## Distinguishing ATN from Prerenal Azotemia **Key Point:** The fractional excretion of sodium (FENa) >2% in the presence of muddy brown casts is the best discriminator between ATN and prerenal azotemia. This combination indicates tubular injury with loss of tubular reabsorptive capacity. ### Pathophysiology of FENa FENa reflects the percentage of filtered sodium that is excreted in urine: $$FENa = \frac{[U_{Na} \times P_{Cr}]}{[P_{Na} \times U_{Cr}]} \times 100$$ - **Prerenal azotemia:** Intact tubules reabsorb sodium avidly → FENa <1% (often <0.1%) - **ATN:** Damaged tubules cannot reabsorb sodium → FENa >2% (often 2–3%) ### Comparison Table: ATN vs Prerenal Azotemia | Feature | Prerenal Azotemia | ATN | |---------|-------------------|-----| | **FENa** | <1% | >2% | | **Urine osmolality** | >500 mOsm/kg | <350 mOsm/kg | | **Urine Na** | <20 mEq/L | >40 mEq/L | | **Urinary casts** | Hyaline casts | Muddy brown, granular casts | | **Urine/plasma Cr ratio** | >40 | <20 | | **Response to fluids** | Rapid recovery | Slow recovery (days–weeks) | **High-Yield:** Muddy brown casts (composed of desquamated tubular epithelial cells and hemoglobin) are pathognomonic for ATN. Combined with FENa >2%, they form the gold-standard discriminator. **Clinical Pearl:** In this case, FENa 2.1% + muddy brown casts = ATN. The patient's diabetes and likely hypotension/sepsis explain the tubular injury. ### Why FENa is Superior to Individual Markers - Urine sodium alone can be misleading (diuretic use, salt wasting) - Casts alone are not quantitative - FENa integrates both filtered load and excretion, making it more reliable **Warning:** FENa loses accuracy in: - Chronic kidney disease (baseline tubular dysfunction) - Contrast-induced nephropathy (may show FENa <1% despite ATN) - Loop diuretic use (falsely elevates FENa) [cite:Harrison 21e Ch 297]
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