Epidemiological investigations on rheumatoid arthritis in Stockholm. The use of different data sources.

The general aim of the thesis was to see how different data sources available in Stockholm County could be used for epidemiological investigations on rheumatoid arthritis (RA). The specific aims were: to study the mortality among persons with RA and the causes of the postulated high mortality in RA, to study the possible preventive effect of oral contraceptive use on the incidence of RA and to study the quality of the in-patient register with regard to RA. Four sources were used: An epidemiological survey of RA in Stockholm, the in-patient register of Stockholm County, the Stockholm County sample of the national health interview survey and the national cause-of-death register. These data sources were compared and validated against each other and other data collected. The possibilities and limitations in the use of these different sources were assessed in a series of epidemiological investigations of which the main findings were the following: Persons with RA had an increased mortality compared to the general population. This applied to persons with RA identified in a random sample from the general population as well as to persons hospitalized with RA. The excess mortality was particularly high in the hospitalized group. RA was rarely recorded as the cause of death. The mortality was strongly correlated to the severity of the disease, and drug use was not found to be the main cause of the high mortality. There was a slight reduction in incidence of RA among oral contraceptive users compared with non users, particularly for women with age at onset of RA over 40. At least 80% of the cases recorded as RA in the in-patient register were judged to be "true" cases of RA. The documentation of the cases as well as the accuracy of the diagnosis varied strongly between different types of departments.