What laboratory finding is indicative of dehydration?

Elevated serum osmolality is a key laboratory finding indicative of dehydration. When the body is dehydrated, there is a decrease in the amount of water relative to solutes in the bloodstream, leading to higher concentrations of these solutes. This results in an increase in serum osmolality, as the osmolality measures the concentration of solutes (such as sodium, glucose, and urea) in the blood.

In a well-hydrated state, serum osmolality is usually within a normal range because there is sufficient water to balance the solute concentration. Conversely, when dehydrated, the kidneys conserve water, leading to a condition in which solute concentration rises, thus elevating serum osmolality. Monitoring serum osmolality is crucial in evaluating a patient's hydration status and determining the appropriate interventions needed to restore fluid balance.

The other options represent different conditions: decreased serum osmolality suggests an overhydrated state, normal serum osmolality indicates hydration levels are within a normal range, and decreased urine output can be a nonspecific response to various conditions but does not directly measure dehydration status like serum osmolality does.