Fractional excretion of sodium greater than 1% is most consistent with which AKI pattern?

• A. Post-renal obstruction
• B. Glomerulonephritis
• C. Pre-renal azotemia
• D. ATN or AIN

Answer: (D)

The key idea is that sodium handling by the tubules distinguishes different AKI patterns. When kidney perfusion is low, the tubules conservatively reabsorb sodium to preserve volume, so the fractional excretion of sodium stays low (less than 1%). In contrast, when the tubules are damaged—as in intrinsic renal AKI from tubular necrosis or interstitial nephritis—their ability to reclaim sodium is impaired, so more sodium is lost in urine and the fractional excretion of sodium rises above 1%. Post-renal obstruction can vary, but a persistently elevated FENa is most consistent with intrinsic tubular injury. Therefore, a FENa greater than 1% points toward ATN or AIN rather than prerenal states or simple obstruction.