Vancomycin therapeutic drug monitoring: trends and seasonal patterns.

Vancomycin is widely used for serious Gram-positive infections, but its narrow therapeutic index necessitates therapeutic drug monitoring (TDM). Over recent decades, monitoring has shifted from trough-based to AUC-guided dosing. This study investigated long-term trends in vancomycin concentrations, seasonal variation and the utility of patient medians as a laboratory quality tool. Serum vancomycin results (n = 59,445) from routine TDM at Akademiska Hospital, Uppsala, Sweden (2006-2024) were analyzed. Data included sampling date, age, sex and concentration. Measurements were performed using validated immunoassays (Architect ci8200 and Cobas Pro c503) with standard quality control. Annual medians and percentiles were calculated, and seasonal variation was assessed. The number of analyses increased from 2,263 (2006) to 4,164 (2015), then declined to 2,414 (2024). Median concentrations and distribution percentiles rose steadily from 2006 to 2015 and remained stable thereafter. Seasonal variation showed a ∼15% decrease in testing volume during July-August, while concentration percentiles remained consistent throughout the year. Vancomycin concentrations in routine TDM increased until 2015 and have since stabilized, despite seasonal variation in test volume. Patient medians were stable over time and may serve as a useful complement to traditional quality control, particularly when standard materials do not fully represent patient samples.