The hDrop wearable hydration monitor generated sweat-loss estimates in fewer than half of training-session observations and demonstrated substantial individual-level error and significant proportional bias, and its hydration score lacked association with urine-specific gravity, findings that 'do not support use of the hDrop as an interchangeable substitute for established methods of individualized sweat-loss or hydration assessment.'
Key Findings
Results
The hDrop monitor generated a sweat-loss estimate in fewer than half of all training-session observations.
Sweat-loss estimates were generated for only 18 of 38 training-session observations (47.4%).
The study involved Division I male collegiate soccer athletes contributing repeated observations across training sessions.
18 complete observations were used for sweat-loss analyses and 36 for hydration-score-USG analyses.
Failure to generate an estimate in over half of sessions represents a fundamental limitation for practical field use.
Results
The hDrop sweat-loss estimates showed large individual-level error with a mean absolute percent error of 50.3%.
MAPE was 50.3%, described as 'reflecting large individual error.'
hDrop estimates averaged 106.2 ± 64.8% of the criterion measure (body mass change adjusted for fluid balance).
The repeated-measures mean bias for sweat loss was -0.423 L (95% CI -1.106 to 0.259 L).
The wide standard deviation of 64.8% around the mean percent of criterion indicates highly variable accuracy across individual observations.
Results
Significant proportional bias was present in hDrop sweat-loss estimates, with greater underestimation occurring at higher sweat-loss values.
Proportional bias slope was -0.790 (95% CI -1.438 to -0.143, p = 0.020) in repeated-measures Bland-Altman analysis.
This indicates the device systematically underestimates sweat loss as true sweat loss increases.
In repeated-measures GEE analysis, criterion-measured sweat loss was positively associated with hDrop-estimated sweat loss (B = 0.204, 95% CI 0.018 to 0.391, p = 0.032).
Generalized estimating equations accounted for observations clustered within athletes.
Results
The hDrop hydration score was not significantly associated with urine specific gravity (USG) either before or after practice.
Pre-practice GEE analysis: B = -407.9 (95% CI -1008.7 to 192.8, p = 0.183).
Post-practice GEE analysis: B = -135.0 (95% CI -803.8 to 533.7, p = 0.692).
USG is an established field-based urine index used as the criterion for hydration status.
Although the hydration score decreased following exercise, the lack of association with USG leaves 'its physiological meaning and criterion validity uncertain.'
Methods
The study design used repeated-measures Bland-Altman analysis and generalized estimating equations to account for the clustered nature of repeated observations within individual athletes.
38 total training-session observations were contributed by Division I male collegiate soccer athletes.
Pre- and post-exercise body mass, fluid intake, urine output, and urine samples were collected during training sessions.
Sweat loss was calculated using changes in body mass adjusted for fluid balance as the criterion measure.
Analyses included mean bias, percent of criterion, MAPE, repeated-measures Bland-Altman analysis, and GEE models.
The authors described this as a pilot study.
Conclusions
The authors concluded that the hDrop should not be used as an interchangeable substitute for established methods of individualized sweat-loss or hydration assessment.
Findings cited include sweat-loss estimates generated in fewer than half of observations, substantial individual-level error, and significant proportional bias.
The uncertain physiological meaning of the hydration score due to its lack of association with USG was also cited.
The hDrop is described as a 'reusable wearable hydration monitor' marketed as a 'non-invasive, quick, and easy to use alternative to existing methods.'
The study framed accurate hydration assessment as having 'potential to optimize athlete performance and reduce risks associated with excessive exercise-related fluid loss.'
What This Means
This pilot study tested whether a wearable sweat sensor called the hDrop could accurately measure how much sweat athletes lose during exercise and assess their hydration levels. Collegiate soccer players wore the device during training sessions, and researchers compared its readings to established methods — specifically, changes in body weight (adjusted for fluid consumed and lost) for sweat loss, and urine specific gravity for hydration status. The results revealed two major problems: the device failed to produce a sweat-loss reading in more than half of all training sessions, and when it did produce a reading, the error was very large (averaging about 50% off from the actual value), with the device tending to underestimate sweat loss more severely when athletes actually lost the most sweat.
For hydration status, the device's built-in 'hydration score' did decrease after exercise as expected, but it showed no meaningful relationship with urine specific gravity — the standard field measure of hydration — either before or after practice. This means it is unclear whether the hydration score is actually measuring anything physiologically meaningful about the athlete's body fluid status.
This research suggests that the hDrop wearable monitor, at least in the form tested, is not yet ready to replace traditional hydration assessment tools for athletes. The combination of frequent data gaps, high individual-level error, and an unvalidated hydration score means coaches, trainers, and athletes should be cautious about relying on this device to make decisions about fluid replacement or hydration management. More research with larger samples and refined technology would be needed before such devices could be confidently recommended for individualized athlete hydration monitoring.
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Snyder J, Toth L, Aguero E, Brennan C, Duarte K. (2026). Evaluation of a Wearable Hydration Monitor for Estimating Sweat Loss and Hydration Status in Collegiate Athletes: A Pilot Study.. Nutrients. https://doi.org/10.3390/nu18193166