IMx Tacrolimus II vs IMx Tacrolimus Microparticle Enzyme Immunoassay Evaluated in Renal and Hepatic Transplant Patients

In 1992, an automated whole-blood microparticle enzyme immunoassay was developed (Abbott Labs.) for the measurement of tacrolimus concentrations (TAC I) on the IMx analyzer (1). This method involves the antitacrolimus monoclonal antibody developed by Fujisawa Pharmaceutical, the company producing the immunosuppressant tacrolimus (2). The assay requires 100 μL of whole blood, and 24 samples can be analyzed in ∼40 min after a rapid organic extraction (200 μL of precipitation reagent: ZnSO4 solution in methanol and ethylene glycol). The capture reagent consists of latex microparticles to which tacrolimus antibodies are attached, the enzyme is tacrolimus-conjugated alkaline phosphatase, and the substrate is 4-methylumbelliferyl phosphate. This method yields CVs of ∼10% (11.8% and 9.6% at concentrations of 15 and 25 μg/L, respectively) (1), but is limited by the detection limit of 5 μg/L. Because in current clinical practice a nonnegligible percentage of transplant patients display low tacrolimus concentrations (<6 μg/L), the immunoassay has recently been modified. A new assay, IMx tacrolimus II (TAC II), with a lower detection limit, has been developed, requiring 150 μL of whole blood and 150 μL of precipitation reagent. Whereas the TAC I has a dynamic range of quantification from 5 to 60 μg/L, the TAC II assay has a range from 1 to 30 μg/L, better corresponding to the therapeutic range of tacrolimus (5–15 μg/L) (3). Because this new assay will replace the TAC I, we have evaluated and compared the two assays in terms of their analytical performances, and their correlation in clinical blood specimens obtained from kidney and liver transplant patients.