Validation of a Portable and Inexpensive Method for Assessing Sweat Sodium Concentration

The accurate assessment of sweat sodium concentration is useful for the treatment of skeletal muscle cramps during exercise, minimizing exercise-induced dehydration, and restoring fluid balance after exercise. However, most methods of sweat sodium analysis require the use of specialized and expensive equipment, and are not readily available to the majority of recreational, elite and professional athletes. PURPOSE: To evaluate the ability of capillary action salt-water testing strips (SS) to accurately measure known [Na] standards, and unknown human sweat [Na] samples. METHODS: The SS used in this study were intended for use in salt-water pools, and indicated [Na] by colour change on a numerical scale. Ten SS were submerged in each of 4 salt-water standards (25, 50, 75, 100 mM [Na]) until the reading had stabilized (6 ± 0 min), and 3 SS served as control (CON; 0 mM [Na]). SS readings were compared to the true [Na] of each aliquot, which was determined by conductivity analysis (WESCOR). Human sweat samples were collected by absorbent patch from the foreheads of 10 subjects (Mean ± SE; 27 ± 1 years, 1.8 ± 0.0 m, 68.6 ± 4.6 kg) during 25 ± 1 min of moderate intensity cycling at 21.2 ± 0.3°C and 26 ± 0% relative humidity. Samples were extracted from the patches by centrifugation, [Na] was determined by WESCOR in a small (5-10 μL) aliquot, and SS were submerged in the remaining sample for 8 ± 2 min. Comparisons of SS and WESCOR measurements were made with Student’s paired t-tests and linear regression analyses. RESULTS: SS underestimated [Na] vs. WESCOR in 25 mM (21 ± 1 vs. 24 ± 1 mM, p < 0.05), 50 mM (41 ± 2 vs. 46 ± 0 mM, p < 0.05) and 75 mM standards (67 ± 3 vs. 71 ± 1 mM, p < 0.05), but was similar in the 100 mM standard (90 ± 4 vs. 90 ± 1 mM). This underestimation was inversely related to [Na] and could be corrected (Correction = −0.002*SS + 1.212, r = 0.99, p < 0.05). In human sweat samples, SS underestimated [Na] (64 ± 6 vs. 78 ± 7 mM, p < 0.05). The underestimation was also related to [Na] (r = 0.75, p < 0.05), and SS measurements could be corrected (Correction = −0.005*SS + 1.75). The SS underestimation of human sweat [Na] was not related to body mass, sweat sample volume or exercise time. CONCLUSIONS: SS consistently underestimated WESCOR [Na] measurements, but the error was inversely proportional to [Na] and predictable for both known standards and unknown human sweat samples. SS could provide a valid method of assessing sweat [Na] for athletes without access to more expensive alternatives.

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