Study Population and Design
We retrospectively reviewed all patients seen in our hospital in Tokyo, Japan, from April 2014 to June 2019. There were 481 patients who underwent serologic testing for anti-GPL-core IgA antibodies. Of them, 119 newly diagnosed NTM-PD patients fulfilled the criteria of the ATS and the IDSA statement on the diagnosis, treatment, and prevention of nontuberculous mycobacterial diseases [8, 26]. Those with a past history of NTM disease, NTM other than MAC, and no abnormal shadow on chest CT were excluded, and the remaining 89 MAC-PD patients were enrolled in our study (Fig. 1). We divided the study patients into two groups according to anti-GPL-core IgA antibody results: an antibody-positive group (n = 59) and an antibody-negative group (n = 30, antibody titer <0.7 U/mL). Then, the antibody-positive group was further divided into a strong positive group (n = 27, antibody titer ≥5 U/mL) and a weak positive group (n = 32, 5.0< antibody titer ≥0.7 U/mL). The antibody titers and the clinical characteristics of the patients, including age, sex, laboratory data, a past history of tuberculosis, comorbidities, and radiological findings, were retrospectively compared. Additionally, we investigated whether treatment was required for up to 1 year after diagnosis to determine if anti-GPL-core IgA antibody titers were associated with disease progression.
Ethics Approval and Consent to Participate
All methods were carried out in accordance with the declaration of Helsinki. The need of informed consent was waived by the Ethics Committee of Toho University Ohashi Medical Center due to the retrospective nature of the study. All protocols were approved by the Ethics Committee of Toho University Ohashi Medical Center (approval no. H20004).
Serological Determination of Anti-GPL-core IgA Antibody Titers
The titer of serum anti-GPL-core IgA antibodies was determined using a Capilia MAC Ab ELISA serodiagnostic kit (TAUNS Laboratories, Inc., Shizuoka, Japan). The cutoff value was defined as 0.7 U/mL, according to the manufacturer’s instructions . The test can be performed using a small amount of serum, and the turnaround time is approximately 4 hours .
AFB were cultured in mycobacteria growth indicator tubes using sputum samples or bronchial washings obtained by bronchoscopy. The sputum samples were obtained on two or more occasions after the initial presentation. The diagnosis of MAC-PD was made when MAC was identified in sputum at least twice or once in bronchial washings. MAC was confirmed when cultures were positive for AFB, and the cultured AFB were subsequently confirmed as MAC by polymerase chain reaction.
Radiological Examination and Measurements
High-resolution chest CT findings were classified as either showing or not showing a cavitary lesion. Additionally, chest CT findings at the time of initial diagnosis were scored, as previously described . Briefly, the lung fields were divided into six lobes based on anatomical structures: right upper, right middle, right lower, left upper (S1+2 and S3), left lingular (S4 and S5), and left lower lobes. We assessed whether, at the time of diagnosis, each lung lobe had a shadow, such as cavities, bronchiectasis, small nodules, consolidations, or atelectasis.
The patients’ characteristics are presented as medians (interquartile range). Numerical data are expressed as numbers (%). Intergroup differences (anti-GPL-core IgA antibody strong positive group vs. weak positive group vs. negative group) were compared using the Kruskal–Wallis test for numerical variables and the chi-square test or Fisher exact test for categorical variables where appropriate. All analyses were performed using SPSS Statistical software v22.0 (IBM Japan, Tokyo, Japan). P-values < 0.05 were considered significant.