Which lab finding can indicate dehydration or hemoconcentration in a child?

• A. Elevated BUN with dehydration
• B. Elevated hematocrit or elevated BUN showing hemoconcentration
• C. Low hematocrit
• D. Low BUN

Answer: (B)

Dehydration causes hemoconcentration because the fluid in the blood vessels (plasma) decreases while the number of blood cells stays about the same. This makes the blood more concentrated. The most direct sign is an elevated hematocrit, since a higher proportion of red blood cells reflects that concentration. BUN can also rise in dehydration because reduced blood flow to the kidneys increases urea reabsorption and concentrates waste products. So an elevated hematocrit or an elevated BUN in the setting of dehydration points to hemoconcentration. Low hematocrit would suggest dilution or anemia, and low BUN is not a marker of dehydration.