Background Portable blood lactate analyzers (PBAs) are increasingly used in sports and clinical contexts to assess physiological responses. Blood lactate is a critical biomarker for evaluating performance and metabolic thresholds. However, variability in device accuracy, sampling procedures, and calibration raises concerns about their validity and reliability. Objectives To systematically review peer-reviewed studies assessing the validity and reliability of commercially available PBAs compared to laboratory reference methods and to evaluate the methodological quality and certainty of evidence using the Consensus-based Standards for the selection of health Measurement INstruments (COSMIN) and Grading of Recommendations Assessment, Development and Evaluation (GRADE) frameworks. Methods Following PRISMA 2020 guidelines, a systematic search was conducted across PubMed, Web of Science, Scopus, and SPORTDiscus. Of 253 records identified, 25 studies met the inclusion criteria. Validity was assessed using statistical measures such as Pearson`s correlation coefficient (r), concordance correlation coefficient (CCC), Bland-Altman limits of agreement (LoA), and mean absolute error (MAE). Reliability was evaluated using intraclass correlation coefficient (ICC), coefficient of variation (CV), and standard error of measurement (SEM). COSMIN and GRADE were used to rate study quality and evidence certainty. Results PBAs (e.g., Lactate Pro, Lactate Scout, Accutrend, Lactate Plus) showed strong correlations with reference standards (r = 0.95-0.99) and high reliability (ICC > 0.90; CV < 5%). However, systematic biases at higher lactate concentrations were common. Thirteen studies had high methodological quality. Conclusions PBAs demonstrate strong reliability and moderate-to-high validity. Standardized protocols and improved calibration are needed to ensure consistency with laboratory-grade analyzers. Key Points Systematic biases at high lactate concentrations undermine device validity: Although most portable blood lactate analyzers (PBAs) showed high correlations with laboratory standards (r = 0.95-0.99), several models demonstrated systematic over- or underestimation at lactate levels above 6 mmol/L. This affects their validity for diagnostic or threshold-based applications, especially when precision is critical. Environmental and procedural factors significantly influence accuracy: The updated review emphasizes that device performance varies with factors like sampling site (earlobe vs. fingertip), post-exercise timing, and environmental conditions (e.g., cold, altitude). These sources of error are often underreported and can meaningfully impact lactate readings. Lack of representation across populations limits generalizability: Major gaps were identified in current literature, particularly regarding female and youth participants, as well as the absence of multicenter trials. This undermines the external validity of PBAs across diverse user groups and settings, despite their growing use in varied athletic and clinical environments. Methodological quality and statistical reporting remain critical limitations: The review highlights frequent misuse or omission of agreement-based metrics like Bland-Altman plots, MAE, and RMSE. Many studies rely heavily on correlation coefficients, which do not assess true agreement. Additionally, few studies conducted a priori sample size estimation for reliability metrics, weakening confidence in reported ICC values.
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