Objectives: The prevalence of rheumatoid arthritis (RA) and its current treatment practices in Japan are poorly documented. Therefore, we examined these factors in a Japanese health insurance database.
Methods: We analyzed reimbursement data provided by health insurance societies for 1 million individuals, including healthy individuals, registered from January 2005 to June 2011. Changes in treatments were determined in 320 thousand individuals originally registered in 2005. The treatment patterns were compared with those of the Institute of Rheumatology, Rheumatoid Arthritis (IORRA) cohort managed by Tokyo Women's Medical University.
Results: The estimated prevalence of RA was 1.24 million (1.0 % of the Japanese population), excluding suspected cases, and 706 thousand (0.6 %) in a sensitivity analysis. Seventy-nine percent of patients were treated for RA. Methotrexate was used by 27 % of patients. In 2005, 5 % of patients were prescribed methotrexate at >8 mg/week, which increased to 13 % in 2011. These rates were lower than those in the IORRA cohort.
Conclusions: Our results indicate that the prevalence of RA in Japan is somewhere between 0.6 and 1.0 %. Considering that methotrexate is infrequently used, the implementation of aggressive treatment regimens such as the 'Treat to Target' strategy is important to achieve tight control of RA in Japan.