Update of lomefloxacin in vitro activity and spectrum. A multicenter trial testing contemporary pathogens following Food and Drug Administration validation guidelines. Lomefloxacin Activity Study Group

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 > or = 20 mm and resistant at < or = 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 > or = 22 mm and resistant at < or = 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-microgram lomefloxacin disks.

Reevaluations of disk diffusion susceptibility testing interpretive criteria for lomefloxacin and norfloxacin using fluoroquinolone-resistant isolates

The current National Committee for Clinical Laboratory Standards interpretive criteria for disk diffusion susceptibility testing of lomefloxacin and norfloxacin were reevaluated using a test panel of 298 bacteria (200 with ciprofloxacin minimum inhibitory concentrations (MICs) at > or = 4 micrograms/ml). MICs were correlated with the diameter of zones of inhibition by regression statistics and error-rate bounding methods. Modifications of the interpretive criteria for lomefloxacin disk susceptibility testing are proposed as follows: susceptible at > or = 20 mm and resistant at < or = 16 mm, a 2-mm decrease of existing break-point zones. These criteria result in an absolute interpretive agreement of 95.3% with a very major (false susceptible) error rate of only 0.7%. The currently used criteria for norfloxacin (susceptible at > or = 16 mm and resistant at < or = 13 mm) were validated, and these break-points had an absolute interpretive correlation between methods of 91.9%. The change proposed for lomefloxacin disk interpretations would minimize minor and major errors most often reported for Staphylococcus saprophyticus isolates.