Macrolide susceptibility and beta-lactamase production among Haemophilus influenzae isolates in the United States, 1996-1997

In 1996 and 1997, 68 hospital laboratories throughout the United States det ermined the beta-lactamase production and susceptibility to macrolides of 1 ,998 isolates of Haemophilus influenzae obtained from patients with communi ty-acquired respiratory tract infections. The MICs at which 90% of the isol ates are inhibited of azithromycin, erythromycin, and clarithromycin were 4 , 8, and 16 mu g/ml, respectively. By National Committee for Clinical Labor atory Standards interpretive criteria, 99 and 78% of the isolates were susc eptible to azithromycin and clarithromycin, respectively. The prevalence of beta-lactamase production was 32%.