Which laboratory finding is most characteristic of prerenal azotemia?

Which laboratory finding is most characteristic of prerenal azotemia?

• A. BUN:Cr ratio < 10
• B. Elevated BUN with creatinine
• C. BUN:Cr ratio > 20
• D. Low BUN with elevated creatinine

Answer: (C)

When kidney perfusion drops, the kidneys try to conserve water and solutes. Urea (BUN) is reabsorbed more than creatinine under these low-flow conditions, while creatinine filtration remains relatively constant. This makes the BUN rise disproportionately compared with creatinine, producing a BUN to creatinine ratio greater than 20:1. That high ratio is the hallmark of prerenal azotemia. In contrast, intrinsic renal disease tends to elevate both BUN and creatinine more evenly, giving a normal or only mildly elevated ratio, and very low ratios argue against prerenal causes. Hence, the most characteristic lab finding is a BUN:Cr ratio > 20.

When kidney perfusion drops, the kidneys try to conserve water and solutes. Urea (BUN) is reabsorbed more than creatinine under these low-flow conditions, while creatinine filtration remains relatively constant. This makes the BUN rise disproportionately compared with creatinine, producing a BUN to creatinine ratio greater than 20:1. That high ratio is the hallmark of prerenal azotemia. In contrast, intrinsic renal disease tends to elevate both BUN and creatinine more evenly, giving a normal or only mildly elevated ratio, and very low ratios argue against prerenal causes. Hence, the most characteristic lab finding is a BUN:Cr ratio > 20.