Validation of the portable MX3 hydration testing system for measuring saliva osmolality

Saliva, specifically salivary osmolality, has gained increasing interest as a non-invasive hydration marker providing rapid results that can be easily obtained in a field setting. However, it is unclear as to whether commercially available hydration devices can accurately measure salivary osmolality. Thus, this study assessed the validity of the MX3 Portable Hydration device against a laboratory-grade freezing-point depression benchtop osmometer (i.e., criterion measure). A total of eight saliva samples were collected from 10 male participants (total saliva samples, n = 80; age, 25 ± 5 y; mass, 79.2 ± 12.1 kg; height, 181.1 ± 6.6 cm) across a two-day period. Samples were stored at 4°C before being centrifuged at 3,000 rpm for 1 min. Samples were simultaneously analyzed via freezing-point depression osmometry (250 μL per sample) and MX3 portable device (~10 μL per sample). Samples were analyzed in duplicate or triplicate, depending on available volume. Results demonstrated moderate agreement between methods, based on Lin's concordance correlation coefficient (CCC = 0.91), and the MX3 demonstrated excellent reliability across repeated measurements of the same sample [ICC (1, 2) = 0.991, (0.988, 0.992)]. A significant proportional bias indicated the MX3 overestimated salivary osmolality values by 24% at the low end [bias = 1.24, 95% CI (1.15, 1.33)] but underestimated osmolality values at higher osmolality values by 25% [bias = 0.75, 95% CI (0.62, 0.90)] when compared to the criterion measure. However, the MX3 demonstrated good validity at mid-range values (average 80 mOsm/kg), where bias decreased to 9% [bias = 1.09, 95% CI (1.05, 1.14)], with LOA from 0.77 to 1.54. These data suggest the MX3 portable device is a reasonable field measure of saliva osmolality, providing a reliable and non-invasive hydration assessment marker for field applications.