Comorbidity in incident rheumatoid arthritis: a nationwide case-control study with bidirectional follow-up

OBJECTIVES: To investigate the association between risk of different concurrent disorders and diagnosis of rheumatoid arthritis (RA) using a temporal approach. METHODS: Retrospective, case-control study. Data were extracted from all available healthcare claims for Poland between 2011-2021. Prevalent RA (n = 262 265) patients were examined to evaluate morbidity patterns. Incident RA (n = 15 879) and age-, sex- and region-matched controls were sampled 1:10 from the general population (GP). Exposure was new-onset RA identified by interspaced, repeat claims and prescription. Follow-up was performed in a bidirectional, five-year timeframe from RA diagnosis (Dx). RESULTS: RA is associated with enhanced risk of multiple concomitant disorders both pre- and post-Dx, especially hematologic (incidence rate ratio (IRR) 1.80, 95% CI 1.58, 2.05), pulmonary (IRR 1.57, 95% CI 1.53, 1.62), gastrointestinal (IRR 1.53, 95% CI 1.46, 1.60) and cardiovascular (IRR 1.25, 95% CI 1.22, 1.27) conditions. RA appears strongly associated with interstitial lung disease (pre-Dx IRR 2.97, 95% CI 2.20, 4.02; post-Dx IRR 4.66, 95% CI 3.91, 5.56) and inflammatory bowel disease (pre-Dx IRR 1.61, 95% CI 1.25, 2.07; post-Dx IRR 2.12, 95% CI 1.70, 2.63). Enhanced post-Dx risk of thyroid cancer (IRR 1.29, 95% CI 1.00, 1.66) and lymphoma (IRR 1.50, 95% CI 1.20, 1.88) was observed, in contrast to reduced risk of colon cancer (IRR 0.77, 95% CI 0.62, 0.94). CONCLUSION: Enhanced risk of multiple concurrent disorders was observed both pre- and post-RA Dx, with varying patterns across different disease groups. These data highlight the multisystemic nature of RA and warrant further research into causal, bidirectional relationships.