Rheumatoid arthritis and interstitial lung disease: the role of comorbidities. Retrospective analysis of two RA inception cohorts in the UK

Abstract Objectives To assess the role of comorbidities measurements in interstitial lung disease (ILD) in patients with rheumatoid arthritis (RA). Methods Data were from two consecutive multicentre, prospective RA inception cohorts, including baseline sociodemographic, clinical, treatment, laboratory and functional features. Comorbidity burden was assessed using major comorbidity count, rheumatic disease comorbidity index (RDCI) and Charlson Comorbidity index (CCI). Associations between comorbidities and development of ILD were explored, adjusted for clinical factors using multivariable logistic regression analyses. Results Data from 2,701 patients were included, median follow-up 6 years. In total, 101 (3.7%) were diagnosed with ILD. Twelve (0.44%) had the diagnosis at baseline, 46 were diagnosed during follow up and 43 identified from death certificates. In multivariable analyses, age at onset (aOR 1.03, 95% CI 1.01 to 1.05), seropositivity (aOR 2.58, 95% CI 1.38 to 4.81), ever smoking (aOR 1.70, 95% CI 1.04 to 2.79) were associated with ILD diagnosis. Higher lung comorbidity burden measured using lung disease component of RDCI (aOR 4.59, 95% CI 2.68 to 7.88) was associated, as was using the entire index (aOR 1.32, 95% CI 1.07 to 1.63). The association did not remain when assessing comorbidity with alternative measures. Conclusion The method chosen to assess comorbidity burden affected whether baseline comorbidity was associated with subsequent ILD development. This study demonstrates that a specific rheumatic disease comorbidity index reveals associations not detected by generic tools or simple counts. Baseline lung comorbidities should be added to the known list of risk factors for ILD and aid targeted screening in the context of RA.