Comparison of five cyclosporin immunoassays with HPLC

High performance liquid chromatography (HPLC) is the reference method for c yclosporin (CyA) measurements but therapeutic monitoring of the drug is fre quently made using the more practical immunoassays. Cross-reactivity with C yA metabolites may compromise the specificity of immunoassays, particularly in liver graft recipients where metabolites may accumulate. The aim of thi s study was to compare with HPLC the performance of two recently introduced CyA immunoassays (the AxSYM fluorescent polarisation immunoassay (FPIA) an d non-extraction CEDIA assay). The comparison was extended to the well-esta blished TDx monoclonal FPIA (TDx mono) and the enzyme multiplied (EMIT)-spe cific assays and to the polyclonal FPIA (TDx poly), in which metabolite cro ssreactivity is extensive. Assays were performed on 106 blood samples (take n 6 days to 118 months post-liver transplant) and results were compared by non-parametric regression analysis and difference plots. AxSYM and CEDIA sh owed both constant and proportional bias against HPLC (unlike EMIT) but the mean difference from HPLC was least for AxSYM (2.7 mug/l vs. 11.7, 9.4 and 54 mug/l for CEDIA, EMIT and TDx mono, respectively. (TDx poly - HPLC) val ues were proportional to all immunoassay results, with slopes of 0.33, 0.38 and 0.45 for EMIT, AxSYM and CEDIA. respectively. Our data suggest close a greement between AxSYM, CEDIA and EMIT results.