IMx tacrolimus II assay: is it reliable at low blood concentrations? A comparison with tandem MS/MS

Objectives: To compare tacrolimus concentrations in blood samples using the MEIA II and MS methods at the lower and higher ends of the clinically relevant range. Methods: MEIA II was compared to our tandem MS/MS procedure by regression and difference plot analysis. Data from recently published CAP surveys comparing MEIA II with MS procedures are also analyzed. Results: Comparison of MEIA II with our tandem MS/MS procedure by regression analysis yielded r values of 0.612 and 0.829 for tacrolimus concentrations below and above 9.0 ng/mL respectively, as determined by MEIA II. Below 9 ng/mL between day imprecision of tacrolimus controls gave CVs of 12.16 for MEIA II and 7.82 for tandem MS/MS. Addition of known amounts of tacrolimus to EDTA whole blood gave percent target values of 148 and 130 at concentrations of 5.0 and 17.5 ng/mL respectively as determined by MEIA II. Difference plots demonstrated variability in the mean bias for MEIA II of 43% and 36% at the lower and higher concentration ranges respectively. Analysis of the CAP surveys suggested that the relative positive bias and inter-laboratory variability with MEIA II was more pronounced when the MEIA II median value was below 9.0 ng/mL. Conclusions: Our results along with the CAP survey data suggest that tacrolimus concentrations below 9.0 ng/mL measured by MEIA II are questionable and should be interpreted with caution.