Multi-center evaluation of analytical performance of the microparticle enzyme immunoassay for sirolimus

OBJECTIVES: This study evaluated the analytical characteristics of the new Abbott microparticle enzyme immunoassay (MEIA) for sirolimus. DESIGN AND METHODS: The protocol consisted of nine sections: evaluation of antibody specificity, linearity, detection limit, quantification limit, endogenous interferents, exogenous interferents, precision, proficiency testing panel, and method comparison. RESULTS: The mean analytical detection limit was 0.68 microg/L. The sirolimus concentration corresponding to a total CV of 20% was 1.5 microg/L. Linearity of response was demonstrated across the dynamic range of the assay. Total precision (CVs) at QC control levels from 5 to 22 microg/L ranged from 5.7 to 12.6%. Assay standardization was found to be in good agreement with LC/MS/MS as compared with target values for spiked sirolimus proficiency samples from an international sirolimus proficiency testing program. Good correlations (R values) of the immunoassay were observed in comparisons to LC/MS/MS. R values tended to be lower in comparisons with LC/UV methods. Across both LC-based methods and all study sites, there was approximately 25% overall positive slope bias due to cross reactivity of the MEIA antibody to metabolites of sirolimus. The assay cross-reactivity to metabolites of sirolimus parent drug ranged from 6 to 63%. Assay interferences were minimal with the exception of hematocrit, which presented a negative relationship to measured sirolimus concentration. CONCLUSIONS: The MEIA demonstrated acceptable analytical characteristics for use for routine monitoring of sirolimus immunosuppressive therapy, and is a viable alternative to HPLC-based methods for sirolimus monitoring.     

Comparative evaluation of digoxin concentrations determined by three assay systems: TDx, IMx and OPUS

Digoxin concentrations measured by three automated immunoassay systems, i.e. OPUS, TDx and IMx assays, were compared in order to evaluate precision and accuracy performance, and data compatibility. Coefficients of variation for all methods in within-run and between-run precision were less than 10% at weighed-in concentrations of 0.545, 1.090 and 2.180 ng/ml. The accuracy relative to the three weighed-in concentrations ranged from 97% to 123% for all methods. One hundred and three plasma samples from 60 patients receiving digoxin were used to evaluate the data compatibility. Digoxin concentrations measured by the three immunoassay systems correlated well with one another. These results suggest that there are few problems when switching between digoxin assay methods, and that IMx and OPUS are more useful than TDx because they do not require sample pretreatment. The digoxin concentrations of the plasma samples from one patient receiving both digoxin and potassium canrenoate were investigated as a case report. The digoxin concentrations measured by TDx and IMx became higher than those measured by OPUS after starting the combination treatment. In another patient suffering from bilirubinaemia, the digoxin concentrations measured by TDx or IMx were higher than those measured by OPUS. These results suggest that OPUS has a higher specificity for measuring the plasma digoxin concentrations compared with TDx or IMx.     

Multi-site analytical evaluation of a chemiluminescent magnetic microparticle immunoassay (CMIA) for sirolimus on the Abbott ARCHITECT analyzer

Objective

This study evaluated a new chemiluminescent magnetic microparticle immunoassay (CMIA) for sirolimus on the ARCHITECT analyzer.

Design and methods

Patient and laboratory proficiency samples were tested at three European sites and one site in the United States.

Results

The CMIA total %CV's were all < 8% and the Limit of Quantification (LOQ) was < 1.52 ng/mL across the four sites. It cross-reacts to sirolimus metabolites F4 and F5 and showed no hematocrit interference over a range of 25% to 55%. Patient specimen correlations to three LC/MS/MS methods gave R ≥ 0.91 at three sites and mean biases of 14%, 25% and 39%. CMIA patient specimen correlations to the Abbott IMx gave R ≥ 0.94 at 2 sites and mean biases of 5.4% and 6.9%.

Conclusions

CMIA is a precise and sensitive immunoassay method without hematocrit interference. It correlates well to both LC/MS/MS and immunoassay results, but shows an expected positive bias to LC/MS/MS.