An ophthalmologist's guide to understanding antibiotic susceptibility and minimum inhibitory concentration data.

PURPOSE

To address the need to establish appropriate evaluation criteria for analyzing in vitro antibiotic susceptibility based on original data.

DESIGN

In vitro laboratory investigation.

PARTICIPANTS

Bacterial isolates from patients with conjunctivitis.

MAIN OUTCOME MEASURES

Minimum inhibitory concentrations (MICs), descriptive statistics, antibiotic susceptibility, potency, and statistical analysis.

METHODS

Minimum inhibitory concentrations were determined for 80 bacterial conjunctivitis isolates to moxifloxacin, gatifloxacin, levofloxacin, ciprofloxacin, and ofloxacin. Using the MIC values, descriptive statistics (median, MIC50, MIC90, mode, range), antibiotic susceptibility, and potency of each antibiotic were calculated for each bacterial group. The data were analyzed statistically using appropriate randomization and nonparametric tests.

RESULTS

The descriptive statistics of gram-positive bacteria (Staphylococcus aureus, Streptococcus pneumoniae) followed a consistent trend where the median, MIC50, MIC90, and mode demonstrated the lowest values, in all instances, for moxifloxacin, gatifloxacin, levofloxacin, ciprofloxacin, and ofloxacin. The descriptive statistics for Haemophilus species (the predominant gram-negative bacteria implicated in conjunctivitis) did not describe any consistent trend. In contrast, antibiotic susceptibility testing did not demonstrate any advantage among the 5 fluorquinolones tested, except for moxifloxacin in the S. aureus fluoroquinolone-resistant group. Potency studies indicated that moxifloxacin and gatifloxacin were the most potent for gram-positive bacteria, whereas gatifloxacin and ciprofloxacin were the most potent for Haemophilus species.

CONCLUSION

In the absence of human clinical trial data to guide care, in vitro susceptibility data should be analyzed with a set of descriptive statistics along with a nonparametric statistical analysis. No single parameter or test should be relied upon in all instances to demonstrate the in vitro superiority of one antibiotic over another. In this study, fourth-generation fluoroquinolones did have some potency advantages over second-generation fluoroquinolones against gram-positive conjunctival bacterial isolates, but not for Haemophilus isolates.