What tests are used to diagnose psoriatic arthritis?

Testing for Psoriatic Arthritis

The diagnosis of psoriatic arthritis is primarily clinical, based on the CASPAR criteria requiring inflammatory arthritis plus at least 3 points from specific features, with no single diagnostic laboratory test available. 1

Clinical Diagnostic Criteria

CASPAR Classification Criteria (98.7% specificity, 91.4% sensitivity) requires established inflammatory arthritis (tender/swollen joints with prolonged morning stiffness) plus ≥3 points from: 1

• Current psoriasis (2 points)
• Personal history of psoriasis (1 point)
• Family history of psoriasis (1 point)
• Current dactylitis or history documented by rheumatologist (1 point)
• Juxta-articular new bone formation on imaging (1 point)
• Rheumatoid factor negativity (1 point)
• Psoriatic nail dystrophy including onycholysis, pitting, hyperkeratosis (1 point)

Laboratory Testing

Serologic Tests:

• No specific diagnostic test exists for psoriatic arthritis 1
• Rheumatoid factor should be negative (though 5-13% of PsA patients may be RF-positive) 1, 2
• Anti-CCP antibodies help differentiate from rheumatoid arthritis but can be positive in similar percentages as RF 2
• Acute phase reactants (ESR/CRP) are elevated in only 50% of patients, limiting their diagnostic utility, though elevated ESR predicts damage progression 3, 2

Important caveat: The presence of RF or anti-CCP does not exclude PsA but should prompt careful diagnostic scrutiny. 4

Imaging Studies

Radiography (Plain X-rays):

• Baseline hand and foot radiographs should be obtained in all suspected inflammatory arthritis cases 4
• Characteristic findings combine destructive changes (erosions, tuft resorption, osteolysis) with bone proliferation (periostitis, ankylosis, spur formation, non-marginal syndesmophytes) 1, 5, 6
• Demonstrates extent and location of joint involvement 1
• For axial disease: radiographs, MRI, and/or CT can detect asymptomatic spine/sacroiliac involvement 1

Ultrasound (Power Doppler):

• Superior to clinical examination for detecting inflammation and structural damage 5, 6
• Identifies synovial hypertrophy, hyperemia, erosions, and enthesitis 5, 6
• Useful for monitoring therapy efficacy and guiding steroid injections 6
• Presence of erosions, synovial hypertrophy, and hyperemia increases post-test probability of inflammatory arthritis to 50-94% 5
• Mild synovial hypertrophy alone is nonspecific and has limited relevance 5

MRI with Intravenous Gadolinium:

• More sensitive than ultrasound, especially for early disease 5, 7
• Directly visualizes inflammation in peripheral/axial joints and entheses 6, 7
• Detects bone marrow edema and synovitis, which predict disease progression 5, 7
• Allows assessment of inflammation and structural damage in detail 7
• Evidence is inconclusive whether MRI or ultrasound is superior overall 5

CT Scanning:

• Limited role in peripheral joints but useful for spine assessment 6
• Similar accuracy to MRI for sacroiliac erosions but less effective for detecting synovial inflammation 6

Bone Scintigraphy:

• Lacks specificity and has been supplanted by ultrasound and MRI 6

Physical Examination Findings

Key clinical features to assess: 1, 3

• Joint examination: 68 joints for tenderness, 66 joints for swelling (including DIP joints of hands and PIP/DIP joints of feet) 1, 3
• Dactylitis (sausage digit) - common in PsA, uncommon in RA 1
• Enthesitis - screen lateral epicondyle of humerus, medial femoral condyle, Achilles tendon insertion 4
• Skin and nail examination for psoriatic plaques, nail pitting, onycholysis, hyperkeratosis 1
• Pattern of joint involvement: asymmetric distribution, DIP involvement, less tender/swollen than RA 1

Baseline Assessment Domains

The following should be documented at baseline: 3

• Peripheral joint assessment (68/66 joint count)
• Patient-reported pain (visual analogue scale)
• Patient global assessment of disease activity
• Physical function (Health Assessment Questionnaire)
• Health-related quality of life (SF-36 or PsAQoL)
• Fatigue assessment
• Acute phase reactants (CRP or ESR)

Differential Diagnosis Considerations

Distinguish from rheumatoid arthritis: 1

• PsA has less symmetric distribution, DIP involvement, dactylitis, enthesitis
• Absence of rheumatoid nodules and high-titer RF
• Presence of psoriatic skin/nail findings

Distinguish from osteoarthritis: 1

• PsA has inflammatory joint swelling vs. Heberden's nodes (bone spurs)
• Morning stiffness improves with activity in PsA vs. worsens with activity in OA

Screening for uveitis: 8

• Pediatric PsA patients have 1.5-25% prevalence of uveitis
• Screen by history and ophthalmologic examination for eye pain, redness, visual loss, photophobia

Referral Recommendations

Refer to rheumatology any patient with suspected inflammatory arthritis and six-week history of painful, swollen joints 4

Preferably, diagnosis should be confirmed by a dermatologist (for psoriasis) and rheumatologist (for inflammatory musculoskeletal disease) 3