In dehydration, which laboratory change is most likely?

• A. Elevated BUN
• B. Decreased BUN
• C. Decreased serum osmolality
• D. Decreased urine specific gravity

Answer: (A)

Dehydration lowers circulating blood volume, so the kidneys receive less blood flow and try to conserve water. That reduced perfusion causes urea to be reabsorbed more in the tubules, which raises the BUN level. At the same time, the blood becomes more concentrated, so serum osmolality increases and the urine becomes more concentrated as well (urine specific gravity goes up). This combination makes an elevated BUN the most likely lab change in dehydration. In contrast, a decrease in BUN would suggest overhydration or other conditions, a decrease in serum osmolality points to dilution from excess fluids, and a decrease in urine specific gravity would indicate dilute urine—not dehydration. Remember, the BUN/creatinine ratio often rises in dehydration (prerenal azotemia), highlighting the volume-status effect on kidney function.