Can antimicrobial susceptibility testing results for ciprofloxacin or levofloxacin predict susceptibility to a newer fluoroquinolone, gatifloxacin?: Report from the SENTRY Antimicrobial Surveillance Program (1997-99)

A serious problem confronting clinical laboratories and hospital formulary practices is the delayed availability of approved, commercially prepared su sceptibility test reagents for newer antimicrobials. A current example is g atifloxacin, a new 8-methoxy fluoroquinolone with expanded potency against many Gram-positive pathogens. This study addresses the use of "surrogate ma rker" fluoroquinolones to predict susceptibility for gatifloxacin. Referenc e broth microdilution MIC results for 29,632 strains isolated in United Sta tes medical centers (SENTRY Antimicrobial Surveillance Program, 1997-99) we re used: staphylococci (9,940 strains), enterococci (2,570), Streprotoccus pneumoniae (3,784), Enterobacteriaceae (10,670) and Pseudomonas aeruginosa (2,668). Gatifloxacin interpretation categories were compared to those of c iprofloxacin and levofloxacin by regression statistics;Ind error rate bound ing analyses. For the Enterobacteriaceae, the absolute categorical agreemen t was 97.9 to 98.7% (false-susceptible or very-major error [VME], 0.03%-0.1 %) for comparisons of both ciprofloxacin and levofloxacin with gatifloxacin . P. aeruginosa testing was more problematic (higher minor error rates), bu t acceptable at 0.6% to 1.1% VME and a 85.7% to 89.9% overall agreement. Ci profloxacin results used to predict gatifloxacin ire Gram-positive species was almost without VME (0.0%-0.2%) because gatifloxacin was significantly s uperior against these species, especially for S. pneumoniae, where gatiflox acin (MIC90, 0.5 mug/ml) was fourfold more potent than levofloxacin (MIC90, 2 mug/ml). The preferred gatifloxacin predictor drug was ciprofloxacin for all species except S. pneumoniae and P. aeruginosa, where levofloxacin res ults had a slightly greater predictive value. Susceptibility testing result s for selected currently available fluoroquinolones can be used to predict susceptibility to gatifloxacin with high confidence. Many Gram-positive coc ci, however, will be categorized as false-resistant by this interim method since gatifloxacin has a 11% to 34% wider spectrum of activity compared to ciprofloxacin when testing staphylococci and enterococci. Clinical laborato ries can reliably use these suggested "surrogate markers" until reliable te sts for gatifloxacin become available. (C) 2001 Elsevier Science Inc. All r ights reserved.