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Abstract 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. 25 X X ¯, 0. 25 X X ¯) 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.
Brown et al. (Sat,) studied this question.