How is psoriatic arthritis diagnosed?

How to Diagnose Psoriatic Arthritis

Psoriatic arthritis diagnosis is based on clinical judgment using the CASPAR criteria, which require established inflammatory arthritis plus at least 3 points from specific features including current psoriasis (2 points), nail changes, dactylitis, negative rheumatoid factor, and radiographic findings. 1, 2, 3

Diagnostic Approach: Step-by-Step Algorithm

Step 1: Confirm Inflammatory Musculoskeletal Disease

Look for these specific inflammatory features:

• Joint pain with tender and swollen joints on examination 1, 2
• Prolonged morning stiffness or immobility-induced stiffness (not just brief stiffness with activity like osteoarthritis) 1, 3
• Spine or enthesis involvement with erythema, warmth, and swelling 2, 3
• Pain that improves with exercise and worsens with rest (suggests inflammatory rather than mechanical cause) 4

Step 2: Apply CASPAR Criteria (Need ≥3 Points)

Score the following features:

• Current psoriasis = 2 points (active skin or scalp lesions on examination) 3
• Personal history of psoriasis = 1 point (only if no current psoriasis) 3
• Family history of psoriasis = 1 point (only if no current or personal history) 3
• Typical nail dystrophy = 1 point (onycholysis, pitting, hyperkeratosis) 3
• Dactylitis = 1 point (current or history of "sausage digit" - entire digit swelling) 1, 3
• Negative rheumatoid factor = 1 point (by any method except latex; ELISA preferred) 3
• Juxta-articular new bone formation = 1 point (on hand/foot X-rays) 3

The CASPAR criteria have 98.7% specificity and 91.4% sensitivity. 1, 3

Step 3: Physical Examination - What to Assess

Perform comprehensive joint examination:

• Examine 68 joints for tenderness and 66 joints for swelling (must include DIP joints of hands and both PIP/DIP joints of feet, as these distinguish PsA from rheumatoid arthritis) 1, 2
• Check for dactylitis - combination of enthesitis and synovitis involving entire digit 2
• Palpate enthesis sites: lateral epicondyle of humerus, medial condyle of femur, Achilles tendon insertion, and plantar fascia 2, 4
• Examine all nails for pitting, onycholysis, and hyperkeratosis (nail disease strongly associated with DIP joint involvement) 2
• Look for asymmetric joint distribution (unlike rheumatoid arthritis which is typically symmetric) 1

Step 4: Laboratory Testing

Order these specific tests:

• Rheumatoid factor (typically negative in PsA - "seronegative arthritis") 1, 2
• ESR and CRP (acute phase reactants to assess inflammation) 2

Important caveat: The presence of rheumatoid factor does not exclude PsA, but should prompt careful scrutiny of the diagnosis. 4 There are no specific serologic tests that confirm PsA. 1

Step 5: Imaging Studies

Obtain baseline radiographs:

• X-rays of hands and feet at baseline for all suspected inflammatory arthritis 4
• Look for juxta-articular new bone formation (characteristic of PsA) 3
• MRI, ultrasound, or HR-pQCT can detect asymptomatic joint inflammation in psoriasis patients before clinical symptoms develop 1, 5
• For axial disease: radiographs, MRI, or CT can detect asymptomatic spine/sacroiliac involvement 1

Key Differentiating Features from Other Conditions

PsA vs. Rheumatoid Arthritis

• PsA has DIP joint involvement, dactylitis, and enthesitis (uncommon in RA) 1
• PsA joints are less tender/swollen and asymmetrically distributed 1
• Psoriatic plaques or nail changes present (absent in RA) 1
• Absence of rheumatoid nodules and extra-articular RA features 1

PsA vs. Osteoarthritis

• PsA has inflammatory joint swelling at DIP joints, not bony Heberden's nodes 1
• Morning stiffness prolonged in PsA vs. brief stiffness with activity in osteoarthritis 1
• PsA improves with movement, osteoarthritis worsens 1

PsA vs. Ankylosing Spondylitis

• Psoriatic plaques or nail changes present in PsA (absent in AS) 1
• PsA has asymmetric sacroiliitis and "skip" pattern spine involvement 1
• PsA is less symptomatic with less severe disease and rarely progresses to ankylosis 1

Critical Clinical Pearls

Timing relationships to know:

• Skin lesions precede arthritis in 72.7% of cases 1
• Arthritis precedes skin in 14.9-19.4% of cases 1, 4
• Average time from psoriasis to PsA is 10-12 years 1, 6
• 30-33% of psoriasis patients develop PsA 1, 6

Common pitfall: Screening tools (PEST, ToPAS, PASE, Early Arthritis questionnaire) have moderate reliability but perform poorly when tested outside their original development populations, so their usefulness in routine practice is limited. 1 Rely on clinical judgment and CASPAR criteria instead.

When to refer: Any patient with suspected inflammatory arthritis and a six-week history of painful, swollen joints should be referred for specialist rheumatology assessment. 4 Early diagnosis is critical because over 40-50% of patients develop radiographic joint destruction if untreated, leading to permanent disability. 6, 7

Baseline Assessment Once Diagnosed

Document these domains at diagnosis:

• Pain assessment using visual analogue or category rating scales 2
• Patient global assessment of disease activity 2
• Physical function using Health Assessment Questionnaire (HAQ) 2
• Quality of life using SF-36 or PsAQoL 2
• Fatigue assessment using FACIT or self-report 2
• Cardiovascular risk factors (hypertension, hyperlipidemia, diabetes) 2
• Screen for comorbidities: metabolic syndrome, inflammatory bowel disease, depression, liver disease 2

Poor prognostic factors requiring aggressive treatment:

• Polyarticular disease (high number of actively inflamed joints) 2
• Elevated ESR 2
• Previous treatment failures 2
• Existing joint damage on clinical or radiographic examination 2
• Diminished quality of life on standardized tools 2