A dehydrated patient develops prerenal acute kidney injury. Which laboratory pattern is most likely?

• A. BUN/creatinine ratio >20:1 with low urinary sodium and concentrated urine.
• B. BUN/creatinine ratio <10:1 with high urinary sodium.
• C. Elevated urinary sodium with dilute urine.
• D. Proteinuria >3 g/day.

Answer: (A)

When the kidney’s blood flow drops from dehydration, it behaves as if it needs to conserve volume. The tubules keep reabsorbing water and sodium, and hormones like the RAAS and ADH are activated. This makes the urine very concentrated and low in sodium, while urea gets reabsorbed along with water. The result is a disproportionately higher BUN compared with creatinine, producing a BUN to creatinine ratio greater than about 20:1. The urine sodium falls (typically below 20 mEq/L) and the urine osmolality is high, reflecting intact tubular reabsorption.

That pattern—BUN/creatinine ratio over 20:1 with low urinary sodium and concentrated urine—best fits prerenal AKI from dehydration.

In contrast, a low BUN/creatinine ratio with high urinary sodium and dilute urine points toward intrinsic renal injury, where the tubules can’t reabsorb sodium effectively. Proteinuria greater than 3 g/day signals glomerular disease rather than prerenal azotemia.