Tentative criteria for determining the in vitro susceptibilities of Haemophilus influenzae, including quality control parameters, to two fluoroquinolones (grepafloxacin and PD 131628).

Grepafloxacin (OPC 17116) and PD 131628 were evaluated against 150 Haemophilus influenzae isolates to propose susceptibility testing criteria for both broth dilution and disk diffusion procedures using haemophilus test medium. Grepafloxacin-susceptible isolates are defined as those for which minimum inhibitory concentrations (MICs) are < or = 0.06 micrograms/ml and zones of inhibition are > or = 27 mm. PD 131628-susceptible strains of H. influenzae included those that exhibited MICs < or = 0.03 micrograms/ml, and the zones were > or = 32 mm. All MICs were well below concentrations that can be achieved in the blood and tissues of patients treated with either study drug. Criteria for defining a resistant category cannot be determined until strains with elevated MICs are available for study. The proposed quality control guidelines for H. influenzae ATCC 49247 and the disk test are 32-39 mm in diameter for grepafloxacin and 34-42 mm for PD 131628. The preliminary broth microdilution MIC control limits for this strain are 0.002-0.016 micrograms/ml for grepafloxacin and 0.002-0.008 micrograms/ml for PD 131628.