Which finding best indicates a pre-renal AKI in the context of FeNa?

Which finding best indicates a pre-renal AKI in the context of FeNa?

• A. FeNa <1% and urine Na <10
• B. FeNa >2% and urine Na >40
• C. FeNa >1% with RBC casts
• D. FeNa <15% with RBC casts

Answer: (A)

Fractional excretion of sodium (FeNa) helps distinguish prerenal AKI from intrinsic renal injury. In prerenal AKI, reduced kidney perfusion triggers the kidneys to avidly reabsorb sodium and water, so the urine contains very little sodium and FeNa stays below 1% (urine Na is typically very low, often under 10 mEq/L). This pattern shows the kidneys are structurally intact and conserving sodium to preserve circulating volume, rather than failing to reabsorb due to tubular damage. In contrast, intrinsic renal injury like ATN or nephritic processes impair tubular reabsorption, leading to higher FeNa (usually >2%) and higher urine sodium (often >40 mEq/L). RBC casts point toward nephritic or intrinsic pathology rather than prerenal. Diuretics can falsely raise FeNa by increasing sodium excretion, so FeNa may be unreliable in that setting and FeUrea can help when diuretics are involved. So the best indication of prerenal AKI here is a FeNa less than 1% with urine sodium less than 10 mEq/L.

Fractional excretion of sodium (FeNa) helps distinguish prerenal AKI from intrinsic renal injury. In prerenal AKI, reduced kidney perfusion triggers the kidneys to avidly reabsorb sodium and water, so the urine contains very little sodium and FeNa stays below 1% (urine Na is typically very low, often under 10 mEq/L). This pattern shows the kidneys are structurally intact and conserving sodium to preserve circulating volume, rather than failing to reabsorb due to tubular damage. In contrast, intrinsic renal injury like ATN or nephritic processes impair tubular reabsorption, leading to higher FeNa (usually >2%) and higher urine sodium (often >40 mEq/L). RBC casts point toward nephritic or intrinsic pathology rather than prerenal. Diuretics can falsely raise FeNa by increasing sodium excretion, so FeNa may be unreliable in that setting and FeUrea can help when diuretics are involved. So the best indication of prerenal AKI here is a FeNa less than 1% with urine sodium less than 10 mEq/L.