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. (C) 2008 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.