Falsely increased blood tacrolimus concentrations using the ACMIA assay due to circulating endogenous antibodies in a liver transplant recipient: a tentative approach to obtaining reliable results.

The presence of endogenous antibodies in the serum of some patients has long been known to be a potential source of interference in immunoassays. We report falsely increased whole blood tacrolimus concentrations using the antibody conjugated magnetic immunoassay from Siemens HealthCare Diagnostics in a liver transplant recipient due to the presence of circulating endogenous antibodies (possibly heterophilic antibodies). Estimation of whole blood tacrolimus concentrations from the washed erythrocytes concentrations is proposed as a tentative approach for obtaining reliable results using the antibody conjugated magnetic immunoassay assay in these cases, leading to analogous blood tacrolimus concentrations to those produced by the microparticle enzyme immunoassay from Abbott Laboratories.