Inter-laboratory variability in vancomycin measurement in an external quality assessment scheme 2016–2025

INTRODUCTION: Vancomycin is a critical antibiotic for treating severe Gram-positive infections. Due to its narrow therapeutic window, reliable plasma concentration measurements are essential for dose adjustment and avoiding toxicity. However, method-dependent analytical variability between immunoassay platforms may compromise result comparability. METHODS: Data from 78 external quality assurance (EQA) samples distributed between 2016 and 2025 by Labquality (Helsinki, Finland) and Equalis (Uppsala, Sweden) were analyzed. Results from laboratories using immunoassays from Abbott Laboratories (n = 1391), Beckman Coulter (n = 172), Roche Diagnostics (n = 3584), Ortho Clinical Diagnostics (n = 78), Siemens Healthineers (n = 1670), Thermo Fisher (n = 152), and ARK Diagnostics (n = 33) were included (total = 7104). The mean consensus value for each sample was used as reference. RESULTS: Across the study period, Abbott Laboratories (+4.7%), Thermo Fisher (+12%), Ortho Clinical Diagnostics (-4.6%), and Siemens Healthineers (-4.9%) showed systematic biases relative to the consensus mean, whereas Beckman Coulter (-0.3%), Roche Diagnostics (<0.1%), and ARK Diagnostics (-2.4%) displayed good agreement. Temporal trends indicated method-specific drifts, most notably for Beckman Coulter (-20.2%) and Siemens Healthineers (+12.5%). Coefficients of variation ranged from 0.9% (ARK Diagnostics) to 12.1% (Beckman Coulter). CONCLUSION: Considerable intermethod bias and temporal drift exist among commonly used vancomycin immunoassays, underscoring the need for improved calibration harmonization and traceability to ensure consistent therapeutic drug monitoring.