Accuracy of Urine Specific Gravity and Osmolality as Indicators of Hydration Status

Abstract

To reduce the adverse consequences of exertion-related and acute intentional dehydration research has focused on monitoring hydration status. This investigation: 1) compared sensitivity of urine specific gravity (U sg ), urine osmolality (U osm ) and a criterion measurement of hydration, plasma osmolality (P osm ), at progressive stages of acute hypertonic dehydration and 2) using a medical decision model, determined whether U sg or U osm accurately reflected hydra-tion status compared to P osm among 51 subjects tested throughout the day. Incremental changes in P osm were observed as subjects dehydrated by 5% of body weight and rehydrated while U sg and U osm showed delayed dehydration-related changes. Using the medical decision model, sensitivity and specificity were not significant at selected cut-offs for U sg and U osm . At the most accurate cut-off values, 1.015 and 1.020 for U sg and 700 m osm /kg and 800 m osm /kg for U osm , only 65% of the athletes were correctly classified using U sg and 63% using U osm . P osm , U sg , and U osm appear sensitive to incremental changes in acute hypertonic dehydration, however, the misclassified outcomes for U sg and U osm raise concerns. Research focused on elucidating the factors affecting accurate assessment of hydration status appears warranted.