Therapeutic monitoring of tacrolimus concentrations in blood of renal and liver transplant recipients: comparison of microparticle enzyme immunoassay and enzyme multiplied immunoassay methods.

Monitoring of tacrolimus blood concentrations is of utmost importance in the management of organ transplant recipients due to narrow therapeutic index of the drug and its considerable interpatient variability in pharmacokinetics. Thus therapeutic monitoring of tacrolimus plays a crucial role not only in the evaluation of the drug efficacy but also in the control of possible side effects. We compared immunoassay-based methods, quantitative enzyme multiplied immunoassay (EMIT) with quantitative microparticle enzyme immunoassay (MEIA), using blood samples from renal and liver transplant recipients (n = 40) treated with tacrolimus. Blood samples were obtained for diagnostic routine measurements. The tacrolimus concentrations measured by EMIT for all the transplant patient samples were higher (2.8 to 28.5 ng/mL) than results obtained in MEIA (3.0 to 25.0 ng/mL). The mean difference expressed in percentage was 13.94% and correlation coefficient EMIT versus MEIA was 0.97. The tacrolimus concentrations measured by EMIT for renal graft recipients (n = 8) were higher (6.0 to 13.3 ng/mL) than those measured by MEIA (6.1 to 12.2 ng/mL), mean difference expressed in percentage was 14.1% and correlation coefficient was 0.85. The tacrolimus concentrations for liver transplant recipients (n = 32) measured by EMIT (2.8 to 28.5 ng/mL) were higher than results obtained in MEIA (3.0 to 25.0 ng/mL), the mean difference expressed in percentage was 13.89%, and the correlation coefficient was 0.98. The results obtained in the study show an insignificant difference in specificity of both methods used to determine the concentration of an active drug. Thus both methods, EMIT and MEIA, seem to have similar diagnostic value.