Use of frequency distribution curves, scattergrams and regression analyses to compare in vitro activities and describe cross-susceptibility and cross-resistance among four quinolones.

Minimum inhibitory concentrations (MICs) of ciprofloxacin, ofloxacin, lomefloxacin, and fleroxacin for 852 recent clinical isolates were determined by a standardized broth microdilution method. Frequency distribution curves, scattergrams and regression analyses were used to compare in vitro activities and describe cross-susceptibility and cross-resistance. All four quinolones were active against most species tested except Xanthamonas maltophilia, enterococci, methicillin-resistant staphylococci and Bacteroides species. Relative in vitro activities were ciprofloxacin > ofloxacin > lomefloxacin = fleroxacin. Some of the in vitro advantages of ciprofloxacin, particularly when compared to ofloxacin, were negated when percents susceptible, based on pharmacokinetically derived breakpoints, were considered. Despite differences in in vitro activities, organisms relatively susceptible to one quinolone were relatively susceptible to all quinolones and organisms relatively resistant to one were relatively resistant to all; using regression analysis, coefficients of determination (R2) were 0.91-0.96 for all drug pairs. There was, therefore, a high degree of cross-susceptibility and cross-resistance. Differential categorizations of susceptibility to the quinolones would be justified only if validated by comparative clinical trials.