Spot Sample Urine Specific Gravity Does Not Accurately Represent Small Decreases in Plasma Volume in Resting Healthy Males.

BACKGROUND

Urine specific gravity (U) is often used to assess hydration status, particularly around athletic competition, but it is unknown whether high U is indicative of plasma volume (PV) reduction (i.e., hypohydration). We tested the hypothesis that if high U is reflective of reduced PV, subsequent fluid ingestion would increase PV.

PURPOSE

The purpose of this study was to examine 24-hour changes in U and PV in individuals presenting with high and low spot U.

METHODS

Nineteen healthy males were provided food and water over 24 hours with a total water volume of 35 ml·kg body mass. Absolute PV and blood volume (BV), measured using the CO-rebreathe technique, along with U were measured before and after a 24-hour intervention period. Based on a preintervention morning spot U, subjects were post hoc assigned to groups according to U (≤1.020 or >1.020; low and high U, respectively).

RESULTS

Despite presenting with an elevated spot U (1.026 ± 0.004), subsequent fluid ingestion over 24 hours did not lead to changes (∆) in PV (-75 ± 234 ml) or BV (-156 ± 370 ml) in the high U group (p > 0.05). However, a spot U after the 24-hour intervention in this group decreased (p = 0.018) to a level indicating improved hydration status (1.017 ± 0.007). In the low U group, there were no changes in PV (-39 ± 274 ml), BV (-82 ± 396 ml), or U (0.003 ± 0.007) over the 24-hour fluid intervention (all p > 0.05).

CONCLUSIONS

Despite a high preintervention U and subsequent decrease after 24-hour fluid intake, measures of PV and BV were not indicative of this seemingly improved hydration status. This suggests that a highly concentrated spot sample U and subsequent changes are not accurately representative of PV or BV.