Validation of a novel handheld lactate analyzer reveals concentration-dependent bias compared with a laboratory reference device

This study aimed to validate a handheld capillary blood lactate analyzer (Lactate Express) against a reference (EKF Biosen C-Line) and assess its impact on lactate thresholds. Thirty-five participants completed incremental running or cycling tests with capillary sampling. Lactate was measured with the Lactate Express and an EKF Biosen C-Line analyzer, yielding 231 paired samples (0.5-14.2 mmol l-1). Agreement was evaluated using intraclass correlation, Lin's concordance correlation, mean absolute error, and Bland-Altman analyses overall and within <1, 1-4, and >4 mmol l-1. Thresholds at 2 and 4 mmol l-1 were compared between devices for 18 participants in both exercise modes. Overall agreement was high (intraclass correlation coefficient 0.98), but Bland-Altman analysis showed a bias of -0.29 mmol l-1 with concentration dependent behavior. Bias was negligible within the 1-4 mmol l-1 range (-0.05 mmol l-1), positive at concentrations <1 mmol l-1 (0.29 mmol l-1) with pronounced underestimation at concentrations >4 mmol l-1 (-1.03 mmol l-1). Differences in 2 and 4 mmol l-1 thresholds were small and non-systematic. The Lactate Express demonstrates good relative and absolute agreement within the 1-4 mmol l-1 range relevant for threshold-based training prescription, but exhibits substantial and systematic bias at both low and high lactate concentrations. The device may provide broadly comparable threshold estimates in some cases but should not be considered interchangeable with the laboratory reference device across the full lactate concentration spectrum.