What is the diagnostic approach for psoriatic arthritis?

How to Diagnose Psoriatic Arthritis

Diagnose psoriatic arthritis using the CASPAR criteria, which requires established inflammatory articular disease plus at least 3 points from specific clinical features, with current psoriasis weighted most heavily at 2 points. 1, 2, 3

Diagnostic Criteria: CASPAR Classification

The CASPAR criteria demonstrate 98.7% specificity and 91.4% sensitivity and should be applied systematically 3:

Required: Established inflammatory articular disease (joint, spine, or entheseal) with erythema, warmth, swelling, and prominent morning/rest stiffness 1

Plus ≥3 points from:

• Current psoriasis (2 points) 3
• Personal history of psoriasis (1 point) 3
• Family history of psoriasis (1 point) 3
• Nail dystrophy (pitting, onycholysis, hyperkeratosis) (1 point) 3
• Negative rheumatoid factor (1 point) 3
• Dactylitis (current or history) (1 point) 3
• Radiographic evidence of juxta-articular new bone formation (1 point) 3

Clinical Assessment Algorithm

Step 1: Identify Inflammatory Features

Look for these specific patterns that distinguish PsA from mechanical joint disease 1, 4:

• Morning stiffness lasting >30 minutes that improves with activity 4
• Dactylitis ("sausage digit"): entire digit swelling from combined enthesitis and synovitis 2
• Enthesitis: tenderness at lateral epicondyle of humerus, medial femoral condyle, Achilles tendon insertion, or plantar fascia 2, 4
• Distal interphalangeal (DIP) joint involvement: uncommon in other arthritides, strongly suggests PsA when present 1, 5
• Asymmetric oligoarthritis: affects 60% in early disease 5

Step 2: Examine for Psoriasis

Ideally confirm psoriasis with dermatology consultation 1:

• Check scalp, intergluteal/perianal areas, and nails (these sites associate with higher PsA risk) 5
• Note that 20% develop arthritis before skin manifestations, sometimes years earlier 4
• Family history of psoriasis counts toward CASPAR criteria even without personal skin disease 3

Step 3: Perform Comprehensive Joint Examination

Conduct a 68/66 joint assessment (68 for tenderness, 66 for swelling), specifically including DIP joints of hands and both proximal interphalangeal and DIP joints of feet 1, 2, 3

Laboratory Testing

Order these specific tests 2, 3:

• ESR and CRP: assess inflammation severity (elevated ESR indicates poor prognosis) 1, 2, 3
• Rheumatoid factor and anti-CCP antibodies: typically negative in PsA (seronegative arthritis), but order to exclude rheumatoid arthritis, especially in symmetric polyarticular presentations 2, 3, 4

Critical pitfall: Presence of RF or anti-CCP does not automatically exclude PsA but demands careful diagnostic scrutiny 4

Imaging Studies

Baseline Radiography

Obtain X-rays of hands and feet at baseline for all suspected inflammatory arthritis 4:

• Look for juxta-articular new bone formation (counts toward CASPAR criteria) 3
• Erosions present in ~50% at 10 years of disease 5
• Use for prognostic assessment and monitoring structural damage 1

Advanced Imaging When Indicated

Consider MRI or ultrasound when 6, 7, 8:

• Isolated enthesitis without obvious joint swelling 6
• Inflammatory spinal pain suspected 6
• Early disease detection needed before radiographic changes 7, 8
• Clinical examination and blood tests are inconclusive 7

Mandatory Baseline Assessment Domains

Once diagnosis is established, document these OMERACT 8 consensus domains 1, 2:

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

Differential Diagnosis Considerations

Distinguish from Rheumatoid Arthritis

PsA differs by 1:

• Less tender/swollen joints, asymmetric distribution 1
• Presence of dactylitis, enthesitis, DIP involvement (uncommon in RA) 1
• Absence of rheumatoid nodules and extra-articular RA features 1
• Negative RF (though 20% of PsA patients, especially women, have symmetric polyarthritis resembling RA) 1

Distinguish from Osteoarthritis

Key differences 1:

• PsA has inflammatory DIP involvement vs. Heberden's nodes (bone spurs) in OA 1
• Morning stiffness in PsA vs. stiffness with activity in OA 1

Distinguish from Ankylosing Spondylitis

When axial disease present 1:

• PsA has asymmetric, less symptomatic sacroiliitis 1
• "Skip" pattern spinal involvement vs. continuous in AS 1
• Presence of psoriatic skin/nail changes 1
• Less progression to ankylosis 1

Prognostic Factors Requiring Documentation

Identify these poor prognosis indicators at baseline (they trigger earlier aggressive therapy) 1, 2, 3:

• Polyarticular disease (high number of actively inflamed joints) 1, 2
• Elevated ESR at presentation 1, 2
• Previous medication failures 1, 2
• Existing structural damage (clinical or radiographic) 1, 2
• Diminished quality of life on standardized measures 1, 2

Comorbidity Screening

Assess these associated conditions that impact treatment decisions 1, 2:

• Cardiovascular risk factors (hypertension, hyperlipidemia, diabetes) 1, 2
• Metabolic syndrome 1, 2
• Inflammatory bowel disease 1, 2
• Depression and anxiety 1
• Non-alcoholic fatty liver disease 1
• Osteoporosis 1

Referral Timing

Refer urgently to rheumatology any patient with suspected inflammatory arthritis and ≥6 weeks of painful, swollen joints 4. Early diagnosis and treatment substantially improve long-term prognosis, as PsA patients have 60% higher premature mortality risk and ~3 years shorter life expectancy than general population 4.

Ideally involve both dermatology (to confirm psoriasis) and rheumatology (to confirm inflammatory musculoskeletal disease) 1, 2.