Multicenter Evaluation of a New Inosine Monophosphate Dehydrogenase Inhibition Assay for Quantification of Total Mycophenolic Acid in Plasma

The performance characteristics of a new inosine monophosphate dehydrogenase inhibition assay for the quantification of total mycophenolic acid (MPA) in plasma (Roche Diagnostics) were assessed in a multicenter evaluation. Validation data were collected from four institutions. Within-run and total imprecision were acceptable (n = 21 for each of 7 materials, coefficients of variation ranging 0.7-9.6%). The lower limit of quantification was 0.31 mg/L. The assay was linear from 0.31 to 15.0 mg/L. Method comparison with validated high-performance liquid chromatography with ultraviolet light or liquid chromatography tandem mass spectrometry methods showed good agreement (coefficients of correlation 0.974-0.994, slopes 1.01-1.17, intercepts -0.17 to 0.06). There was no difference found between results from different transplant types (cardiac vs. renal) or comedications (cyclosporine vs. tacrolimus). The recovery of samples from a proficiency testing scheme was acceptable. The cross-reactivity of AcMPAG, an in vitro active metabolite of MPA, was examined by adding AcMPAG to a pool of patient samples and subsequent quantification. MPA overestimation by AcMPAG cross-reactivity was found to be low (< 5%). Thus, this interference is expected to be clinically irrelevant. In conclusion, the Roche Total MPA assay is a promising alternative for MPA quantification where chromatographic methods are not available.