What are the most reliable measures of dehydration?

What are the most reliable measures of dehydration?

• A. Hematocrit
• B. Serum osmolality
• C. BUN
• D. Plasma and urine osmolarity

Answer: (D)

Hydration status is best judged by how the body manages water and solutes, which is reflected in osmolality. Plasma osmolality rises when body water is low, showing how concentrated the blood is. Urine osmolality, on the other hand, shows how well the kidneys are conserving water by concentrating the urine. When both are measured, you get a reliable picture: high plasma osmolality confirms a water deficit, and high urine osmolality confirms an appropriate renal response to conserve water. Hematocrit can go up with dehydration but is influenced by many other factors like red blood cell mass and fluid shifts, so it isn’t specific. BUN can increase for reasons beyond dehydration (renal function, protein intake, GI losses), making it less reliable on its own. Serum osmolality alone indicates overall solute load but doesn’t reveal how the kidneys are handling water reabsorption, which is crucial to assessing true dehydration. By assessing both plasma and urine osmolarity, you capture both the systemic water deficit and the kidney’s concentrating response, making it the most informative approach.

Hydration status is best judged by how the body manages water and solutes, which is reflected in osmolality. Plasma osmolality rises when body water is low, showing how concentrated the blood is. Urine osmolality, on the other hand, shows how well the kidneys are conserving water by concentrating the urine. When both are measured, you get a reliable picture: high plasma osmolality confirms a water deficit, and high urine osmolality confirms an appropriate renal response to conserve water.

Hematocrit can go up with dehydration but is influenced by many other factors like red blood cell mass and fluid shifts, so it isn’t specific. BUN can increase for reasons beyond dehydration (renal function, protein intake, GI losses), making it less reliable on its own. Serum osmolality alone indicates overall solute load but doesn’t reveal how the kidneys are handling water reabsorption, which is crucial to assessing true dehydration. By assessing both plasma and urine osmolarity, you capture both the systemic water deficit and the kidney’s concentrating response, making it the most informative approach.