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

Abstract

Background: Urine specific gravity (U_SG) is often used to assess hydration status, particularly around athletic competition, but it is unknown whether high U_SG is indicative of plasma volume (PV) reduction (i.e., hypohydration). We tested the hypothesis that if high U_SG 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_SG and PV in individuals presenting with high and low spot U_SG.

Methods: Nineteen healthy males were provided food and water over 24 hours with a total water volume of 35 ml·kg^-1 body mass. Absolute PV and blood volume (BV), measured using the CO-rebreathe technique, along with U_SG were measured before and after a 24-hour intervention period. Based on a preintervention morning spot U_SG, subjects were post hoc assigned to groups according to U_SG (≤1.020 or >1.020; low and high U_SG, respectively).

Results: Despite presenting with an elevated spot U_SG (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_SG group (p > 0.05). However, a spot U_SG 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_SG group, there were no changes in PV (-39 ± 274 ml), BV (-82 ± 396 ml), or U_SG (0.003 ± 0.007) over the 24-hour fluid intervention (all p > 0.05).

Conclusions: Despite a high preintervention U_SG 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_SG and subsequent changes are not accurately representative of PV or BV.

Keywords: Hydration assessment; blood volume; carbon monoxide rebreathing; hypohydration; serum osmolality.