CheckAP: Prevalence of psoriatic arthritis (PsA) and performance evaluation of the EARP questionnaire in the population of Portuguese patients with psoriasis followed in a dermatology setting.

Background: The percentage of Portuguese psoriasis patients with psoriatic arthritis is unknown but musculoskeletal complaints related to PsA affect up to a third of patients. Dermatologists can identify early PsA as skin symptoms often precede joint symptoms in 80% of patients. Efficient and easy to perform screening tools are needed to help dermatologists effectively discriminate between Pso and PsA patients. The present study aims to evaluate the prevalence of PsA in Pso patients followed in Portuguese dermatology clinics. Additionally, it aims to evaluate the EARP-PT performance (validity, sensitivity, specificity) and the best cut-off point to allow an early identification of PsA potential patients.

Methods: A multicentre national, cross-sectional, observational study with two independent assessments (dermatologist and rheumatologist), was performed. A PsA case was defined by a combination of expert opinion and classification criteria for psoriatic arthritis (CASPAR). The EARP-PT questionnaire screening performance was evaluated.

Results: Pso patients (n=172) were included with a mean age of 53.8+/-14.5 years, 53.5% were male with a mean time of diagnosis of 17.4+/-14.9 years. The prevalence of PsA in patients with Pso in our sample was 8.70% (95% CI: 4.8-14.2). The EARP-PT questionnaire displayed good internal consistency (Cronbach's α=0.81) and, using a validated initial cut-off point of 3, demonstrated a sensitivity of 71.4% and specificity of 40.1%.

Conclusion: The estimated prevalence of PsA in a population of Pso patients followed in Portuguese dermatology clinics, is 8.7%. The EARP-PT questionnaire appears to be a useful tool for dermatologists in the early detection of PsA.