Long-Term Stability of Cyclosporine Blood Concentrations Assessed by Patient-Based Percentiles over 20 Years

Background/Objectives: Therapeutic drug monitoring (TDM) of cyclosporine is essential due to its narrow therapeutic index and pronounced pharmacokinetic variability. Long-term surveillance of patient results may provide insight into analytical stability and clinical practice patterns beyond conventional quality control approaches. Methods: This retrospective observational study included 48,835 routine whole blood cyclosporine concentrations analyzed at a tertiary university hospital laboratory between January 2006 and December 2025. Yearly patient percentiles (10th, 25th, 50th, 75th and 90th percentiles) were calculated to assess longitudinal trends, variability, and potential effects of analytical platform transitions. Results were analyzed overall and by sex. Results: The yearly number of reported cyclosporine results declined modestly over the study period. The overall median cyclosporine concentration was 134.4 µg/L, with negligible differences between female and male patients. The 10th, 25th, and 50th percentiles remained highly stable across the 20-year period, with coefficients of variation between 6.1% and 6.8%. Upper percentiles exhibited greater variability, but the total coefficient of variation for the 90th percentile remained below 8%. No systematic shifts associated with analytical platform transitions were observed. Conclusions: Long-term patient median and percentile analysis demonstrated remarkable temporal stability of cyclosporine concentrations over two decades, despite changes in analytical platforms and clinical practice. Continuous monitoring of patient medians and percentiles may serve as a valuable complementary quality indicator, particularly for assays with limited commutable quality control materials.