Provisional quality control parameters and interpretive criteria for testing susceptibility ofStreptococcus pneumoniae andHaemophilus influenzae to quinupristin/dalfopristin (RP59500)

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Abstract
Studies were undertaken to select tentative criteria for susceptibility testing of quinupristin/dalfopristin againstStreptococcus pneumoniae andHaemophilus influenzae. Against 612 isolates ofStreptococcus pneumoniae, MICs of quinupristin/dalfopristin were ≤1.0 μg/ml for all but one strain. With a tentative MIC breakpoint of either ≤ 1.0 μg/ml or ≤2.0 μg/ml for susceptible, a disk diffusion zone diameter breakpoint of ≥19 mm embraced all but two of the susceptible pneumococci; ≥16 mm included all strains. ForHaemophilus influenzae, MICs of quinupristin/dalfopristin clustered near the tentative breakpoints; 91.5% of the MICs were 2.0 to 8.0 μg/ml. This precluded satisfactory performance of the disk diffusion test in discriminating between resistant and susceptible isolates unless MIC breakpoints are modified for this species: clinical experience will be needed before that can be justified. Based on data from a multilaboratory study, the following quality control limits are proposed forStreptococcus pneumoniae ATCC 49619 when testing quinupristin/dalfopristin: 0.25 to 1.0 μg/ml for broth microdilution tests and 19 to 24 mm for disk diffusion tests. For tests ofHaemophilus influenzae ATCC 29247, MIC limits are 2.0 to 16 μg/ml; disk tests were very reproducible but are not yet recommended.