The objective of the study is to investigate pain mechanisms, comorbidity status, biomarkers, patient reported outcome measures, ultrasonographic (US) inflammatory activity and association between these features in patients with psoriatic arthritis (PsA) intensifying anti-rheumatic treatment. Furthermore, to assess the predictive value of baseline pain profile, comorbidity status, and US joint/entheses activity on treatment outcome after 4 months. Finally, we aimed to compare baseline characteristics with I) patients with skin psoriasis without arthritis and II) healthy controls.

Patients with psoriatic arthritis, who initiate or switch anti-rheumatic treatment (conventional disease modifying drugs or biologic drugs) in routine care due to an active disease state, will be enrolled in the observational study.

The overall aim is to investigate pain mechanisms, comorbidities and US psoriatic changes and elucidate if these factors - independently or by interaction - influence treatment response after 4 months. Patients will have a baseline visit, a follow up visit after 4 months and yearly thereafter. Examinations will be performed at all time points (except stated otherwise) and include:

1. Assessment of pain mechanisms by clinical evaluation (swollen/tender joint ratio, tender points) and pain questionnaires (widespread pain index, PainDETECT).
2. Ultrasonography of joints and entheses by two trained assessors
3. Clinical examination of all psoriatic manifestations
4. Interview and questionnaires regarding lifestyle, comorbidity status, function, quality of life and the impact of psoriatic manifestations.
5. Blood samples for standard rheumatic monitoring and biobank
6. X-ray of hands and feet (only at baseline)
7. AMPS test (assessment of motor and process skills) will be performed and interpreted by an certified ergo therapist (only at baseline visit and at 34-months follow-up and oly for the first included 20 PsA patients)

Clinical as well as patient-reported and observer-based outcomes will be described for the overall study population and the prognostic influence of US, comorbidities, and pain mechanisms will be analysed. Subsequently, the analyses will be repeated for certain subgroups of patients (e.g., conventional drug therapy vs. biologic drug intervention) in an exploratory manner. We will compare baseline data including pain, fatigue, work, life style and comorbidity status of PsA patients before initiating new treatment with patients with skin psoriasis without arthritis and healthy controls

Inclusion Criteria:

• Diagnosed with PsA according to the CASPAR (The Classification of Psoriatic Arthritis) criteria
• Peripheral joint involvement.
• Minimum 18 years of age.
• Initiating or switching anti-rheumatic treatment due to active PsA.
• Signing a written informed consent.

Exclusion Criteria:

• Pregnancy
• Peripheral neuropathy
• Demyelinising disease
• Recent stroke
• Other rheumatic inflammatory diseases.
• Oral, intra-articular or intra-muscular glucocorticoids within 3 weeks prior to baseline
• Treatment with centrally acting analgesics (opioids, anti-depressants, anticonvulsants) within 1 week prior to baseline
• Treatment with mild analgesics (non-steroidal anti-inflammatory drugs, acetylsalicylic acid, acetaminophen) within 24 hours prior to baseline

> 18 years of age with PsA according to the CASPAR criteria who initiate or switch anti-rheumatic treatment (biologics and/or conventional synthetic DMARDs) due to active PsA in routine care.