A comparison of the two case-referent studies included in the pooled analysis
| Study of incident cases of RA | Study of prevalent cases of RA | |
|---|---|---|
| *American Rheumatism Association (ARA) 1987 Revised Criteria for Rheumatoid Arthritis.22 | ||
| †Morning stiffness >60 minutes and symmetrical arthritis and arthritis of small joints. | ||
| ‡Cases and referents in the study using prevalent cases were recruited from the catchment area of the University Hospital in Linköping, situated in southeastern Sweden. Cases and referents in the study using incident cases were recruited from this area as well as from the catchment areas of nine other rheumatology units located in Southeastern Sweden. | ||
| §Age when responding to the postal questionnaire. | ||
| Exposure assessment | Questionnaire based | Questionnaire based |
| Occupational history | Lifelong | Lifelong |
| Cases | ||
| Inclusion criteria | ||
| ARA criteria* (n) | ⩾4 or 3 specific† | ⩾4 |
| Age at diagnosis, range (years) | 16–75 | 25–75 |
| Year of diagnosis, range | 1996–98 | 1980–95 |
| Residency | Southeastern Sweden‡ | Southeastern Sweden‡ |
| Ethnicity | Nordic | Nordic |
| Origin of cases | Hospital based | Hospital based |
| No. included (n) | 298 | 422 |
| Age at diagnosis, mean (years) | 55 | 54 |
| Age at participation§, mean (years) | 55 | 62 |
| Referents | ||
| Inclusion criteria | ||
| Age at enrolment, range (years) | 16–75 | 25–75 |
| Year of enrolment, range | 1996–1998 | 1980–1995 |
| Residency | Southeastern Sweden‡ | Southeastern Sweden‡ |
| Ethnicity | Nordic | Nordic |
| Origin of referents | Population based | Population based |
| No. included (n) | Cases ×4 | Cases ×2 |
| Age at enrolment, mean (years) | 47 | 50 |
| Age at participation§, mean (years) | 47 | 58 |