False positive blood tacrolimus concentration in microparticle enzyme immunoassay

The difference in the blood concentration of tacrolimus between the assay methods, microparticle enzyme immunoassay (MEIA) and enzyme linked immunosorbent assay (ELISA) was observed in a liver transplant recipient with anemia. MEIA provided significantly higher concentration than those of ELISA (7.8+/-1.9 vs. 5.0 +/- 1.8nglml, p<0.05) while the patient had low haematocrit <25%. The difference, however, was not observed during the periods with haematocrit >25%. This observation suggested that unknown tacrolimus levels generated from difference in assay methods gave incorrect blood tacrolimus during anemia. False positive concentration of tacrolimus ranging 0.1-3.3 ng/ml was observed in MEIA applying to the blood samples obtained from the patients without receiving tacrolimus. The false positive tacrolimus increased in the samples with lower hematocrit, suggesting that MEIA gave incorrect blood tacrolimus during anemia. Since MEIA potentially overestimates the tacrolimus levels, ELISA should be used for blood tacrolimus monitoring in the patients with anemia.