Day-to-day variability and method agreement of sweat sodium concentration during pilocarpine iontophoresis and exercise

Purpose Sweat sodium testing is increasingly used to estimate electrolyte losses during exercise; however, the reliability of sweat sodium assessment and the influence of methodological factors remain unclear. We evaluated the day-to-day variability and agreement between common sweat induction, collection, and analytical approaches, including pilocarpine iontophoresis and exercise-induced sweating. Methods Twenty physically active adults (10 males 33 ± 11 y, 10 females 33 ± 10 y) completed two experimental trials separated by 48 h. Local sweat was induced at rest using pilocarpine iontophoresis. Participants then performed 60 min of treadmill running or cycling in 27 °C, 50% relative humidity, during which sweat was collected. Sweat sodium concentration was then analyzed and agreement between measurements was evaluated using Bland-Altman analysis. Results Day-to-day variability was good, with coefficients of variation of 13.8% for iontophoresis, 14.9% for exercise-derived Macroduct samples, and 9.9% for patch-based collection. Iontophoresis-derived sweat sodium concentrations were higher than exercise-derived values (42.0 ± 9.3 vs. 33.5 ± 11.0 mmol·L − 1 ) with a mean bias of -8.5 mmol·L − 1 (level of agreement (LoA): -32.2 to 15.2). Conclusion Sweat sodium concentration exhibits moderate day-to-day variability under controlled laboratory conditions. Exercise-derived values were lower than those obtained via pilocarpine iontophoresis, with iontophoresis overestimating sweat sodium by ~ 8.5mmol·L − 1 (~ 195mg·L − 1 ) on average. Although differences in methodological approaches were relatively small, these methodological factors contribute additional variability. Collectively, these findings indicate that both biological and methodological factors contribute to variability in sweat sodium assessment, and iontophoresis-derived values should be interpreted cautiously when estimating sodium losses during exercise.