In prerenal acute kidney injury, what lab pattern best distinguishes it from intrinsic AKI?

• A. Low urine sodium (<20 mEq/L) and FENa <1%
• B. High urine sodium (>40 mEq/L) and FENa >2%
• C. Urine sodium around 20-40 mEq/L and FENa 1-2%
• D. Urine osmolality low with FENa <1%

Answer: (A)

Prerenal AKI happens when the kidney is underperfused, so the kidneys respond by holding onto water and sodium. That means the urine becomes very concentrated and sodium reabsorption stays strong, yielding a urine sodium value under 20 mEq/L and a fractional excretion of sodium (FENa) under 1%. This pattern shows the tubules are functioning to conserve Na and water, which is characteristic of a prerenal process. In intrinsic AKI, the tubular damage impairs reabsorption, so the urine sodium tends to be higher (often >40 mEq/L) and the FENa exceeds about 2%, with less concentrated urine (urine osmolality closer to plasma). If diuretics are used, FENa can be unreliable, and FEUrea may be preferred. Thus, the low urine sodium and very low FENa best distinguish prerenal AKI from intrinsic AKI.