What lab value is commonly indicative of dehydration?

The lab value commonly indicative of dehydration is elevated serum sodium or an elevated BUN to creatinine ratio. When a patient is dehydrated, the concentration of sodium in the serum tends to increase due to a decrease in the total fluid volume in the body. This results in hypernatremia, or elevated serum sodium levels.

Additionally, in dehydration, the blood urea nitrogen (BUN) often increases more significantly than creatinine because urea is typically more sensitive to changes in hydration status. An elevated BUN to creatinine ratio greater than 20:1 can indicate a prerenal cause of azotemia, often associated with dehydration, as the kidneys retain urea in response to low blood volume.

Understanding the physiological responses during dehydration helps clarify why the elevated serum sodium and BUN to creatinine ratio are relevant markers. These values signal that there is a relative concentration of solutes in the blood as a result of reduced volume, which is a critical sign for assessing a patient's hydration status.