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

Abstract 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 153 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 19% (29/153). The 3-sputum diagnosed group had a lower body mass index (18.0±2.3 vs. 19.7±3.0 kg/m2; p=0.002) and higher chest CT score (10.7±4.9 vs. 7.2±4.0; p=0.001) including cavitary lesions (51.7% vs 20.2%; p=0.002) compared with the non-3 sputum diagnosed group. When the number of sputum specimens was increased to 6, the diagnostic yield increased to 26.1% (40/153). Conclusion: Diagnostic yield with 3 sputum specimens was 19%. 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 7.1%.