Falsely abnormally elevated blood trough concentration of tacrolimus measured by antibody-conjugated magnetic immunoassay in a renal transplant recipient: a case report

This report presents a falsely abnormally elevated blood trough concentration (C_t) of tacrolimus measured by antibody-conjugated magnetic immunoassay (ACMIA) methods in a renal transplant recipient. Because the C_t of tacrolimus was 78.5 ng/mL at day 2 after a 52-year-old man underwent renal transplantation, we stopped the tacrolimus extended-release formulation. However, because the abnormally elevated blood C_t continued in the range of 41.1-59.1 ng/mL, we then measured the tacrolimus concentration in a stored blood sample before renal transplantation, it was 43 ng/mL. Consequently, the day-7 blood sample was measured with both ACMIA and enzyme-linked immunoassay, showing C_t values of 42.8 ng/mL and 0.89 ng/mL, respectively. Because the abnormally elevated C_t was falsely measured by the ACMIA method, we restarted tacrolimus However, the calcineurin inhibitor was subsequently converted to cyclosporine at day 21 after renal transplantation. Although cyclosporine was also measured by ACMIA, there was not an abnormally elevated C_t. Subsequently, the tacrolimus concentration ratio in plasma and whole blood (P/B-tacrolimus concentration ratio) was measured by ACMIA in a posttacrolimus blood sample. The P/B-tacrolimus concentration ratio was 100%. In contrast, the P/B-tacrolimus concentration ratio was <30% in 2 control patients administered tacrolimus. It has been reported recently that there were cases showing falsely slightly elevated C_t of tacrolimus within the therapeutic range of concentrations. Therefore, we must be careful not to reduce the tacrolimus dose falsely. We consider confirmatory methods for a falsely abnormally elevated C_t of tacrolimus measured by ACMIA to (1) measure P/B-tacrolimus concentration ratio, (2) compare ACMIA with another measurement, and (3) evaluate a blood sample stored before tacrolimus administration.