Performance of the IMx Tacrolimus II Assay and Practical Limits of Detection

Recent correspondence to Clinical Chemistry (1)(2) addressed the performance of the second generation Tacrolimus assay for the IMx analyzer (Abbott Diagnostics), with the former letter identifying nonequivalence in results from its predecessor and the latter considering performance approaching the lower limits of detection. Our own published results (3) have considered these points, and we report here our additional experience. In their comparison of the second- vs the first-generation assays, Garg et al. (1) described comparable coefficients of variation (CVs), but this is not the case when identical control samples are used in each assay at low concentrations (≤5 μg/L), e.g., 14.2% vs 42.4% at 4.2 μg of tacrolimus per liter of blood (1). In common with previous findings (3)(4)(5), Garg et al. (1) reported lower values with the second-generation assay using 36 samples of undefined origin. The slopes and intercepts reported for these various comparisons differed (as did the comparison methods applied), but Garg et al. (1) did not relate these data into the practical measurement of mean differences in assay results. Mean underestimates of 1–2 μg/L were reported for tacrolimus concentrations of 3–35 μg/L both by Wallemacq et al. (4), who used renal and liver recipients, and ourselves (3) (adult and pediatric liver and adult renal transplant recipients and patients with autoimmune disease). Of the explanations proposed for these differences by the various investigators, the lower recovery of the second-generation assay experienced by Garg et al. (1) in 1998 is not consistent with 1998 data from the Tacrolimus International Proficiency Testing Scheme (coordinator, Dr. D.W. Holt, St George’s Hospital Medical School, London, UK), from which can be calculated a positive bias and mean recovery of 114% (range, 102.1–121.7%) in 20 samples to which 3–28 μg/L tacrolimus was added and a similar overestimate relative to the results reported by the small number of centers using HPLC/mass spectrometry. A bias in assay calibrators (3) or differences in the contribution of tacrolimus metabolites (3)(4) may be a more likely explanation for the differences between the first- and second-generation assay results. Given the inherent variability in assay performance and tacrolimus pharmacokinetics, we still doubt whether a difference of 1–2 μg/L in assay results would have major practical impact on management by the realistic clinician. This is true both early after transplantation, when tacrolimus trough concentrations usually exceed 10 μg/L (but are subject to variability because of alterations in graft function, drug dosage, and coadministered medication), and later in clinically stable patients, when tacrolimus concentrations are often below 10 μg/L (and pharmacokinetic variability is lower but still subject to the influence of food intake) (6). In this lower range, where the increased sensitivity of the second-generation assay is advantageous, the CVs in tacrolimus measurements will span the differences of 1–2 μg/L between assay results.