Validity and Reliability of a Reusable Hydration Wearable During Exercise in the Heat

Sweat rate and sweat sodium concentration [Na + ] are useful measures to have to guide replacement during exercise. However, methods of sweat testing can be expensive or unfeasible for many. The hDrop device is a reuseable sweat monitoring device that bluetooth connects to an associated phone application. No previous study has investigated the validity and reliability of the hDrop for sweat [Na + ] and rate. PURPOSE: Test the validity of the hDrop system for sweat rate and sweat [Na + ] against a lab-grade, standard ion-sensitive electrolyte analyzer. In addition, test the reliability of hDrop from inter-day measures. HYPOTHESIS: There would be no difference between the hDrop device and lab measures for sweat rate and sweat [Na + ]. hDrop would provide consistent and repeatable sweat [Na + ] and sweat loss values. METHODS: Nineteen healthy active participants (9 females; 33 ± 10 y; 170.5 ± 10.9 cm; 69.8 ± 13.9 kg) participated in this study. Participants completed two matched trials that were separated by 48 hours. Prior to exercise, a Tegaderm+ patch was placed on the left forearm, and an hDrop sensor on the upper left arm. Participants entered the environmental chamber (27°C, 50% relative humidity) and self-selected a pace on the treadmill or cycle ergometer they could maintain for 1 hour of exercise. Sweat rate was calculated based on pre- and post-exercise nude body mass, and whole sweat samples were analyzed for sweat [Na + ] using the Medica EasyLyte analyzer. Data were analyzed for reliability and validity using paired t-tests, coefficients of variation (CV), mean bias, standard error of the mean (SEM), and 95% limits of agreement (LoA) to assess inter-day reliability and agreement between methods. RESULTS: WBGT was matched for both trials (T1: 22.4±0.1; T2: 22.4±0.1; p=0.56). Pre-exercise hydration was matched for both trials (T1 USG: 1.011±0.007; T2 USG: 1.011±0.007; p=0.73). There was no difference in exercise intensity between trials (T1 HR: 128±29 bpm, T2 HR: 129±28 bpm, p=0.50; T1 RPE: 10±2, T2 RPE: 10±2, p=0.47). Lab sweat rate for day 1 was 1.1±0.4 L and 1.1±0.4 L on day 2 (mean bias = 0±0.3 L [95% LoA: -0.5 to 0.6]; CV: 15.9±0.1%; SEM: 0.1 L). HDrop fluid loss for day 1 was 1.0±0.3 L and 0.9±0.3 on day 2 (mean bias = -0.1±0.2 L [95% LoA: -0.6 to 0.4]; CV: 14.7±0.1%; SEM: 0.1 L). hDrop reported fluid loss was not different than lab sweat rate 1.0±0.3 L vs 1.1±0.4 L, (p=0.1); mean bias = 0.1±0.3 L [95% LoA = -0.4 to 0.6]). Lab sweat [Na + ] for day 1 was 41±11 mmol•L-1 and 41±9 mmol•L-1 on day 2 (mean bias = -1±5 mmol•L-1 [95% LoA: -11 to 9]; CV: 7.4±0.1%; SEM: 2 mmol•L-1). hDrop sweat [Na + ] for day 1 was 52±13 mmol•L-1 and 51±14 mmol•L-1 on day 2 (mean bias = -0.3±11.1 mmol•L-1 [95% LoA: -22 to 22]; CV: 11.2±0.1%; SEM of 3 mmol•L-1). hDrop reported a higher sweat [Na + ] compared to the patch (52±13 mmol•L-1 vs 40±10 mmol•L-1, (p< .001); mean bias = 12±11 mmol•L-1 [95% LoA = -11 to 34]). CONCLUSIONS: Sweat [Na + ] for hDrop and lab measures demonstrated similar reliability, however, hDrop overestimated sweat [Na + ] by ~ 11 mmol•L-1. hDrop demonstrated reliability for sweat rate, and validity between hDrop and lab measures. This could result in athletes ingesting excessive Na + beyond sweat losses, which could result in hypernatremia. Future research should investigate hDrop in various climates or event durations. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.