Satisfactory performance of the Abbott ARCHITECT i2000 tacrolimus Immunoassay against the LC-MS/MS tacrolimus assay demonstrated in kidney transplant patients from the Northern Territory, Australia

Background Tacrolimus, a post-transplant immunosuppressive drug with a narrow therapeutic window requires close monitoring of levels to avoid under-immunosuppression or toxicity. Top End Renal Services in the Northern Territory (NT) refer to South Australia (SA) for drug levels as there was no local service. We aimed to evaluate the Abbott ARCHITECTi2000 immunoassay against the liquid chemistry tandem mass spectroscopy (LC-MS/MS) used in SA for measuring tacrolimus levels to provide on-site service in NT. Methods 465 specimens were collected over 5 months and analyzed over several reagent lots. We used Passing-Bablok regression plots and Bland–Altman plots to assess the agreement between tacrolimus levels on both platforms. Results The Passing-Bablok regression plot demonstrated a slope of 1.172 (CI 1.136 to 1.207) with an intercept of 0.262 (CI 0.040 to 0.472). In Deming analysis, the slope was 1.095 (CI 1.074 to 1.116) with an intercept of 0.773 (CI 0.592 to 0.955), correlation coefficient (r) was 0.9782. Bland-Altman plot demonstrated positive bias for Abbott ARCHITECT results. The mean absolute bias was 1.494 ug/L and the mean percentage bias was 18.78%. Within run imprecision, Co-efficient of Variation (%) was 5.1, 2.7, 4.3, 3.4 and 3.5 at tacrolimus concentration levels of 4.2, 6.5, 9.5, 17.2 and 24.4 µg/L. Results turnaround time improved by 80%. Conclusion The results demonstrate that Abbott ARCHITECT i2000 is an acceptable method to monitor levels of tacrolimus. The positive bias could be justifiable if the drug levels are initially based and then monitored on results from the same platform.