What is the diagnostic approach for a patient with suspected psoriatic arthritis, considering their medical history and potential autoimmune conditions?

How to Diagnose Psoriatic Arthritis

Diagnose psoriatic arthritis using the CASPAR criteria, which requires established inflammatory arthritis (tender/swollen joints with prolonged morning stiffness) plus at least 3 points from a 7-feature scoring system, achieving 98.7% specificity and 91.4% sensitivity. 1

Apply the CASPAR Diagnostic Criteria

The CASPAR classification system is the gold standard for PsA diagnosis and requires you to first establish inflammatory articular disease, then score the following features 1, 2:

• Current psoriasis = 2 points 1, 3
• Personal history of psoriasis (if no current psoriasis) = 1 point 1
• Family history of psoriasis (if no current or personal history) = 1 point 1
• Dactylitis (current or history documented by rheumatologist) = 1 point 1, 2
• Juxta-articular new bone formation on imaging = 1 point 1
• Rheumatoid factor negativity = 1 point 1, 2
• Psoriatic nail dystrophy (onycholysis, pitting, hyperkeratosis) = 1 point 1, 2

A total of ≥3 points confirms PsA diagnosis 1, 3.

Establish Inflammatory Arthritis First

Before applying CASPAR criteria, you must document inflammatory features including 1, 2:

• Joint pain with erythema, warmth, and swelling 1
• Prominent morning stiffness lasting >30 minutes 1
• Rest-induced stiffness that improves with activity 4, 5
• Tender and swollen joints on examination 1

Common pitfall: Do not confuse osteoarthritis with PsA—osteoarthritis causes brief stiffness (<30 minutes) that worsens with activity, while PsA causes prolonged morning stiffness that improves with movement 1.

Perform Targeted Clinical Examination

Peripheral Joint Assessment

Examine 68 joints for tenderness and 66 joints for swelling, focusing on 1, 2:

• Distal interphalangeal (DIP) joints—highly characteristic of PsA, uncommon in rheumatoid arthritis 1, 6, 7
• Asymmetric oligoarthritis pattern (≤4 joints)—present in 60% of early PsA 7, 5
• Look for less tender/swollen joints compared to rheumatoid arthritis 1

Enthesitis Screening

Palpate these specific sites for tenderness 2, 4, 5:

• Achilles tendon insertion 2, 4
• Plantar fascia insertion 2
• Lateral epicondyle of humerus 4
• Medial femoral condyle 4

Enthesitis is characteristic of PsA and helps differentiate it from rheumatoid arthritis 1, 2.

Dactylitis Assessment

Check for "sausage digits"—diffuse swelling of entire fingers or toes from combined enthesitis and synovitis 1, 2, 7. Present in approximately one-third of PsA patients 7, 5.

Axial Disease Evaluation

Ask about inflammatory back pain features 1, 4:

• Early morning stiffness in the spine 4
• Pain relieved by exercise and worsened by rest 4
• Asymmetric sacroiliitis pattern (unlike ankylosing spondylitis) 1

Skin and Nail Examination

Document psoriatic features, as nail dystrophy, scalp lesions, and intergluteal/perianal psoriasis strongly associate with PsA development 7:

• Nail pitting, onycholysis, hyperkeratosis—especially correlates with DIP joint involvement 1, 2, 7
• Psoriatic plaques anywhere on body 1

Critical caveat: In 20% of cases, arthritis precedes skin manifestations by years, and 16% develop joint and skin symptoms simultaneously 1, 4. Do not exclude PsA based solely on absent skin findings if family history exists 1, 5.

Order Appropriate Laboratory Tests

Mandatory Baseline Tests

• ESR and CRP—the only laboratory tests specifically recommended for baseline PsA evaluation to assess inflammation severity 1, 2, 8

Important: There are no diagnostic laboratory markers for PsA—diagnosis relies on clinical criteria 3, 8, 4.

Differential Diagnosis Tests

Order rheumatoid factor (RF) and anti-CCP antibodies to exclude rheumatoid arthritis, especially in polyarticular presentations 3, 8. PsA is typically seronegative, though RF presence does not absolutely preclude PsA diagnosis 1, 4.

Obtain Imaging Studies

Baseline Radiographs

Obtain X-rays of hands and feet at baseline for all suspected inflammatory arthritis cases 4. Look for juxta-articular new bone formation, which scores 1 point in CASPAR criteria 1.

Key finding: Radiological erosions occur early—present in nearly 50% at 10 years of disease 7.

Advanced Imaging When Indicated

Use MRI or ultrasound for 6:

• Isolated enthesitis without obvious joint swelling 6
• Inflammatory spinal pain to detect axial involvement 6
• Early disease detection before radiographic changes appear 6

Coordinate Multidisciplinary Diagnosis

Ideally, confirm psoriasis diagnosis with a dermatologist and inflammatory musculoskeletal disease with a rheumatologist 1, 2. Refer any patient with suspected inflammatory arthritis and a 6-week history of painful, swollen joints for specialist assessment 4.

Complete Baseline Assessment Domains

Once PsA is diagnosed, document these OMERACT 8 consensus domains 1, 2:

• Pain on visual analogue scale 1, 2
• Patient global assessment of disease activity 1, 2
• Physical function using Health Assessment Questionnaire (HAQ) 1, 2
• Quality of life using SF-36 or PsAQoL 1, 2
• Fatigue assessment using FACIT or patient self-report 1, 2

Identify Poor Prognostic Factors

Document these features that predict progressive joint damage and warrant aggressive treatment 1, 2:

• Polyarticular disease (high number of actively inflamed joints) 1, 2, 3
• Elevated ESR at baseline 1, 2, 3, 8
• Previous treatment failures 1, 2
• Existing joint damage on clinical or radiographic examination 1, 2
• Diminished quality of life on standardized measures 1, 2

Clinical significance: Patients with polyarticular disease and elevated inflammatory markers represent severe PsA warranting earlier biologic therapy 3, 8. Early aggressive treatment substantially improves long-term prognosis 4.