Prevalence of resistance to three fluoroquinolones : Assessment of levofloxacin disk test error rates and surrogate predictors of levofloxacin susceptibility

More than 3,000 consecutive clinical bacterial isolates from 10 U.S. medical centers were subjected to standard broth microdilution and disk diffusion tests to determine their susceptibilities to levofloxacin, ofloxacin, D-ofloxacin, and ciprofloxacin. Levofloxacin was confirmed to be twice as active as ofloxacin and to have activity comparable to that of ciprofloxacin, with minor variations in activity against some species. The prevalence of resistant isolates was 7.1% to levofloxacin, 9.3% to ciprofloxacin, and 11.2% to ofloxacin. The susceptibilities of some species to the quinolones were less than those reported in previous studies. Pseudomonas aeruginosa isolates had the greatest variability in their susceptibilities to the three drugs between the participating centers. Two proposed zone size breakpoints for levofloxacin disk tests yielded similar low error rates. Ofloxacin and ciprofloxacin susceptibility test results correlated reasonably well with those of levofloxacin and could be used as surrogate indicators of levofloxacin susceptibility, but that resulted in some serious errors, and thus, direct testing of levofloxacin susceptibility is preferable. Replicate testing of standard quality control strains confirmed the established and proposed quality control parameters for all three quinolones tested.