What Tests Are Needed to Diagnose Psoriatic Arthritis?

There is no single test that can confirm psoriatic arthritis (PsA). The diagnosis is made through a series of tests. In addition to a physical exam and patient history, physicians use blood tests, imaging scans, and biopsies when diagnosing PsA. Because of the many variations in how PsA appears in different people, the diagnosis process can be extensive and at times challenging.^1,2

Blood tests

Rheumatoid arthritis also involves the joints and causes inflammation, although in rheumatoid arthritis, the joints are mostly affected symmetrically (same joints on both sides of the body). To distinguish between rheumatoid arthritis and PsA, physicians use a blood test that looks for rheumatoid factor, which is present in the majority of people with rheumatoid arthritis. While it is possible to have both rheumatoid arthritis and PsA, that is rare.¹

Gout is another condition that can cause swelling and pain in the joints in the feet, particularly in the joint of the big toe. Physicians use a serum uric acid test to determine if the joint swelling is gout. It is possible to have both gout and PsA.¹

Imaging

There are several imaging techniques that are used to view the joints and soft tissue around the joints:

• X-ray – Using a small amount of electromagnetic radiation, an x-ray can show erosive changes in the joints, which occur in PsA. A classic sign of PsA is the “pencil-in-cup” deformity, in which the bones of the digits collapse into each other. Other features of PsA seen on x-ray include new bone formation and bone resorption.3 
• Ultrasound – Ultrasound uses sound waves to produce an image and is useful in differentiating inflammation in the synovial capsule of the joint (synovitis) from the attachment points of the tendons or ligaments (enthesitis).³
• MRI (magnetic resonance imaging) – MRI uses powerful magnets and radio waves to create pictures of the interior structures of the body. MRI is standard to detect PsA in the spine and may also be used to view the peripheral joints. MRI can view the soft tissues and detect inflammatory changes in the synovial capsule and entheses.^1,3,4
• CT Scan (computed tomography) – CT scans uses special x-ray technology to create cross-sectional images of the body. Micro-CT has been used as a research instrument in PsA and can detect small bone lesions and measure the effectiveness of treatment. CT scan is not generally used in diagnosis of PsA in clinical practice.³

Biopsy

In a biopsy, a small amount of fluid or tissue is removed and examined under the microscope. In the diagnosis of PsA, physicians may use biopsy to test synovial tissue or skin lesions.

Synovial tissue is found between joints that are bound by a fibrous capsule whose inner membrane secretes a lubricating fluid, known as synovial fluid. These types of joints allow for a wide range of movement and are found in the wrist, elbow, shoulder, knee, and hip.⁵ In PsA, inflammation occurs due to an increase in immune system cells, particularly white blood cells (T-cells, B-cells, and some macrophages). ³ While there are no specific diagnostic tests for PsA, one abnormality that can be definitively tested is increased presence of lymphocytes in the synovial fluid.⁶

Because PsA most often occurs in people with psoriasis, skin biopsy of a lesion may be used to confirm psoriasis in cases where the person has not been previously diagnosed.¹