A BUN-to-creatinine ratio greater than 20:1 with a normal creatinine level is most consistent with which condition?

• A. Pre-renal Azotemia
• B. Intrinsic Renal Failure
• C. Post-renal Obstruction
• D. Acute Tubular Necrosis

Answer: (A)

When the kidneys aren’t getting enough blood flow, they try to conserve volume and upregulate reabsorption of urea in the tubules. This increases the BUN more than the creatinine, so the BUN:creatinine ratio climbs above 20:1 while creatinine may still be normal if the filtration rate isn’t yet impaired. That pattern points to prerenal azotemia. In intrinsic renal disease like acute tubular necrosis, the tubules can’t reabsorb efficiently and creatinine rises, so the ratio tends to be normal or lower (roughly 10–15:1) with an elevated creatinine. In postrenal obstruction, the trend can vary, but obstruction that advances typically leads to rising creatinine as well. So the combination of a high BUN:Cr ratio with a normal creatinine is most consistent with prerenal azotemia.