Which renal function lab value is most sensitive to dehydration?

The blood urea nitrogen (BUN) level is most sensitive to dehydration because it reflects the body's urea concentration, which is influenced significantly by hydration status. When a person is dehydrated, the kidneys conserve water, leading to less urine output. As a result, urea becomes more concentrated in the blood, causing an increase in BUN levels.

Conversely, creatinine levels typically remain stable until there is significant impairment in kidney function, making them less responsive to changes due to dehydration. The glomerular filtration rate (GFR) also does not respond directly to hydration status in the same immediate way as BUN, as it is a measure of kidney function rather than hydration. Electrolytes can change with dehydration, but their levels are influenced by a variety of factors, such as food intake and various medical conditions, making them less direct indicators of dehydration specifically. Thus, among these options, BUN is the most sensitive marker for detecting changes related to dehydration.