Cross-validation of equations to predict whole-body sweat sodium concentration from regional measures during exercise

We have previously published equations to estimate whole-body (WB) sweat sodium concentration ([Na⁺ ]) from regional (REG) measures; however, a cross-validation is needed to corroborate the applicability of these prediction equations between studies. The purpose of this study was to determine the validity of published equations in predicting WB sweat [Na⁺ ] from REG measures when applied to a new data set. Forty-nine participants (34 men, 15 women; 75 ± 12 kg) cycled for 90 min while WB sweat [Na⁺ ] was measured using the washdown technique. REG sweat [Na⁺ ] was measured from seven regions using absorbent patches (3M Tegaderm + Pad). Published equations were applied to REG sweat [Na⁺ ] to determine predicted WB sweat [Na⁺ ]. Bland-Altman analysis of mean bias (raw and predicted minus measured) and 95% limits of agreement (LOA) were used to compare raw (uncorrected) REG sweat [Na⁺ ] and predicted WB sweat [Na⁺ ] to measured WB sweat [Na⁺ ]. Mean bias (±95% LOA) between raw REG sweat [Na⁺ ] and measured WB sweat [Na⁺ ] was 10(±20), 0(±19), 9(±20), 22(±25), 23(±24), 0(±15), -4(±18) mmol/L for the dorsal forearm, ventral forearm, upper arm, chest, upper back, thigh, and calf, respectively. The mean bias (±95% LOA) between predicted WB sweat [Na⁺ ] and measured WB sweat [Na⁺ ] was 3(±14), 4(±12), 0(±14), 2(±17), -2(±16), 5(±13), 4(±15) mmol/L for the dorsal forearm, ventral forearm, upper arm, chest, upper back, thigh, and calf, respectively. Prediction equations improve the accuracy of estimating WB sweat [Na⁺ ] from REG and are therefore recommended for use when determining individualized sweat electrolyte losses.