How Is Psoriatic Arthritis Diagnosed?

Psoriatic arthritis (PsA) is an autoimmune condition, meaning your immune system attacks healthy cells in your body by mistake. Those who live with PsA develop symptoms such as joint pain, inflamed skin patches, and swelling.

Generally, PsA affects people who already have pre-existing psoriasis—a condition that causes raised and discolored patches on the skin. In some cases, however, you might experience joint symptoms first and skin concerns later.

PsA can progress quickly, so it’s important to get tested early. But, the most challenging part of getting a diagnosis for PsA is that there is no single definitive test that checks for PsA.

PsA can also mimic symptoms of other joint diseases (e.g., rheumatoid arthritis) or skin conditions (e.g., eczema). Your primary healthcare provider will likely work with a care team that will include a rheumatologist (a doctor who specializes in the joints and bones) and a dermatologist (a doctor who specializes in the skin) for proper testing. Your care team can use a combination of physical exams, blood tests, and imaging tests to give you an official diagnosis.  

Medical History

Your healthcare provider may use a written or oral medical history questionnaire to learn more about your background. They will likely ask questions such as:

• Do you or anyone in your family have psoriasis?
• Does anyone in your family have psoriatic arthritis? 
• What symptoms are you experiencing?
• How long have you had these symptoms?
• Have you had any infections or illnesses recently?

Questions can also be specific to certain symptoms. Your healthcare provider can ask you about: 

• Skin concerns: About 30% of people with psoriasis develop psoriatic arthritis—although it can take an average of 10 years or longer to show up.
• Joint pain: Most people with PsA experience joint stiffness. About half of all people with PsA also experience morning soreness that can last for 30 minutes or more. Dactylitis, or sausage like swelling of the fingers and toes, and enthesitis, inflammation where ligaments and tendons insert into bone, are often seen in people with psoriatic arthritis.
• Fatigue: If you have PsA, feeling tired is normal. About 22% of people with PsA experience extreme fatigue with the condition.

Physical Exam

After learning about your medical history, your healthcare provider will use a physical exam to assess the appearance and mobility of your joints. During the exam, they may:

• Measure your vitals (e.g., temperature, blood pressure, heart rate)
• Take a look at your joints, such as the hands, feet, elbows, and knees 
• Look for signs of inflammation or stiffness in your joints 
• Make a note if you have joint symptoms on one side of the body and not the other (known as asymmetrical joint involvement)
• Press on the skin around your joints to see if it’s tender 
• Check for discoloration or raised patches on your skin 

While PsA typically affects the larger joints, this condition can also cause inflammatory symptoms in other parts of the body. Your healthcare provider may also examine your fingers, toes, spine, nails, and eyes.

Imaging Tests

If your care team suspects signs of PsA, they may order imaging tests to check which joints are affected. Common imaging tests include:

• X-ray: An X-ray is a radiographic imaging tool that takes detailed photos of your joints. In the very early stages of PsA, it may be hard for an X-ray to detect any joint changes. In later stages, an X-ray can show signs of damage to the bones.  
• Magnetic resonance imaging (MRI): An MRI uses radio waves and magnets to make a 3D model of your joints. The images from an MRI can help your care team identify bone damage, joint inflammation, and changes to the tissue around the joints.
• Computer tomography (CT scan): A CT scan uses several X-rays placed at different angles to take a detailed and holistic image of your joints. This test helps depict changes in your bones and tissue.
• Ultrasound: Also known as a sonogram, an ultrasound is a more accessible exam than other imaging tests—as it does not need a lab and can be done at your provider’s office. This test can show signs of inflammation, changes to your soft tissue, and joint and bone erosion.    

Blood Tests

Your care team will use blood tests to check for certain proteins, antibodies, and cells in your blood. These tests can help them make a PsA diagnosis and rule out other conditions.

All of your blood tests can be done at the same appointment and multiple tests can be performed using the same sample of blood. Some common blood tests for PsA include:

• C-reactive protein (CRP): Your liver produces higher amounts of CRP when there’s inflammation in your body. A normal amount of this protein is less than 0.3 milligrams (mg) of CRP per deciliter (dL) of blood. Those with 0.3 to 1.0 mg/dL may have other conditions such as a common cold or diabetes. A result greater than 1.0 mg/dL can indication infection, autoimmune disease, or cancer.
• Erythrocyte sedimentation rate (ESR): This blood test checks for inflammation by examining how quick red blood cells sink into your blood. The faster the ESR, the higher the amount of inflammation in your body.
• Rheumatoid factor (RF): PsA and rheumatoid arthritis (RA) often share symptoms. Your care team will use an RF test to rule out RA. This test measures the amount of RF, or antibodies in your immune system that attack healthy cells and lead to RA symptoms. A negative RF test means you have little or no RF in your blood, while a positive RF test indicates a high level of RF in your blood. It is normal for people with RA to have RF in their bloodstream, but much less so for those with PsA. F can be seen in other diseases, such as lupus and Sjogren's syndrome.
• Anti-cyclic citrullinated peptide antibodies (Anti-CCP): This test measures the amount of cyclic citrullinated peptide (CCP) antibodies in your blood, which is another protein that attacks healthy joint cells. Research estimates that 8% to 16% of all people with PsA have high levels of this antibody, which can also mean that you have a more advanced stage of PsA. Although, this is also used for RA, and high titers of anti-CCP typically indicate an RA diagnosis. 
• Human leukocyte antigen B27 (HLA-B27): This protein is found in white blood cells and is a genetic marker for PsA and other autoimmune diseases. When in a normal range, this protein helps your immune system understand the difference between healthy and harmful cells. If you receive an abnormal test result for HLA-B27, it can indicate a presence of an autoimmune condition.

Your care team may order a variety of blood tests to help confirm a PsA diagnosis. If you have PsA, you may be asked to come in for routine blood testing so your care team can check how you are responding to treatment. 

Diagnostic Criteria 

Historically, PsA was hard to diagnose because healthcare providers there was no standard criteria that everyone agreed upon. In 2006, a global group of rheumatologists convened to publish the CASPAR criteria—the most commonly used diagnostic criteria for PsA today. While these criteria have been used for diagnosis, they were actually developed as a case-definition for inclusion in clinical trials, and not for diagnosis in everyday practice.

There are five categories in the CASPAR criteria. A total score of three or more points can indicate a PsA diagnosis.