Assessment of the MX3 hydration testing system in adults at rest and following exercise in the heat

This study examined the performance of MX3 hydration testing system for assessing hydration status at rest and following exercise. Twenty adults completed two matched exercise trials in warm-humid conditions. Pre- and post-exercise saliva osmolality (S_osm) were assessed using MX3. Reliability was assessed using mean difference (MD), absolute difference (ABD), coefficient of variation (CV), standard error of the measurement (SEM), intraclass correlation coefficient (ICC), and 95% limits of agreement (LoA). Diagnostic accuracy was assessed using a crosstab analysis relative to urine specific gravity (USG) and body mass loss (BML). Pre-exercise, MX3 showed a MD of 4 mOsm/kg (95% LoA: -46, 54), ABD of 23 mOsm/kg, CV of 31%, SEM of 18 mOsm/kg, and ICC of 0.198. Post-exercise, MX3 demonstrated a MD of 5 mOsm/kg (95% LoA: -78, 68), ABD of 28 mOsm/kg, CV of 29%, SEM of 26 mOsm/kg, and ICC of 0.391. Pre-exercise, S_osm demonstrated a sensitivity of 0.09, specificity of 0.82, and LR⁺ = 0.51, LR^- = 1.11 relative to USG >1.020. Post-exercise, S_osm showed a sensitivity of 0.57, specificity of 0.63, and LR⁺ = 1.52, LR^- = 0.69 relative to >2% BML. MX3 has low reliability and poor-to-weak predictive value in identifying hypohydration at rest and following exercise.