Falsely Abnormally Elevated Blood Trough Concentration of Tacrolimus Measured by Antibody-Conjugated Magnetic Immunoassay in a Renal Transplant Recipient: A Case Report

Abstract 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 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.