Baseline Disease Activity and Moderate-to-High Disease Activity at 6 Months in Rheumatoid Arthritis-Associated Interstitial Lung Disease: A Multicentre Real-World Study from the ANSWER Cohort

Background/Objectives: In rheumatoid arthritis-associated interstitial lung disease (RA-ILD), factors associated with moderate-to-high disease activity (MDA/HDA) during follow-up are not well defined. We examined baseline factors associated with MDA/HDA at 6 months. Methods: We analyzed 278 patients with RA-ILD from the multicentre ANSWER cohort. At 6 months, patients were classified by the 28-joint Disease Activity Score using erythrocyte sedimentation rate (DAS28-ESR) as remission/low disease activity (LDA) or MDA/HDA. Logistic regression used age and baseline DAS28-ESR as continuous variables (n = 178 with complete baseline DAS28-ESR); discrimination, calibration and optimism (1000 bootstrap resamples) were assessed. A sensitivity analysis used the ESR-free Clinical Disease Activity Index (CDAI) as the outcome. Results: In all, 148 patients achieved remission/LDA and 130 had MDA/HDA. Baseline DAS28-ESR was independently associated with MDA/HDA (odds ratio 2.01 per point, 95% CI 1.57–2.57), as was age (1.05 per year, 1.01–1.08). The area under the curve was 0.768 for DAS28-ESR alone and 0.776 with age added; internal validation of the two-variable model showed little estimated optimism (corrected area 0.772, calibration slope 0.984, 95% CI 0.693–1.396). With the CDAI-based outcome, the association with age was not reproduced (odds ratio 1.00, 0.97–1.04). Conclusions: Baseline disease activity is strongly associated with MDA/HDA at 6 months in RA-ILD. The incremental value of age was small, and the association with age was not reproduced when the outcome was defined by the CDAI. The age finding should therefore be regarded as secondary and dependent on the disease activity measure used. External and prospective validation is required before these findings are applied clinically.