19th January 2024

Diagnosis

The diagnosis of psoriatic arthritis is clinical and is based largely on history and examination. Blood tests and imaging are performed either to establish severity of symptoms or exclude other diagnoses, particularly rheumatoid arthritis. Inflammatory joint pain and stiffness lasting more than 30 minutes, and joint or digit swelling are highly suggestive features of psoriatic arthritis in patients with pre-existing skin disease.

Investigations

Blood tests:

• ESR and CRP – can be normal or raised in polyarticular and progressive arthritis
• Rheumatoid factor – a negative result can be used to distinguish between rheumatoid arthritis and psoriatic arthritis
• Anti-CCP – useful for excluding rheumatoid arthritis (although note that anti-CCP is not always positive in RA)
• Lipid profile, fasting blood glucose and uric acid level – metabolic screen due to the increased risk of cardiovascular disease in patients with psoriatic arthritis

Imaging:

Patients who present with evidence of joint inflammation should receive plain x-rays of the hands and feet. Early disease may not produce characteristic radiological changes, and x-rays may, therefore, only reveal soft tissue swelling. The following radiological features can be demonstrated in patients with psoriatic arthritis:

• DIP joint erosions
• Joint subluxation
• Periarticular new-bone formation
• Periostitis
• Pencil-in-cup deformity: due to periarticular erosions and bone resorption in advanced disease
• Arthritis mutilans: osteolysis and articular collapse can cause “telescoping fingers” in advanced disease

MRI or CT scanning may be a more specific and sensitive method of investigating subtle psoriatic arthritis signs, especially in the hands and feet. MRI imaging of the sacroiliac joints can detect early pre-radiographic sacroiliitis, which is characteristically asymmetric and can be asymptomatic. Spinal changes tend to mostly affect the cervical spine, and subchondral bone oedema is a characteristic finding.

Other investigations:

Synovial fluid aspiration can be performed if there is a high suspicion of gout. The aspirate should not contain crystals; however, the white cell count is often high. Ultrasound of the joints can also be used to demonstrate involvement of entheses in psoriatic arthritis.