Comparisons of the in vitro susceptibility testing results for garenoxacin using six different national methods: report from the garenoxacin international bridging study.

BACKGROUND

Laboratories worldwide carry out MIC/disc diffusion (DD) tests using various national methods, often published by local organizations. Potentially different results may complicate drug development or registration between the US methods and those used in the European Union (EU). This study uses a new des-F(6)quinolone, garenoxacin, to compare in vitro results among the most utilized antimicrobial susceptibility testing methods.

METHODS

Investigators in France, Germany, Spain, Sweden and the UK tested two bacterial collections designated, challenge (CC; n = 330) containing quinolone-resistant strains and national (NC; n = 540) that were recent clinical isolates (2000-2001). Results were compared with values derived from the National Committee for Clinical Laboratory Standards (NCCLS) methods, carried out by the US coordinating site. Discords (greater than four-fold) between MIC method results were repeated. Results were analysed for variation from the NCCLS results (+/-1-2 dilutions or greater than 3-6 mm) and by regression statistics. Ciprofloxacin was used as the control quinolone agent.

RESULTS

CC and NC testing compliance averaged 98.4% and 86.4% among participating centres, respectively, and correlation (r) of the national method to the NCCLS MIC was: France (0.98), Germany (0.95), Spain (0.98), Sweden (0.96) and UK (0.95). CC produced MIC results that were identical to the NCCLS (Spain) to 0.6 x log(2) greater (Germany), but the percentage of strains +/-2 dilutions versus NCCLS MIC results was 98% (Germany)-100% (Sweden, UK). Similar patterns were observed for the NC (r = 0.93-0.98), and all nations had >99% of results within +/-2 dilutions (+/-6 mm for disc tests) of NCCLS values. MIC results from four national methods were slightly elevated compared to the NCCLS result (average 0.2 x log(2)). Control ciprofloxacin MIC and DD test results showed similar patterns.

CONCLUSIONS

Inter-method garenoxacin susceptibility test results indicate that MIC and zone diameter endpoints derived from five EU methods compare favourably to the NCCLS method results, and generally were identical or a fraction of a log(2) dilution step higher. In contrast, zone diameters for garenoxacin and ciprofloxacin were routinely larger for the EU methods. This unique inter-method 'bridging' experiment allows regulatory agencies to better correlate in vitro testing results derived from procedures that use different national methodologies.