Microscopic colitis and risk of incident rheumatoid arthritis: A nationwide population‐based matched cohort study.

Summary: Background: Microscopic colitis (MC) has been linked to several autoimmune conditions. Results from previous studies on the association with rheumatoid arthritis (RA) have been inconsistent. Aim: To assess the risk of future RA in MC. Methods: We conducted a nationwide matched cohort study in Sweden of 8179 patients with biopsy‐verified MC (diagnosed in 2007–2017), 36,400 matched reference individuals and 8202 siblings without MC, with follow‐up until 2021. Information on MC was obtained from all of Sweden's regional pathology registers (n = 28) through the ESPRESSO cohort. Data on incident RA were collected from the National Patient Register. Using Cox regression, we calculated adjusted hazard ratios (aHRs) and 95% confidence intervals (CIs). Results: During a median follow‐up of 9.1 years (interquartile range = 6.7–11.7), 73 MC patients and 183 reference individuals from the general population were diagnosed with RA (99 vs. 55 events per 100,000 person‐years), equivalent to one extra case of RA in 226 patients with MC followed for 10 years. These rates corresponded to an aHR of 1.83 (95% CI = 1.39–2.41). The aHR was highest during the first year of follow‐up (2.31 [95% CI = 1.08–4.97]) and remained significantly elevated up to 5 years after MC diagnosis (aHR 2.16; 95% CI = 1.42–3.30). Compared to siblings, without MC, the aHR was 2.04 (95% CI = 1.18–3.56). Conclusion: Patients with MC are at a nearly two‐fold risk of developing RA compared to the general population. Knowledge of this increased risk may expedite evaluation for RA in patients with MC presenting with joint symptoms and/or arthralgia, thus preventing delay until RA diagnosis. [ABSTRACT FROM AUTHOR]