Which lab pattern best suggests pre-renal AKI rather than intrinsic AKI?

• A. FeNa >2% and BUN:Cr <10
• B. FeNa <1% and BUN:Cr <10
• C. FeNa >2% and BUN:Cr >20
• D. FeNa <1% and BUN:Cr >20

Answer: (D)

The key idea is using how the kidney responds to reduced blood flow to tell prerenal from intrinsic AKI. In prerenal AKI, reduced perfusion triggers the kidney to hold onto sodium and water. That means the fractional excretion of sodium is very low, typically less than 1%, and urea is reabsorbed more, so the BUN rises relative to creatinine, giving a BUN:Cr ratio greater than about 20.

So the pattern of a very low FeNa (<1%) together with a high BUN:Cr (>20) fits prerenal AKI best, because it reflects both sodium and water conservation and increased proximal reabsorption of urea.

High FeNa (>2%) points toward intrinsic renal injury, where damaged tubules fail to reabsorb sodium efficiently, and BUN:Cr is not typically elevated in the prerenal range. A low BUN:Cr with a low FeNa would be atypical for prerenal AKI, since prerenal states usually show the opposite BUN behavior.