Quantification of cyclosporine and tacrolimus in whole blood. Comparison of liquid chromatography–electrospray mass spectrometry with the enzyme multiplied immunoassay technique

Objectives The aim of this work was to compare a validated liquid chromatography–mass spectrometry (LC–MS) method with the commercial enzyme multiplied immunoassay technique (EMIT) for cyclosporine and tacrolimus whole blood quantification. Design and methods Samples of transplant patients receiving cyclosporine ( n = 38) or tacrolimus ( n = 41) were analyzed successively by LC–MS and EMIT. Several statistical approaches for method comparison were evaluated and Passing–Bablok and Bland–Altman analyses chosen. Results Overestimations of the concentrations measured with EMIT compared to LC–MS were observed with means of 23% (range: 6% to 46%) for cyclosporine and 30% (range: − 3% to 73%) for tacrolimus. Conclusion The EMIT demonstrated significant positive biases due to cross-reactions with metabolites. This indicates that, in some clinical situations, a selective method such as LC–MS is preferable for therapeutic drug monitoring in transplant patients.