Limit of Quantification (Functional Sensitivity) of the New IMx Tacrolimus II Microparticle Enzyme Immunoassay

In kidney and liver organ transplant recipients, tacrolimus concentrations of 5.0–15.0 μg/L and 3.0–8.0 μg/L, respectively, should be achieved during long-term course. Measuring with the old IMx® tacrolimus assay, the laboratory could not provide tacrolimus concentrations in the subtherapeutic range because of its poor sensitivity. The new IMx Tacrolimus II microparticle enzyme immunoassay (Abbott) was introduced in clinical practice 1 year ago. The imprecision and detection limit of this new assay have already been evaluated and compared with the old assay. Wallemacq et al. (1) reported in the October 1997 issue of Clinical Chemistry that the IMx Tacrolimus II assay is characterized by a much better analytical performance. The detection limit of this new assay was reported to be 1.2 μg/L or 1.5 μg/L (1)(2). Thus far, the interassay imprecision of the new IMx tacrolimus assay has been determined with the help of kit controls. The concentration of the low control (5 μg/L) is within the therapeutic range of tacrolimus. The authors did not publish data on the imprecision of the assay in the subtherapeutic range (1)(2)(3).