Abstract

Objective: 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.

Methods: 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 (M&W) and CASPAR criteria based on interview, clinical examination, and scrutiny of medical records.

Results: A total of 228 twin individuals reported PsA and 184 (81%) participated in clinical validation. By using the M&W 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 M&W 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 pyr (95% CI: 3/100,000 pyr, 11/100,000 pyr).

Conclusion: 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.