Performance and specificity of monoclonal immunoassays for cyclosporine monitoring: How specific is specific?

Background: Immunoassays designed for the selective measurement of cyclospo rin A (CsA) inadvertently show cross-reactivity for CsA metabolites. The ex tent and clinical significance of the resulting overestimation is controver sial. A comprehensive assessment of old and new methods in clinical specime ns is needed. Methods: In a comprehensive evaluation, CsA was analyzed in 145 samples wit h the new CEDIA(R) assay and compared with the Emit(R) assay with the old a nd new pretreatments, the TDx(R) monoclonal and polyclonal assays, the AxSY M(R), and HPLC. All samples were from patients with liver and/or kidney tra nsplants. Results: The CEDIA offered the easiest handling, followed by the AxSYM, whi ch showed the longest calibration stability. The TDx monoclonal assay provi ded the lowest detection limit and the lowest CVs. The mean differences com pared with HPLC were as follows: Emit, 9-12%; CEDIA, 18%; AxSYM, 29%; and T Dx monoclonal, 57%. The CycloTrac(R) RIA paralleled the Emit results. In co ntrast to the mean differences, substantial (>200%) and variable overestima tions of the CsA concentration were observed in individual patient samples. Metabolic ratios, estimates of the overall concentrations of several cross -reacting metabolites (nonspecific TDx polyclonal/specific reference method ), correlated with the apparent biases of the various monoclonal assays. Me tabolic ratios varied up to 10-fold, which translated into biases for indiv idual samples between -7% and +174%. The higher the cross-reactivity of an assay was, the higher was the range of biases observed. The interindividual differences markedly exceeded other factors of influence (organ transplant ed, hepatic function). Conclusion: Because assay bias cannot be predicted in individual samples, s ubstantially erratic CsA dosing can result. The specificity of CsA assays f or parent CsA remains a major concern. (C) 1999 American Association for Cl inical Chemistry.