Whether two-drug therapy (clarithromycin and ethambutol) for Mycobacterium avium complex (MAC) pulmonary disease contributes to the development of macrolide-resistant MAC is unclear.

To compare the incidence of macrolide-resistant MAC between patients treated with two-drug therapy (clarithromycin and ethambutol) and the standard three-drug therapy (clarithromycin, ethambutol, and rifampicin) for MAC pulmonary disease.

We retrospectively reviewed 147 patients with treatment-naive MAC pulmonary disease who had received two-drug therapy (n = 47) or three-drug therapy (n = 100) between 1997 and 2016 at National Hospital Organization, Tenryu Hospital, Hamamatsu, Japan. The risk of development of macrolide-resistant MAC was evaluated by calculating the cumulative incidence rate using Gray's test.

The median follow-up period was 74.5 months. During the follow-up period, one of the 47 patients (2.1%) in the two-drug group developed macrolide-resistant MAC, compared to 12 of the 100 patients (12.0%) in the three-drug group. The cumulative incidence rate of macrolide-resistant MAC was lower in the two-drug group than in the three-drug group (0.0023; 95% confidence interval, 0.002 to 0.107 versus 0.200; 95% confidence interval, 0.100 to 0.324, p = 0.0593).

These results suggest that two-drug treatment with clarithromycin and ethambutol for MAC pulmonary disease does not lead to a higher incidence of resistance acquisition to clarithromycin than the standard three-drug treatment.