BACKGROUND: Assessing hydration status outside of labs (e.g., field settings) has presented a long-standing challenge given that most hydration status measures are invasive, stationary, costly, or have poor validity. Technological advances, however, are quickly allowing these barriers to be overcome. One of these recent advancements is the development of a cost-effective, noninvasive, and portable device to measure saliva osmolality (SOSM), which is used as a spot assessment of hydration status. This device, the MX3 Hydration Testing System (HTS, MX3 Diagnostics, Austin, TX), is currently available commercially for purchase. The purpose of this investigation was to determine the test-retest reliability of the MX3 HTS. METHODS: A sample of 35 subjects (30.4 ± 10.6 y; 20 men, 15 women) participated in this study. After avoiding any fluid intake for at least 5 minutes, participants were asked to generate a fresh saliva sample (i.e., new saliva generated after swallowing all existing saliva) and present the sample on their tongue with an open mouth. Using the MX3 HTS, a researcher then tapped the tip of a test strip to the saliva. Participants were asked to repeat this process after a short period of time (less than 5 minutes apart). The intra-class correlation (ICC) and minimal detectable change (MDC) were calculated as measures of reliability. A paired t-test was used to determine whether there were any significant differences in SOSM between the two timepoints. RESULTS: The means of the first and second SOSM measures were 61.4±21.3 mOsm and 64.2±18.9 mOsm, respectively. Based on a t-test, the mean difference of 2.8±14.1 mOsm was not significant (p=0.253). Within-day repeat SOSM measures yielded an ICC of 0.75. Using this ICC, an MDC at the 90% confidence level was calculated to be 23 mOsm. CONCLUSION: With moderate-to-good reliability, the MX3 HTS appears to be a reasonable and practical choice for reliably measuring moderate-sized changes (greater than 20 mOsm) in saliva osmolality outside of laboratory constraints. These results suggest this tool may be useful in field settings where hydration status is crucial to performance, such as athletics, military, and the workforce.

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