Clinical performance of the EMIT vancomycin assay.

In evaluating the EMIT system for vancomycin used in the Cobas Bio centrifugal analyzer, we found two potential problems, each of which may have important clinical ramifications. First, precision, though acceptable in concentrations up to 30 mg/L, was marginal in the range above 30 mg/L. Second, when EMIT values were compared with those by fluorescent polarization immunoassay (TDx), we found a good correlation but a significant proportional bias: [EMIT] = (0.877)[TDx] + 0.435 mg/L (r = 0.971). The proportional bias was much more pronounced in specimens with high creatinine values than in specimens with normal values for creatinine. It remains to be determined which method is more nearly accurate. This imprecision and proportional bias in specimens with increased creatinines lead us to conclude that the EMIT vancomycin system should be used with caution.