Multi-center validation of proposed disk diffusion susceptibility testing interpretive criteria for lomefloxacin using more than 1,500 clinical isolates

The accuracy of disk diffusion susceptibility testing of lomefloxacin was evaluated using 1,555 recent clinical isolates from ten medical centers. The isolates were rapidly growing nonfastidious aerobic species (1,501 isolates) and Haemophilus species (54 isolates), each found as an indicated species in the product package insert. Applying the recently proposed modification of disk diffusion interpretive criteria (susceptible at ≤20 mm and resistant at ≤16 mm), absolute categorical agreement for nonfastidious aerobes (1,501 strains) was 95.5% with 0.5% very major and 0.1% major errors (error rates calculated using all tested strains as the denominator). The intermethod discord (MIC vs disk diffusion) was 0.5%. This contrasts to the current NCCLS recommended criteria (susceptible at ≤22 mm and resistant at ≤18 mm) where the absolute categorical agreement was significantly less (89.6%) with 0.2% very major, and 0.3% major errors, and the intermethod discord was 7.1%. For Haemophilus species (54 strains), intermethod agreement was complete using either the current NCCLS interpretive criteria or the modified criteria. These multicenter (ten laboratories) data support the acceptance of the proposed modification of disk diffusion interpretive criteria for 10-μg lomefloxacin disks.