A 1994-95 survey of Haemophilus influenzae susceptibility to ten orally administered agents. A 187 clinical laboratory center sample in the United States.

During August, 1994 to April, 1995, a total of 2278 clinical isolates of Haemophilus influenzae were obtained from 187 clinical laboratories in the United States (U.S.). The vast majority of these isolates (75%) were from respiratory sites, and the remaining organisms were from blood, ear, eye, and spinal fluid sources. The overall rate of beta-lactamase production and ampicillin resistance was 36%. The antimicrobial susceptibility of isolates was determined by reference broth microdilution testing against ten orally administered agents. MIC values were compared according to 12 geographical regions, inpatient or outpatient status, gender, and eight age groupings. Modest and occasionally significant differences were observed: 1) greater numbers of beta-lactamase-producing strains among outpatients, in males, in the mid-Atlantic region, and in children < or = 12 years of age; 2) lower prevalence of beta-lactamase-producing isolates in the Southeast and Pacific regions; 3) cefaclor, cefprozil, and loracarbef activity was lowest among the younger children (< or = six years); and 4) macrolide in vitro efficacy was lowest in patients > 50 years of age and in three eastern regions. Overall, more than 99% of the strains were susceptible to amoxicillin/clavulanic acid, cefixime, and cefpodoxime (e.g., widest potential clinical use). Susceptibilities using National Committee for Clinical Laboratory Standards (NCCLS) breakpoint criteria for the other agents were: 96.6% to cefuroxime, 86.5% to loracarbef, 84.0% to clarithromycin, 81.8% to cefaclor, and 80.7% to cefprozil. Non-beta-lactamase mechanisms of resistance to ampicillin were rare (0.2%) or episodic and were attributed to altered penicillin-binding proteins. Although there is an increased prevalence of beta-lactamase production among H. influenzae isolates compared to prior years, four beta-lactams remain highly active (> 95% susceptibility) against contemporary strains of H. influenzae. Other monitored compounds seem to have declined in spectrum and surveillance trials for resistance among H. influenzae isolates should continue in an effort to identify trends in the U.S.