Therapeutic monitoring of Tacrolimus (FK506) with the first-and second-generation microparticle enzyme immunoassays : Performance and results in four patient populationsThe performance of the first- and second-generation microparticle enzyme immunoassays for tacrolimus (Tacrolimus I and Tacrolimus II, respectively) was compared during 8 months routine drug monitoring. The minimum detection limit of the Tacrolimus II assay was lower: 1.4 versus 4.0 ?g tacrolimus/l. There was also no overlap between results using 10 samples containing 0 and 1 ?g tacrolimus/l in the Tacrolimus II assay (p 0.001), with a corresponding significant difference achieved for the Tacrolimus I assay only at 0 and 4 ?g/l. For control specimens, within-assay precision was superior for Tacrolimus II at 5 ?g/l (7.3% vs. 31.5%), similar at 10 and 15 ?g/l (6.3% versus 6.6% and 4.4% vs. 4.6%, respectively) but worse than Tacrolimus I at 25 ?g/l (8.0% vs. 4.0%). Using three pools of blood samples from recipients of liver transplants (containing approximately 4, 10, and 20 ?g tacrolimus/l) interassay precision (n 20) was 14.2%, 10.4%, and 8.0% for Tacrolimus II and 42.4%, 13.8%, and 7.1%, for Tacrolimus 1. The analytical times and stability of the calibration curves were similar for the two assays. Tacrolimus I and Tacrolimus II results were closely correlated using patients' blood samples (r ? 0.8 in 249 adult and 168 pediatric liver transplant samples, 161 renal transplant samples, and 61 samples from patients with autoimmune diseases). However, Tacrolimus II assay results were consistently lower (by a mean of 1.02 to 2.05 ?g/l). The authors conclude that the Tacrolimus II assay retains the speed, accuracy, and precision of its predecessor and demonstrates an improved sensitivity, which should facilitate monitoring at less than 5 ?g tacrolimus/l.
