Background/Purpose: Rheumatoid arthritis (RA) is
one of more than 80 different types of autoimmune diseases, many of which,
including RA, share common pathogenic mechanisms, resulting in overlapping
syndromes. Some autoimmune diseases, such as lupus and psoriasis, may also
occur as a side effect of treatment. The purpose of this study was to quantify
the incidence of autoimmune
diseases in an RA population. For comparison, a population of patients was
identified who had osteoarthritis (OA) and similar health care encounters to
the RA population.  

Methods: Patients were identified from the US MarketScan^® Commercial
and Supplemental Medicare databases. Patients with RA were age- and sex-matched
1:5 with a comparison cohort of patients with OA. Patients were required to
have ≥180 days of continuous health plan enrollment before the qualifying
RA or OA diagnosis between January 1, 2006 and September 30, 2013. Study index
date corresponded to the first qualifying RA or OA diagnosis date during the
study period. A single International Classification of Diseases, Ninth Revision
diagnosis code of an autoimmune disease after the index date was considered an
event. Over 28 autoimmune diseases were pre-specified. Patients were followed
from their index date until the end of study period, end of enrollment, or
occurrence of autoimmune disease, whichever occurred first. For the RA cohort,
patients were censored if they developed OA during the follow-up period. Patients with an autoimmune disease in their 6-month
baseline period were excluded when calculating incidence rates (IRs) for that
specific autoimmune disease during follow-up. IRs with 95% CI for 28 pre-specified
autoimmune diseases during the follow-up period were computed for both RA and
OA populations.

Results: A
total of 208,397 and 1,031,784 patients with RA and OA, respectively, were evaluated.
Of the 28 pre-specified autoimmune diseases evaluated, lupus, psoriatic
disease, and Sjögren’s syndrome occurred most frequently among patients with RA,
occurring in 5%, 3%, and 3% of the population, respectively, with corresponding
IRs of 29, 19 and 17/1000 person-years. Corresponding IRs for the matched OA
population were 2, 6 and 2/1000 person-years for lupus, psoriatic disease, and
Sjögren’s syndrome, respectively. Chronic urticaria and type 1 diabetes were
the most frequently reported autoimmune diseases in the OA population (2% and
1.9%, respectively), with IRs of 8.7 and 8.4/1000 person-years, respectively. The IRs of autoimmune diseases were generally higher among the RA
patients than the matched OA patients (except for type 1 diabetes). Selected co-morbid autoimmune diseases in either
cohort are shown in the Table.   Conclusion: Previous work showed that the prevalence of certain autoimmune
diseases was greater in RA patients compared to those with OA. These results
suggest that some autoimmune diseases occur more frequently among patients with
RA than in closely matched patients with OA.