Diagnostic value of nonpurulent sputum in nontuberculous mycobacterial lung disease: A cross-sectional study based on the Miller and Jones classification.

BACKGROUND

Sputum quality is widely considered a determinant of microbiological diagnostic yield in respiratory infections. However, its significance in the context of nontuberculous mycobacterial (NTM) lung disease remains unclear. This study aimed to investigate the association between sputum quality, as assessed using the Miller and Jones classification, and the detection rates of NTM via smear microscopy, polymerase chain reaction (PCR), and culture.

METHODS

We conducted a cross-sectional study of 54 patients diagnosed with NTM lung disease at a single institution in Japan between September 2021 and September 2022. A total of 158 sputum samples were visually classified into five categories (M1-P3). Diagnostic yield was assessed using auramine-O fluorescence staining, Mycobacterium avium complex-polymerase chain reaction, and mycobacterial culture. Bacterial co-detection and treatment status were also analyzed.

RESULTS

Nonpurulent sputum samples (M1 and M2) comprised over 60 % of all specimens and demonstrated clinically relevant positivity rates: 33 % for M1 and over 50 % for M2 by culture. No significant association was found between sputum quality and smear, PCR, or culture positivity. Purulent sputum (P2-P3) was significantly associated with bacterial co-detection (p = 0.0008), particularly Pseudomonas aeruginosa.

CONCLUSIONS

Sputum purulence was not associated with diagnostic yield for NTM. Nonpurulent samples should not be excluded from diagnostic evaluation. These findings support current guideline recommendations and highlight the importance of repeated sputum collection and adjunctive diagnostic tools in NTM lung disease.