Sweat testing has moved from the lab into the hands of everyday athletes. That sounds exciting because sweat sodium levels vary widely among runners, and for long races in hot conditions, knowing whether you are a salty sweater can be genuinely useful. But there is always a catch with consumer-friendly sports tech. Just because a device gives you a number does not mean that number is reliable.
A new study tested two portable sweat sodium analyzers,1Zhao, X., McKenna, Z., Wierick, S. C., Perez, R. I., Sellner, R., Allen, B., & McDermott, B. P. (2026). Reliability and Validity of Portable Sweat Sodium Analyzers during Exercise in the Heat. Applied Physiology, Nutrition, and Metabolism = Physiologie Appliquee, Nutrition et Metabolisme, 10.1139/apnm2025-0482. https://doi.org/10.1139/apnm-2025-0482 the MX3 and LAQUAtwin-Na-11, against a reference analyzer in warm, humid exercise conditions. Nineteen healthy, active adults completed two 60-minute running or cycling trials in the heat, around 81 degrees Fahrenheit and 51% relative humidity. Sweat was collected using an absorbent patch on the forearm and then analyzed using portable devices and the reference method.
The question was twofold: are these devices reliable from trial to trial, and are they valid compared with the reference analyzer?
Both devices performed reasonably well, but not perfectly. The MX3 showed acceptable-to-high between-trial reliability, while the LAQUAtwin showed acceptable-to-moderate reliability. The coefficients of variation were still fairly large, at around 23–24%, indicating that individual readings can vary widely. That matters because athletes often want sweat sodium testing to give them a precise hydration prescription. The reality is probably more like a useful estimate than a laboratory-grade truth.
On validity, both devices looked better. They were strongly associated with the reference method. Overall, the authors concluded that both portable analyzers provide good validity for lab-based sweat sodium assessment, with MX3 showing somewhat better reliability.
The study is useful, but it also reinforces an important limitation: this was a controlled lab-style setup using forearm sweat patches, not a chaotic race-day environment with changing pace, wind, fluid intake, aid stations, and hours of exercise. Sweat sodium itself also varies by body site, sweat rate, heat acclimation, and collection method. So, while these tools can be helpful, I would be cautious about building an overly precise sodium plan based on a single test.
What this means for runners
Portable sweat sodium testing can be useful, especially if you are training for long races in the heat or have a history of heavy salt loss, cramping, or white salt marks on clothes. But treat the number as a starting range, not a commandment.
If a device tells you that you lose a lot of sodium, it may justify practicing with higher-sodium fluids or gels during long runs, but the real test is how your gut, thirst, performance, and post-run symptoms respond. I’d use sweat sodium data alongside sweat rate, race duration, heat exposure, and fueling tolerance rather than turning it into a rigid formula.
