Efficacy of three sputum specimens for the diagnosis of Mycobacterium avium complex pulmonary disease

Background In Mycobacterium avium complex pulmonary disease (MAC-PD), diagnosis requires a positive culture from at least two separate expectorated sputum specimens. The optimal number of sputum examinations remains unclear. Objective This study sought to elucidate the diagnostic yield of acid-fast bacilli in MAC-PD using 3 sputum specimens and to clarify the clinical characteristics of patients with MAC-PD diagnosed using 3 sputum specimens. Furthermore, we investigated the correlation between increased number of sputum specimens and diagnostic yield. Methods We reviewed the medical records of 139 patients with MAC-PD diagnosed at Toho University Omori Medical Center for whom at least three sputum specimens were examined before treatment from November 2014 through June 2021. Patients were classified into the 3-sputum diagnosed and the non-3 sputum diagnosed groups based on diagnostic procedure; clinical and radiological characteristics were compared. We also assessed diagnostic yield with the increased number of sputum specimens. Results Diagnostic yield with 3 sputum specimens was 16.5% (23/139). The 3-sputum diagnosed group had a lower body mass index [18.6(17–19.5) vs. 19.5(18–21.5); p = 0.014], and higher chest CT score [9(6.5–13) vs. 6(4–9); p = 0.011] including cavitary lesions (39.1% vs. 19%; p = 0.037) compared with the non-3 sputum diagnosed group. When the number of sputum specimens was increased to 6, the diagnostic yield increased to 23.7% (33/139). Conclusion Diagnostic yield with 3 sputum specimens was 16.5%. Patients diagnosed using 3 sputum specimens had more severe chest CT findings including cavitary lesions. Increasing the number of sputum specimens to 6 improved diagnostic yield by 7.2%.