The variability in measurements of sweat sodium concentration using the regional absorbent patch method
Creators: name: Dziedzic, Christine
Publisher: name: University of the Sunshine Coast, Queensland
Year: 2013
Subjects: sweat testing
salt
fluid balance
hydration
electrolytes

description: There is recognition that replacement of sodium lost in sweat plays an important role in the restoration of fluid balance following exercise, and that it be included in individualised hydration plans for athletes. While more precisely measured by the whole-body washdown technique, the regional absorbent patch method provides a practical approach to undertaking sweat testing in the field. The effects of differing protocols for the collection and analysis of sweat sodium using the regional absorbent patch method have not previously been examined. The aim of this research was to identify and quantify the sources of variability in sodium concentration associated with the collection of sweat via the regional absorbent patch method by mimicking conditions or variations to the protocol that might occur with field testing. The regional absorbent patch method was used to collect sweat samples from the forearms, chest, scapula and thigh of 12 trained cyclists during four standardised cycling time trials. The sweat collection protocol was employed to allow site selection and application time, plus storage time and temperature to be investigated in either hot or temperate conditions. Single measure analysis of sodium concentration was conducted immediately by ion-selective electrodes (ISE). A subset of 30 samples was frozen for re-analysis of sodium concentration using ISE, flame photometry (FP) and conductivity (SC). A series of linear mixed models revealed that sweat samples collected in hot conditions produced significantly higher sweat sodium concentration than those from the temperate environment (F1,40.44 = 9.83, p = 0.0032). A significant effect of application site (F4,84.25 = 56.00, p &lt; 0.0001) was evident, with the estimated mean sodium concentration of samples taken from the chest found to be higher than sweat sampled from the back, and left and right forearm and thigh. A significant difference (F1,18.99 = 10.21, p = 0.0048) in estimates of sweat sodium concentration was evident when calculated from the forearm average (mean±95% CI; 64±12 mmol.L-1) compared to using a weighted four-site equation (70±12 mmol.L-1). There was a high correlation between the values produced using different analytical techniques (r2 = 0.95), however estimated mean values were statistically different between treatments (frozen-FP , frozen-SC &gt; ISE &gt; frozen-ISE; p &lt; 0.0001). This study highlights a significant difference in the estimation of sweat sodium concentrations depending on the environmental testing conditions, and number of sites included in the estimation of whole-body sweat sodium concentration. The impact of sample freezing and subsequent analytical technique was small but significant. There is potential for individuals' sweat sodium concentration to be misclassified when measured values are within the margins of arbitrary thresholds. Nevertheless, when undertaken by an experienced tester using a standardised protocol related to patch application time and sample handling, the regional absorbent patch method appears to be a relatively robust field test. It is now important to determine what is seen as a practically worthwhile or physiologically significant change in sweat sodium concentration in order to truly reflect on the value of this method in estimating whole-body sweat sodium concentration and losses.