Tacrolimus (FK-506) levels determined by 2 different chemiluminescence immunoassay-based analytical methods: A comparative cross-sectional study.

Several assays have been used to monitor tacrolimus levels. Automated immunoassay systems are preferred routinely because of their simplicity and rapid turnaround times. However, variations in the measured results for the same sample by different assays necessitate the evaluation of various immunoassay systems for consistency. This study aimed to compare whole-blood tacrolimus concentrations measured using 2 different chemiluminescence analytical methods: chemiluminescent microparticle immunoassay (CMIA; Abbott Diagnostics) and chemiluminescent immunoassay (CLIA; Snibe Diagnostics). A total of 257 patients who had undergone kidney transplantation were included in this study. Whole-blood tacrolimus concentrations were measured using the MAGLUMI 800 analyzer and compared with the ARCHITECT i1000SR analyzer using correlation analysis and Bland-Altman plots. There was a strong correlation between tacrolimus concentrations measured using MAGLUMI and those measured using ARCHITECT (R = 0.916, P < .001). An intraclass correlation coefficient (ICC) value of 0.934 (CI = 0.915-0.949) indicated excellent reliability of the measurements. Comparison of samples with a routine ARCHITECT assay for tacrolimus showed a minimal bias of 0.385 ng/mL (95% CI = 4.46%-3.69%), but scattering analysis indicated that the method agreed when the tacrolimus levels were low but disagreed at high levels. Overall, the measured tacrolimus level in patient samples was higher by MAGLUMI than by ARCHITECT with a median difference of 0.17 ng/mL and a median percentage difference of 3.4% respectively. Snibe MAGLUMI is a reliable assay with excellent correlation, but a small positive bias against Abbott ARCHITECT in measuring whole-blood tacrolimus levels. This bias is most likely due to the cross-reactivity of the metabolites in the Snibe MAGLUMI platform.