Reliability and validity of portable sweat sodium analyzers during exercise in the heat

We aimed to evaluate reliability and validity of MX3 and LAQUAtwin-Na-11 (LA_Na) analyzers in measuring sweat [Na⁺]. Nineteen healthy, active adults (9 females, 33 ± 10 years) completed two matched, self-paced 60 min cycling or running trials in warm-humid conditions (27.3 ± 0.4 °C, 50.9 ± 1.8% RH). Sweat was collected from an absorbent patch on the left ventral forearm. Sweat was analyzed using MX3, LA_Na, and Medica analyzers. The accuracy of both portable analyzers was compared to Medica results. Reliability was assessed using coefficient of variation (CV) and intraclass correlation coefficient (ICC). MX3 demonstrated acceptable-to-high between-trial reliability (CV: 23%; ICC: 0.826), while LA_Na showed acceptable-to-moderate between-trial reliability (CV: 24%; ICC: 0.744). Validity was evaluated using CV and linear regression. Both analyzers demonstrated strong associations with the reference method (MX3: R² = 0.867; LANa: R² = 0.852), with good CV values (MX3: 17.5%; LANa: 13%). Overall, MX3 demonstrated higher reliability than LA_Na, and both analyzers provide good validity for laboratory-based sweat [Na⁺] assessment. Clinical registration number: NCT07490756.