Association of ICD-10-Coded Pneumonia Events with Interstitial Lung Disease Outcomes in Patients with Rheumatoid Arthritis: A Large Database Retrospective Cohort

Introduction: Patients with rheumatoid arthritis (RA) have higher risk for pneumonia, interstitial lung disease (ILD) and pulmonary fibrosis (PF). However, the association between an ICD-10-coded pneumonia event (CPE) and the incidence of ILD or PF in the RA population remains unclear. Methods: We conducted a retrospective cohort study using the TriNetX database. Patients with ICD-10 for RA aged 50 or older who had a CPE within one year of RA diagnosis (CPE cohort, n = 4553) were matched 1:1 by propensity score for key factors, including demographics, comorbidities (i.e., COPD), and medication use (DMARDs, corticosteroids) to RA patients without a CPE (Control cohort, n = 4553). Cox proportional hazard models assessed the incidence of a composite ILD outcome, PF, and secondary complications over a 4-year follow-up after the index event defined as 1-year after RA diagnosis for both cohorts. Results: The CPE cohort showed an increased risk for all outcomes. Patients with CPE had a 2.48-fold increased risk for PF (HR = 2.48; 95% CI, 1.78–3.45; p < 0.01) and a 2.87-fold increased risk for the composite ILD outcome (HR = 2.87; 95% CI, 2.18–3.80; p < 0.01). The risk of rheumatoid lung disease was 4.15 times higher (HR = 4.15; 95% CI, 2.30–7.50; p < 0.01). Furthermore, the CPE group had a higher risk for all-cause mortality (HR = 1.82; 95% CI, 1.58–2.09; p < 0.01). Conclusions: The CPE within one year of RA diagnosis is associated with an increase in subsequent ILD-coded outcomes. While this retrospective design cannot establish causality, an unspecified pneumonia code in early RA may represent an early clinical manifestation of unrecognized ILD, serving as a high-risk marker that warrants pulmonary surveillance.