Because the presentation of psoriatic arthritis symptoms can vary among individuals, there is no one specific diagnostic test for psoriatic arthritis. Doctors including dermatologists and rheumatologists will typically settle on a diagnosis after taking a detailed medical history and performing a physical exam.
Medical history. This will give the physician an indication of when the patient first started noticing symptoms, and also if there are members of the patient’s family that have experienced similar symptoms in the past. If a patient has first-degree relatives (mother, father, sibling) with a history of psoriasis or related joint pain, there is a higher likelihood the patient will also have the condition. When the patient already has a known history of psoriasis, the later psoriatic arthritis is easier to diagnose.
Physical examination. The physician will first check the skin for signs of psoriasis. If the identifying patches are not readily apparent on exposed areas of skin, the physician may check the scalp, umbilicus, genitalia, and perianal areas where psoriasis can sometimes go undetected. If psoriasis is found on the skin, the fingernails and toenails will be examined next, as this is where signs of psoriatic arthritis first appear.
The physician will also examine the joints to look for tenderness, swelling or stiffness—signs of inflammation, which may indicate the presence of arthritis.
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Diagnostic Testing. If inflammation is suspected, diagnostic tests may be performed to assess levels of inflammation and also to rule out other diseases that can mimic psoriatic arthritis. Common diagnostic tests include:
- X-ray. X-rays provide an image indicating which joints are affected and the extent of joint damage, although are typically only useful if the damage is extensive. MRI, ultrasound and CT scans may also be used.
- Blood tests. Blood tests are used to rule out other conditions. They can show levels of inflammation and also detect the presence of rheumatoid factor in the blood. If rheumatoid factor is present, further testing may be performed to correctly diagnose rheumatoid arthritis and thus rule out psoriatic arthritis.
- Aspiration. A small amount of fluid may be removed from the affected joint to rule out gout and other types of arthritis.
The following diseases can closely mimic psoriatic arthritis, so obtaining an accurate diagnosis is crucial to receiving appropriate and timely treatment:
- Ankylosing spondylitis
- Reactive arthritis (typically triggered by infection)
- Rheumatoid arthritis.
Patients with psoriatic arthritis can also develop additional symptoms of inflammation, including inflammation of the tendons and around the cartilage, inflammation of the chest wall, inflammation in and around the lungs, and inflammation of the colored portion of the eye.
The combination of medical history, physical exam, and possibly diagnostic testing, all inform the diagnosis of psoriatic arthritis.