To apply and compare different classification criteria on a representative nationwide sample of psoriatic arthritis (PsA) twins and to estimate the prevalence and incidence of PsA.
The study comprised three Danish nationwide twin cohorts. In 1994 37 388 Danish twin individuals and in 2002 46 418 twin individuals received a questionnaire, including questions on rheumatic diseases. Twins reporting PsA and their co-twins were classified according to the Moll and Wright and CASPAR (ClASsification criteria for Psoriatic ARthritis) criteria based on interview, clinical examination and scrutiny of medical records.
228 twin individuals reported PsA and 164 (72%) participated in clinical validation. By using the Moll and Wright and CASPAR criteria, 54 and 50 cases were diagnosed with PsA respectively. The positive predictive value of self-reported PsA was 31%. According to the Moll and Wright and CASPAR criteria the prevalence was 0.15% (95% CI: 0.13%, 0.22%) and 0.14% (95% CI: 0.11%, 0.19%) respectively. The annual incidence rate based on new self-reported cases in 2002 was 6/100 000 person-years (95% CI: 3/100 000 person-years, 11/100 000 person-years).
The positive predictive value of self-reported PsA was 31%. The prevalence and incidence figures of PsA were equivalent to the previously reported occurrence in population- and hospital-based studies.