Which urinary indices favor ATN over prerenal AKI?

• A. Urine Na >40 mEq/L and osmolality <350 mOsm/kg
• B. Urine Na <20 mEq/L with osmolality >500 mOsm/kg
• C. BUN:Cr ratio >20:1
• D. Concentrated urine with low sodium

Answer: (A)

When the tubules are damaged, as in ATN, they lose the ability to reclaim filtered sodium and water. This leads to a urine that's relatively rich in sodium and not very concentrated. So you see a high urine sodium (>40 mEq/L) and a low urine osmolality (<350 mOsm/kg). That combination is classic for ATN because it reflects tubular dysfunction and an impaired concentrating ability.

In contrast, prerenal AKI shows the opposite pattern: the kidneys conserve sodium and water to preserve intravascular volume, yielding a low urine sodium (<20 mEq/L) and a high urine osmolality (>500 mOsm/kg). A high BUN-to-creatinine ratio (>20:1) and a concentrated urine with low sodium also point toward prerenal.