** (dilute) | | **Urine casts** | **Absent or hyaline only** | **Muddy brown + epithelial casts** | | **Urine sediment** | Benign | Granular, epithelial cells | | **BUN:Cr ratio** | **> 20:1** | **10–15:1** | | **Mechanism** | Renal hypoperfusion (reversible) | Direct tubular epithelial injury | ### Why Muddy Brown Casts Are Diagnostic **Key Point:** Muddy brown (or "dirty brown") casts are **pathognomonic for ATN**. They consist of: - Sloughed tubular epithelial cells - Myoglobin or hemoglobin (from rhabdomyolysis or hemolysis) - Tamm-Horsfall protein matrix These casts form only when tubular epithelial cells are actively necrotic and being shed into the tubular lumen—a hallmark of ATN, not prerenal azotemia. ### Pathophysiology **High-Yield:** - **Prerenal azotemia** = intact tubular function responding to decreased perfusion → avid sodium reabsorption (FENa < 1%) and concentrated urine (osmolality > 500). - **Contrast-induced ATN** = direct epithelial toxicity from contrast osmolality and reactive oxygen species → loss of tubular concentrating ability and impaired sodium reabsorption (FENa > 2%, dilute urine). ### Clinical Pearl **Clinical Pearl:** The presence of muddy brown casts on urine microscopy in a patient with acute kidney injury post-contrast is virtually diagnostic of ATN. Prerenal azotemia does not produce these casts because the tubular epithelium is intact. **Mnemonic: Muddy Brown = Epithelial Necrosis (MBE)** — if you see muddy brown casts, think ATN, not prerenal. ### Why Recovery Pattern Differs Prerenal azotemia reverses within hours of fluid resuscitation (restoring perfusion). Contrast-induced ATN requires days to weeks for epithelial regeneration, even with supportive care. [cite:Harrison 21e Ch 297]