This case–control study investigated comorbidities associated with NTM infection according to sex and age using national claims data in Korea. Consistent with previous studies, respiratory diseases (bronchiectasis, COPD, interstitial pneumonia, asthma, and pan-bronchiolitis) and non-respiratory conditions (GERD and malignancies) were founded to be associated with NTM infection in this study [1, 2, 6]. A cohort study matched by sex, age, and insurance benefit coverage in the United States (U.S.) also reported that several respiratory diseases (such as asthma, bronchiectasis, and COPD) and systemic diseases such as mental disorders, metabolic diseases (diabetes mellitus and hyperlipidemia), musculoskeletal disorders, cardiovascular diseases (such as coronary artery diseases, myocardial infarction, and arrhythmia) and neoplasm were associated with NTM infection, similar to our findings [21]. In the unstratified analysis in our study, we identified significant associations between diffuse pan-bronchiolitis, interstitial pneumonia, osteoporosis, and chronic kidney disease and NTM infection, consistent with the results of a cross-sectional study in Japan [6].
Previous studies have reported that structural lung disease including bronchiectasis, COPD, and interstitial lung disease are risk factors for NTM infection [3, 22]. In structural lung disease, local damage in the lungs can result in non-clearing infections, leading to an excessive inflammatory response with further lung damage, a decline in lung function, and more severe infection [23]. In addition, patients with asthma and COPD treated with inhaled corticosteroids and patients with interstitial lung disease treated with systemic immunosuppressants were more likely to have NTM infection than those not treated with corticosteroids and systemic immunosuppressants [1,2,3, 24, 25]. These studies suggest that the use of corticosteroids and systemic immunosuppressants is associated with NTM infection.
No significant differences were observed in comorbidities between men and women in our study. Traditionally, the phenotype of NTM infection is considered to differ between men and women. The typical phenotype of right middle lobe and lingular segment involvement in NTM pulmonary disease is termed ‘Lady Windermere syndrome’ and is considered a phenotype that occurs frequently in post-menopausal, slender women [26, 27]. In contrast, the fibro-cavitary form of NTM pulmonary disease is considered a phenotype in men with COPD [28]. However, previous reports suggest a similar NTM phenotype between men and women [29, 30]. Although we could not confirm the phenotype in our study, we did not identify a significant difference in the distribution of comorbidities related to NTM infection between men and women.
Compared to a previous Japanese study [6], our study revealed several distinct findings regarding comorbidities in the sex-stratified analysis. Ischemic heart disease was associated with NTM infection in women in our study but not in the previous study in Japan. Conversely, rheumatoid arthritis was not associated with NTM infection in women in our study but was associated with NTM infection in the Japanese study. The study conducted in Japan only included individual aged < 75 years and dataset used was not the national representative dataset, but they had stricter definition of NTM than our study as those who were claimed 3 or more times as an A31 disease code, and the number of NTM patients as subjects was relatively small (men = 134, women = 285) compared to our study. The discrepancies in results between these studies could be partly underpinned by differences in race, lifestyle, study design, sample size, characteristics of claims data, and the prevalence of each comorbidity.
Among the 20–39 years age group, the prevalence of systemic diseases such as hypertension, diabetes mellitus, and malignancy, as well as respiratory diseases, which are traditional risk factors for NTM infection, was higher than that in the control group. Hypertension was associated with NTM infection in the U.S. study but not in the Japanese study [6, 31]. In our study, hypertension and NTM infection were associated with NTM infection only in the 20–39 years age group. Although we did not conclusively determine the reason for the association between hypertension and NTM infection only in younger age groups, a possibility is that NTM infection is associated with diastolic blood pressure (DBP) rather than systolic blood pressure (SBP). Indeed, DBP increases in most hypertensive patients before 50 years of age, after which SBP continues to rise and DBP tends to decrease due to a decrease in elasticity of blood vessel walls and blood volume with aging. This condition is referred to as isolated systolic hypertension [32, 33]. A relationship between hypertension and NTM infection may not be observed in individuals over the age of 50 years, the timepoint at which isolated systolic hypertension begins to increase [32, 33].
Consistent with our findings, depression was associated with NTM infection in the U.S. study, but this association was not observed in the Japanese study [1, 6]. Anxiety, depression, and poor sleep quality are frequently observed in patients with NTM infection [34,35,36]. Our study suggests that mental disorders such as depression may increase the risk of NTM infection.
Several limitations of the present study should be acknowledged. First, the diagnosis of NTM infection and comorbidities in this study was based on insurance claims, which rely on ICD-10 codes. The use of administrative claims data may have resulted in issues such as coding accuracy (over-coding, under-coding, or miscoding), diagnostic inertia, and lack of disease specificity. Thus, we defined the disease based on at least two claims, similar to most previous studies analyzing claims data [1, 2, 31, 37, 38]. Second, it was impossible to classify and analyze cases of infectious diseases such as tuberculosis, mental/behavioral disorders, or cancer in the NHIS–NSC 2.0 database because the detailed disease code was partially disclosed to protect personal information. In addition, human immunodeficiency virus and cystic fibrosis are known risk factors for NTM infection [23], but these diseases are classified as personal sensitive information in the NHIS–NSC 2.0 database and are therefore not disclosed. As such, we were unable to confirm their relevance to NTM infection. Third, the mycobacterium species could not be distinguished because of the unavailability of microbial information. Fourth, possible residual confounding factors, including dietary, personal lifestyle, clinical, or other environmental factors may affect the association between NTM infection and comorbidities. Finally, the study population was homogeneously Korean, which may limit the generalizability of the findings to other ethnic populations.
Despite these limitations, our study has several strengths. First, this study used the NHIS–NSC 2.0 database of South Korea, which is representative of the entire population and contains healthcare utilization information from all settings in South Korea. Second, to our knowledge, this is the first study in Korea to examine the comorbidities associated with NTM infection according to sex and age. Region of residence and income level were also considered to be factors that would affect the relationship between NTM infection and comorbidities; hence, matching and logistic regression analysis were conducted taking this into account. Third, we investigated the ranking of comorbidities with high frequency in patients with NTM infection using preliminary analysis of claims data and added chronic sinusitis, acute sinusitis, and skin diseases to the comorbidities investigated in previous studies.