In the present study, the average dosage of MTX used for the enrolled patients was lower than that commonly used in Western countries. Ten patients were newly diagnosed with MAC-PD. Eight individuals who already had diagnoses of MAC-PD at the time of enrollment and nineteen who had negative expectorated sputum cultures for MAC and positive CT images compatible with MAC-PD and who refused bronchoscopy were excluded from the following analysis. Anti-GPL antibodies were detected in 12 of 369 patients. Eight of the ten patients with MAC-PD and 4 of 359 patients without MAC-PD tested positive for the anti-GPL antibodies. The specificity and sensitivity were 99 % and CC0651 80 %, respectively. Positive and negative predictive values were 67 %, and 97 %, respectively. When we analyzed diagnostic performance of the antibodies in 57 patients with RA who had abnormal shadows on chest x-rays, the positive and negative predictive values were 100 %, and 96 %, respectively. Twelve patients underwent bronchoscopy. Bronchoalveolar lavage fluid (BALF) samples from six patients were positive for MAC, and BALF samples from the remainder were negative. Anti-GPL antibodies were detected in the sera of all six patients with positive results for MAC by BALF sampling, whereas the antibodies were not detected in the sera from the remainder with negative results for MAC CC0651 by BALF CC0651 sampling. == Conclusions == The measurement of anti-GPL antibodies is useful as a supplementary diagnostic tool for MAC-PD in patients with RA and may provide a new DLL1 strategy, in combination with chest x-ray and CT, for differentiating MAC-PD from other pulmonary comorbidities in patients with RA. == Electronic supplementary material == The online version of this article (doi:10.1186/s13075-015-0787-y) contains supplementary material, which is available to authorized users. == Introduction == Although the CC0651 emergence of biologic disease-modifying antirheumatic drugs (DMARDs) has markedly changed the course of rheumatoid arthritis (RA) and outcomes for patients, concerns have been raised regarding the higher risk of infection. Researchers in recent studies have reported an increase in the prevalence of diseases caused by nontuberculous mycobacteria (NTM) [14]. Eighty percent of patients with NTM diseases in Japan have been infected withMycobacterium aviumcomplex (MAC) [5]. MAC CC0651 is now widely recognized as an important pathogen that causes chronic and progressive pulmonary diseases in both immunocompetent and immunosuppressed patients. The diagnosis of MAC-PD is complicated because, in contrast toMycobacterium tuberculosis, the contamination of clinical specimens by MAC can come from environmental sources such as water, dust, and soil, and this organism may colonize the respiratory tract without any accompanying invasive disease [6]. A diagnosis of MAC-PD requires clinical findings and its repeated isolation from sputum. Difficulties have also been associated with discriminating MAC-PD from rheumatoid lung disease in a clinical setting, because RA could develop pulmonary manifestations, including rheumatoid nodules, cryptogenic organizing pneumonia, bronchiolitis obliterans, and bronchiectasis [7,8]. The most frequent finding by computed tomography (CT) in patients with RA in one study was bronchiectasis [8]. Although only 13 % of patients with RA clinically exhibit bronchiectasis, as many as 30 %30 % of patients with RA manifest bronchiectasis on high-resolution computed tomography (HRCT) [9]. Bronchiectasis is considered to be both a risk factor for and a consequence of NTM infections [9]. Kitada et al.[6,10] recently established an enzyme immunoassay (EIA) for the serological diagnosis of MAC-PD by examining serum immunoglobulin A (IgA) antibody levels against the glycopeptidolipid (GPL) core antigen, which is a MAC-specific antigen. Unlike bronchoscopy and sputum tradition examinations, this test is definitely less invasive and provides more rapid diagnostic info on MAC-PD. In the present study, we focused on MAC-PD in individuals with RA and carried out a cross-sectional observational study to investigate the medical usefulness of measuring anti-GPL antibodies with this patient population. == Methods == == Individuals == A cross-sectional observational study was conducted. The study sample consisted of 396 individuals who have been treated for RA between May and October 2013 in the Hirose Medical center of Rheumatology, which is an outpatient medical center located in Saitama prefecture in Japan. The inclusion criteria for individuals were fulfillment of the 2010 American College of Rheumatology/Western Little league against Rheumatism classification criteria for RA [11]. Individuals younger than 20 years of age and those.