Tacrolimus (FK506) Levels in Transplants by two MethodsCOMPARISON BETWEEN WHOLE BLOOD TACROLIMUS BY ABBOTT IMx AND PLASMA TACROLIMUS BY ENZYME- LINKED IMMUNOASSAY. S. Zuckerman, T. McKaveney, S. Mehta, K. Velmer, J. Chao, R. Venkataramanan, A. Jain, V. Warty and (Depts. of Path. and Surg., Univ. of Pittsburgh School of Medicine, Pittsburgh, PA 15213) Tacrolimus is a potent immunosuppressive agent used in preventing organ rejection. Due to its narrow therapeutic index and highly variable pharmacokinetics, routine monitoring of tacrolimus concentrations in blood/plasma is required. We compared two distinct methodologies, Abbott IMx for whole blood and enzyme-linked immunosorbent assay (ELISA) for plasma using multiple trough samples from 15 patients who underwent liver, kidney of heart transplantation. The IMx whole blood tacrolimus assay is based on micro-particle enzyme immunoassay technology . Its linearity range is 5-60 ng/mL with an inter-assay CV of 9.6% and intra-assay CV of 9.9%. The ELISA method for plasma tacrolimus requires a solid-phase extraction by Sep-pak before analysis. The assay linearity is 0.1-8.0 ng/mL with an inter-assay CV of 17% and intra assay CV of 7.1%. The therapeutic range for whole blood tacrolimus is between 10-20 ng/mL whereas for plasma the patients were maintained between 0.5-2.0 ng/mL . The comparison between these two methods resulted in following correlations. For Liver Tx patients, IMx =5.5 ELISA + 3.6, r2 =0.48 (n=101); for kidney Tx patients, IMx =8.9 ELISA +8.2, r2 =0.76 (n=48) and for heart Tx patients, IMx =9.8 ELISA + 3.2, r2 =0.72 (n=53). The average blood to plasma ratio of tacrolimus is 8.6 +/- 3.8 in liver, 15.1 +/- 5.3 in kidney and 14.7 +/- 4.9 in heart transplant patients, suggesting that this ratio is dependent on the nature of the organ transplanted.