Rheumatoid arthritis and interstitial lung disease: the role of comorbidities—a retrospective analysis of two RA inception cohorts in the UK

Objective: The objective of this study was to assess the role of comorbidities measurements in interstitial lung disease (ILD) in patients with RA. Methods: The data were from two consecutive multicentre, prospective RA inception cohorts, including baseline socio-demographic, clinical, treatment, laboratory and functional features. Comorbidity burden was assessed using a count of major comorbidities, the Rheumatic Disease Comorbidity Index (RDCI) and the Charlson Comorbidity Index (CCI). Associations between comorbidities and the development of ILD were explored, adjusted for clinical factors using multivariable logistic regression analyses. Results: The data from 2701 patients were included, and the median follow-up time was 6 years. In total, 101 patients (3.7%) were diagnosed with ILD. Twelve (0.44%) had the diagnosis at baseline, 46 were diagnosed during follow-up, and 43 were identified from death certificates. In multivariable analyses, age at onset [adjusted odds ratio (aOR) 1.03, 95% CI 1.01–1.05], seropositivity (aOR 2.58, 95% CI 1.38–4.81) and ever smoking (aOR 1.70, 95% CI 1.04–2.79) were associated with ILD diagnosis. Higher lung comorbidity burden measured using the lung disease component of the RDCI (aOR 4.59, 95% CI 2.68–7.88) was associated, as was using the entire index (aOR 1.32, 95% CI 1.07–1.63). The association did not remain when assessing comorbidity with alternative measures. Conclusion: The method chosen to assess the 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.