Reliability and validity of the MX3 portable sweat sodium analyser during exercise in warm conditions

Purpose: Accurately measuring sweat sodium concentration ([Na⁺]) in the field is advantageous for coaches, scientists, and dieticians looking to tailor hydration strategies. The MX3 hydration testing system is a new portable analyser that uses pre-calibrated biosensors to measure sweat [Na⁺]. This study aimed to assess the validity and reliability of the MX3 hydration testing system. Methods: Thirty-one (11 females) recreationally active participants completed one experimental trial. During this trial, participants exercised at a self-selected pace for 45 min in a warm environment (31.5 ± 0.8 °C, 63.2 ± 1.3% relative humidity). Sweat samples were collected from three measurement sites using absorbent patches. The samples were then analysed for sweat [Na⁺] using both the MX3 hydration testing system and the Horiba LAQUAtwin-NA-11. The reliability of the MX3 hydration testing system was determined following two measurements of the same sweat sample. Results: The mean difference between measurements was 0.1 mmoL·L⁻¹ (95% limits of agreement (LoA): − 9.2, 9.4). The analyser demonstrated a coefficient of variation (CV) of 5.6% and the standard error of measurement was 3.3 mmoL·L⁻¹. When compared to the Horiba LAQUAtwin-NA-11, there was a mean difference of − 1.7 mmoL·L⁻¹ (95% LoA: − 0.25X¯, 0.25X¯) and the CV was 9.8%. Conclusion: The MX3 hydration testing system demonstrated very good single-trial reliability, moderate agreement and a very good CV relative to the Horiba LAQUAtwin-Na-11. To further validate its performance, the MX3 hydration testing system should be compared with analytical techniques known for superior reliability and validity.