Macrolide Susceptibility and β-Lactamase Production among Haemophilus influenzae Isolates in the United States, 1996–1997

In 1996 and 1997, 68 hospital laboratories throughout the United States determined the β-lactamase production and susceptibility to macrolides of 1,998 isolates of Haemophilus influenzae obtained from patients with community-acquired respiratory tract infections. The MICs at which 90% of the isolates are inhibited of azithromycin, erythromycin, and clarithromycin were 4, 8, and 16 μg/ml, respectively. By National Committee for Clinical Laboratory Standards interpretive criteria, 99 and 78% of the isolates were susceptible to azithromycin and clarithromycin, respectively. The prevalence of β-lactamase production was 32%.