Comparative evaluation of five tacrolimus assays in transplant recipients: implications for optimizing therapeutic drug monitoring

Tacrolimus is a widely used immunosuppressive therapy in transplant recipients, but its narrow therapeutic index necessitates accurate monitoring. Tacrolimus levels can be quantified using immunoassays (IAs) and liquid chromatography-tandem mass spectrometry (LC-MS/MS), however, differences between these methods may influence clinical decision-making. In this study, we compared two IAs, i.e., chemiluminescent (CMIA) and electrochemiluminescent (ECLIA), with three LC-MS/MS assays in 181 clinical specimens. When compared with the overall mean concentration, all five assays showed strong correlations, though with variability across methods: three LC-MS/MS assays demonstrated correlation coefficients of 0.9927, 0.9612, and 0.9920, while two immunoassays yielded coefficients of 0.9938 and 0.9857. Deming regression analysis revealed slopes of 0.96, 0.94, and 0.93 for the three LC-MS/MS, while the immunoassays showed higher slopes of 1.032 (ECLIA) and 1.21 (CMIA). Bland–Altman analysis indicated systematic underestimation by the LC-MS/MS methods (–7.5%, −18.7%, and −8%) and overestimation by the immunoassays (ECLIA +9.7%, CMIA +18.4%), relative to the overall mean. The two immunoassays showed only moderate agreement with each other (slope = 0.85, intercept = 0.49), and even the LC-MS/MS assays were not fully concordant. Among 47 patients within 3 months post-transplantation and 134 patients beyond 3 months, clinically relevant discrepancies (≥2 ng/ml) between LC-MS/MS and immunoassay results were observed in 13 patients (28%) and 49 patients (37%), respectively. These findings underscore the substantial impact of assay-dependent variability on tacrolimus monitoring and emphasize the need for standardized laboratory practices as well as assay-specific therapeutic ranges to prevent underexposure with rejection or overexposure with toxicity.