Comprehensive evaluation of lactate enantiomers in human sweat: Stability, sampling sites, and correlation with serum...

The enantiomeric analyses of lactate (LA) are useful for disease diagnosis and pathophysiological understanding because d-lactate (D-LA), a minor enantiomer in the human body, is associated with several diseases. Although various biofluids contain LA, LA enantiomers in human sweat exhibit potential as noninvasive biomarkers. For clinical applications, sweat sampling should be performed rapidly, and the samples must be appropriately stored before analysis. However, the stability of sweat LA enantiomers, appropriate sweat sampling sites, and substitutability of blood LA enantiomers with those in sweat have not been sufficiently investigated. In this study, the stability, sweat sampling sites, and correlation between sweat and serum LA enantiomers were evaluated using a previously developed two-dimensional liquid chromatography method. Although sweat LA enantiomers were stable at -30 and 4 °C for 14 days, they significantly decomposed at 25 °C within seven days. This degradation was suppressed under acidic conditions by adding trifluoroacetic acid, suggesting the involvement of enzymatic processes in instability. Sweat from fingertips, forearms, and cheeks were compared; fingertip sweat showed higher secretion amounts and relatively good reproducibility of LA enantiomer concentrations. No significant correlation was observed between sweat and serum D-LA, suggesting that sweat D-LA might reflect local metabolism rather than systemic D-LA levels. In contrast, the secretion amount of L-LA in sweat positively correlated with its serum concentration, which suggests that L-LA in sweat could serve as a surrogate marker for L-LA in serum. These findings contribute to the applications of sweat LA enantiomer analysis in clinical disease diagnosis.