Provisional interpretive criteria for quinupristin/dalfopristin susceptibility tests

For testing bacterial susceptibility to the streptogramin quinupristin/dalfopristin, it seems reasonable that the test reagent should contain a similar ratio of the two compounds as in the therapeutic material, i.e. 30% quinupristin and 70% dalfopristin. The precise ratio of quinupristin to dalfopristin is not critical as long as threshold concentrations of both drugs are present. In-vitro tests were not altered by minor changes in pH, incubation in CO2 or by addition of lysed horse blood to the medium. Filter paper discs containing 15 mu g of the quinupristin/dalfopristin reagent are preferred for agar diffusion susceptibility testing in order to optimize disc tests with enterococci. Provisional interpretive criteria for pathogens not requiring blood or increased COP are: susceptible, inhibition zone diameters of greater than or equal to 19 mm or MIC less than or equal to 1.0 mgiL; intermediate, 16-18 mm or MIC 2.0 mg/L; resistant, less than or equal to 15 mm or MIC greater than or equal to 4.0 mg/L. Those criteria will need to be reassessed as clinical experiences become available. Of particular concern is the role of quinupristin/dalfopristin in the treatment of infections due to Haemophilus influenzae or Enterococcus faecalis: the clinically relevant interpretive category for each of those two species is yet to be determined.