Different sample source indicated different colonization or infection sites, which might be significant in the diagnosis of infectious disease such as NTM infection, since it exists in living environment. Therefore, the patients with NTM isolates from different sample sources mainly including sputum and BALF were retrospectively analyzed in this study. Our results showed that the sample source was associated with different gender, age, acid fast assay, T. spot assay, COPD and bronchiectasis among NTM patients
Sputum and BALF are common samples collected for the diagnosis of respiratory infectious disease. Expectorated sputum is easier to collect, but it is more likely contaminated by environmental factor. The BALF is collected from the lower respiratory tract and free of contamination from mouth, but it is only available in patients who are subjected to bronchoscopy and it is forbidden in some patients with severe respiratory diseases. As suggested in the principles of diagnosis of pulmonary NTM tuberculous by ATS/IDSA, repeated positive culture from sputum sample or single positive culture from BALF is an indication of the presence of NTM [7]. Our results showed that the patients with sputum positive for NTM strain were older than those with BALF positive, suggesting that bronchoscopy might be more tolerated by younger inpatient as they were usually hospitalized because of suspected infection accompanied with coexisting mild diseases. Indeed, coexisting diseases influence the choice in the selection of the sample source. As suggested by our study, BALF positive samples were closely associated with patients with bronchiectasis and sputum positive samples were associated with patients with COPD. In clinical practice, when bronchiectasis is suspected, a bronchoscopy test is suggested, while patients with COPD are not subjected to bronchoscopy especially when they suffer from exacerbation [15].
The association between COPD and pulmonary NTM disease has been increasingly reported, but whether NTM infection increases the risk of COPD or COPD results in high risk of NTM infection has not been clearly clarified [2, 16–19]. Patients with COPD are more susceptible to NTM infection than those with other chronic pulmonary diseases [20, 21] (except cystic fibrosis), and inhaled corticosteroids increase the risk of NTM infection [19]. Chronic inflammation and airway remodeling may contribute to higher susceptibility to NTM infection. Among patients with NTM in this study, the patients with COPD were mainly older and males, as it was also previously reported [11]. Limited data are available regarding the influence of NTM infection on COPD, but our previous work and another study suggested that repeated isolation of samples in patients was associated with a higher percentage of COPD [10, 11]. In a follow up study, COPD patients with NTM-positive sample more likely experience exacerbation than those with NTM-negative samples, and patients with multiple positive samples had a more pronounced pulmonary function decline than those with single or no positive samples [22]. In addition, our results showed that male patients with COPD had an increased risk of prostatic hyperplasia than those without COPD, which could be explained by the fact that male patients with COPD are at higher risk of benign prostatic hyperplasia despite of the presence of NTM [23, 24]. The relationship between COPD and prostatic hyperplasia might be explained by the physical inactivity and chronic inflammation [25, 26].
Nodular-related bronchiectasis is of significance in the diagnosis and treatment of pulmonary NTM infection. Although bronchiectasis has been observed in 20%-30% of patients with NTM, NTM infection seems not common in patients with bronchiectasis [10, 27]. Indeed, 34–50% of patients with multiple small nodules (and sometimes cavity or cavities) combined with their diffusion (or widespread) indicate the presence of active pulmonary NTM infection [28]. In our previous study, bronchiectasis was observed mainly in female patients with NTM, result that is similar to another study from Singapore [11]. Among patients with NTM, the percentage of patients with tuberculosis history was higher in the ones with bronchiectasis than that in patients without bronchiectasis, meaning that pulmonary tuberculosis might result in anatomic changes of the respiratory tract and reinfection by other pathogens such NTM [29]. The percentage of tuberculosis history between patients with COPD or without COPD was not significantly different, suggesting that previous tuberculosis infection might not be associated with the pathogenesis of COPD. Previous works showed that the percentage of patients with bronchiectasis presenting multiple NTM positive isolations is comparable to the percentage of patients with bronchiectasis presenting a single NTM positive isolation [10, 11], indicating the limited impact of NTM infection on bronchiectasis. Instead, repeated isolations of NTM positive strains partly contributes to the progression of COPD [10, 11].
After excluding patients with COPD and bronchiectasis, sputum resource from patients with positive NTM was associated with age, acid fast assay and T. spot assay. Higher percentage of positive result in acid-fast assay could be explained by the fact that the repeated sample collection was more common when sputum was collected than when BALF was collected. The higher percentage of positive results of T. spot assay in patients with positive sputum than that in patients with positive BALF might represent a warning that this result could be more likely misdiagnosed as MTBC infection before species identification.