The Effects Of Handling And Analysis Techniques Used In The Measurement Of Sweat Sodium Concentration

Abstract

The development of individualized hydration plans requires protocols to accurately assess fluid and sodium balance during exercise. While sweat sodium concentration and losses are most precisely measured by whole-body washdown techniques, the regional absorbent patch method provides a practical approach for field testing. Common laboratory-based techniques used to analyse sodium concentration of sweat samples include ion-selective electrode (ISE) and flame photometry. In addition, a desktop device assessing sweat conductivity may be appropriate for 'on the spot' analysis. The effects of differing handling protocols and analysis techniques on measurements of sweat sodium concentrations have not previously been examined. PURPOSE: To determine the effects of freezing and thawing sweat samples prior to analysis and of using different analytical techniques to measure sodium concentrations of sweat samples. METHODS: The regional absorbent patch method was used to collect sweat samples from the forearm during a standardized exercise protocol undertaken by 12 trained cyclists on four occasions. We undertook immediate single measure analysis of sodium concentration by ISE (Hitachi Model 911) and selected 30 samples covering a range of concentrations for further analysis. Three aliquots from each sample were frozen (-80°C) for re-analysis 4 - 12 weeks later using ISE (FISE), flame photometry (FFP) (Corning Model 410C) and sweat conductivity (FCON) (SweatChek 3120). RESULTS: Immediate analysis by ISE resulted in a mean sweat sodium concentration of 58 ± 19 mmol/L (range: 17 - 97 mmol/L). Re-analysis of stored samples by ISE produced a mean of 54 ± 18 mmol/L (range: 22 - 89 mmol/L). A mean of 66 ± 22 mmol/L (range: 23 - 109 mmol/L) and 62 ± 19 mmol/L (range: 25 - 97 mmol/L) were found using flame photometry and sweat conductivity, respectively. There was a high correlation between the values produced under each condition (R2 = 0.95), however mean values were statistically different between all treatments (FFP > FCON > ISE > FISE; p < 0.0001). CONCLUSIONS: The difference in values produced after freezing and by various analytical techniques is small but significant. There is potential for individuals' sweat sodium concentration to be misclassified when measured values are within the margins of arbitrary thresholds.