Therapeutic Monitoring of Tacrolimus (FK506) With the First- and Second-Generation Microparticle Enzyme Immunoassays: Performance and Results in Four Patient Populations

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Abstract
The 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 microg tacrolimus/l. There was also no overlap between results using 10 samples containing 0 and 1 microg 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 microg/l. For control specimens, within-assay precision was superior for Tacrolimus II at 5 microg/l (7.3% vs. 31.5%), similar at 10 and 15 microg/l (6.3% versus 6.6% and 4.4% vs. 4.6%, respectively) but worse than Tacrolimus I at 25 microg/l (8.0% vs. 4.0%). Using three pools of blood samples from recipients of liver transplants (containing approximately 4, 10, and 20 microg 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 I. 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 approximately equal to 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 microg/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 microg tacrolimus/l.

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