Interference of hematocrit in the tacrolimus II microparticle enzyme immunoassay

The authors studied the effect of hematocrit, bilirubin, and alkaline phosphatase on microparticle enzyme immunoassay for tacrolimus II (MEIA II) using specimens of whole blood obtained from 33 patients undergoing cyclosporine treatment. Tacrolimus was added to these samples at a final concentration of 7.5 μg/L and 15 μg/L. Both coefficients of variation were over 20% (21% at 7.5 μg/L of tacrolimus and 22% at 15 μg/L of tacrolimus). No correlation was found between bilirubin and tacrolimus concentrations or between alkaline phosphatase and tacrolimus concentrations. On the other hand, negative correlations were found between hematocrit values and tacrolimus concentrations (r² = 0.47; P < 0.0001 at 7.5 μg/L tacrolimus, r² = 0.54; P < 0.0001 at 15 μg/L tacrolimus). Negative correlations were also found between hematocrit and the tacrolimus concentration using normal human red blood cells diluted with physiological saline solution (r² = 0.93; P < 0.0001 at 7.5 μg/L tacrolimus, r² = 0.91; P < 0.0001 at 15 μg/L tacrolimus). The results showed that the hematocrit interferes with the MEIA II for tacrolimus, and the magnitude of the interference is clinically significant. Beyond the normal range of hematocrit values, caution should be exercised in interpreting results as one may need to compensate for the levels of tacrolimus.