Which finding is most consistent with intrinsic AKI over prerenal AKI?

• A. FENa >2% and urine sodium >40 mEq/L
• B. FENa <1% and urine sodium <20 mEq/L
• C. Urine osmolality >500 mOsm/kg with low sodium
• D. Normal creatinine

Answer: (A)

The key idea is using urinary indices to tell apart intrinsic (intrarenal) AKI from prerenal AKI. When the tubules are damaged in intrinsic AKI, they lose the ability to reabsorb sodium effectively and to concentrate urine. That leads to a high fractional excretion of sodium and a high urine sodium concentration (FENa over about 2% and urine sodium over 40 mEq/L). In contrast, prerenal AKI from reduced renal perfusion keeps tubular function intact, so the kidneys avidly reabsorb sodium and water, resulting in a very low FENa (often under 1%) and low urine sodium (typically under 20 mEq/L), with the urine often being concentrated due to preserved concentrating ability.

So the finding of FENa greater than 2% with urine sodium over 40 mEq/L is most consistent with intrinsic AKI because it reflects impaired tubular sodium reabsorption. A low FENa with low urine sodium would point toward prerenal AKI, not intrinsic. A urine osmolality that’s still very high with low urine sodium also suggests prerenal physiology. A normal creatinine alone doesn’t distinguish between the two patterns.