Background and aims: The therapeutic monitoring of Tacrolimus (FK506) is necessary since low doses may cause graft rejection while overdosage is linked to nephrotoxicity, neurotoxicity, and many other adverse effects. Occasional notices of elevated values recorded in patients under maintenance regimen have prompted us to record all results exceeding the therapeutic range (>15 ng/mL) with no clinical signs or explanation and to compare the routine method (Siemens-Dade Dimension XPand) with other assays.
Methods: Eighty-four whole blood samples from 8 patients have been assayed by Dimension and by one or more of three other commercial assays (CMIA and MEIA, Abbott; EMIT, Siemens-Dade). As a reference, an automated LC-MS/MS method has been performed.
Results: In all cases the raised Tacrolimus values were observed only by ACMIA, while the correlation (r2) of the other assays with LC-MS/MS was excellent for CMIA (0.97) and good for MEIA (0.88) and EMIT (0.83). The aberrant results were often recorded over a span of several weeks or months and could not be ascribed to a common cause.
Discussion: Abnormally high Tacrolimus results by the ACMIA method have been observed in 1% of the patients currently followed up in our Center. Since these results may lead to erroneous adjustments of the drug dosage, we suggest checking any elevated or clinically unexplained Tacrolimus result by the ACMIA assay with other method(s) requiring an external pretreatment.