Objectives: The aims of this study were to determine the prevalence of polyautoimmunity, i.e., the presence of more than one autoimmune disease in seropositive compared with seronegative patients with rheumatoid arthritis (RA) at the time of diagnosis; and to estimate the incidence of polyautoimmunity in the first five years of the disease, again comparing seropositive with seronegative RA patients.
Methods: The study was register-based and used the nationwide DANBIO rheumatology register and the national administrative registers including the Danish National Patient Registry. The prevalence of polyautoimmunity was determined as the proportion of patients with at least one autoimmune disease in addition to RA at the time of RA diagnosis, and the odds ratio of having another autoimmune disease in seropositive compared with seronegative patients was determined using logistic regression. To estimate the incidence and hazard ratio (HR) of being diagnosed with yet another autoimmune disease in the subsequent 5 years, cause-specific Cox regression models accounting for the competing risk of death was performed.
Results: The study analysed data from 12 517 patients recently diagnosed with RA. The groups were almost identical regarding treatment, DAS28-CRP and HAQ-DI but patients with seropositive RA were younger (median age 59.4 years vs. 63.0 years) and included more women (68.6 % vs. 63.0 %). At time of diagnosis, patients with seropositive RA had an OR of 0.88 (95% CI 0.77-1.02) for polyautoimmunity compared to patients with seronegative RA. Likewise, patients with seropositive disease had a HR of 0.83 (95% CI 0.62-1.11) in the first five years of follow-up compared with seronegative RA. However, patients with seropositive RA had a higher mortality rate than patients with seronegative RA (5.2 % vs. 3.9 %) during follow up.
Conclusion: The study showed that patients with seronegative RA had higher risk of polyautoimmunity compared to patients with seropositive RA at time of diagnosis as well as in the following five years regardless of sex, age, treatment and clinical disease activity measures.