Plasma and serum volume remain unchanged following a 12-h fast from food and drink despite changes in blood and urinary hydration markers

Dehydration did not affect plasma or serum volume. Therefore, a 12-h fast from food and water as done in this study will not likely affect laboratory test results of biomarker concentration. All 3 urinary measures were able to detect changes in hydration status, whereas only 2 blood measures were able to detect changes in hydration status. This may indicate that urinary measures are best at detecting small changes in hydration status.