The epidemiology of established rheumatoid arthritis.

No published epidemiological study has specifically focused on the prevalence of established rheumatoid arthritis (RA), as epidemiologists do not study established RA separated from RA as a whole; especially no incidence studies can be found, as incidence refers to new cases (early RA). Such a study, if it existed, would find a prevalence much larger than that of recent-onset RA, and should be planned based on clear definitions that currently do not exist in epidemiology. As a result, any study addressing RA as a whole, leaving aside early arthritis, would be actually studying established RA. This work reviews the epidemiology of RA, in contraposition of early RA, and tried to highlight epidemiological characteristics of established RA in published studies as well as methodological issues, including differences between recent-onset and established RA regarding the prevalence of comorbidities and other characteristics, and differences across countries. The global epidemiology of established RA teaches us that long-term outcomes could largely depend on health care models and are modifiable.