We aimed to investigate the natural history of non-cavitary nodular bronchiectatic (NC-NB) Mycobacterium avium complex (MAC) lung disease (LD).

Among all patients diagnosed with NC-NB MAC LD between March 2000 and December 2013, 551 patients who were followed until December 2017 were enrolled at a tertiary referral center in South Korea. Patients were subdivided into progressive and stationary groups, depending on whether treatment was initiated within 3 years after diagnosis. We investigated the proportion of patients not receiving anti-MAC treatment within 3 years after the diagnosis. The rate for spontaneous sputum conversion in the untreated group was also calculated.

The mean age of 551 patients was 61.1 years. During the median 5.8 years of follow-up, 323 (58.6%) patients received treatment within 3 years (progressive group), whereas the remaining 228 (41.4%) patients did not (stationary group). Multivariate analysis revealed that age ≤ 60 years, a positive sputum smear, the presence of systemic symptoms, body mass index >18.5 kg/m², and the involvement of ≥ 4 lobes were predictors of receiving treatment. Additionally, during the median of 5.0 years follow-up, spontaneous sputum conversion occurred in 52.2% of patients who did not receive treatment.

Among patients diagnosed with NC-NB MAC LD, approximately 40% did not receive antibiotic treatment within 3 years of diagnosis. Moreover, about 50% of untreated patients experienced spontaneous sputum conversion.