How Is Psoriatic Arthritis Diagnosed?

The most challenging part of getting a diagnosis for PsA is that there is no single definitive test that checks for psoriatic arthritis (PsA). A care team can use a combination of physical exams, blood tests, and imaging tests to diagnose PsA. The care team will include a rheumatologist (who specializes in the joints and bones) and a dermatologist (who specializes in the skin).

PsA is an autoimmune disorder, meaning your immune system mistakenly attacks healthy cells. Those with PsA develop symptoms like joint pain, inflamed skin patches, and swelling. PsA can progress quickly, so it's important to get tested early.

Medical History

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

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

Questions can also be specific to certain symptoms. A healthcare provider can ask you about the following: 

• Fatigue: Feeling tired is normal if you have PsA. About 22% of people with PsA have extreme fatigue with the condition.
• Joint pain: Most people with PsA experience joint stiffness. About half of all people with PsA also have morning soreness that can last for 30 minutes or more. Dactylitis (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.
• Skin concerns: About 30% of people with psoriasis develop PsA. It can take an average of 10 years or longer to show up.
  

Physical Exam

A healthcare provider will use a physical exam to assess the appearance and mobility of your joints. They may:

• Check for discoloration or raised patches on your skin.
• 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 (asymmetrical joint involvement).
• Measure your vitals (e.g., temperature, blood pressure, heart rate).
• Press on the skin around your joints to see if it's tender.
• Take a look at your joints, such as the hands, feet, elbows, and knees.  

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

Imaging Tests

A healthcare provider may order imaging tests to check what joints are affected if they suspect PsA. Common imaging tests include:

• 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.
• 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.
• Ultrasound: An ultrasound is a more accessible exam than other imaging tests. It does not need a lab and can be done at a healthcare provider's office. This test can show signs of inflammation, changes to your soft tissue, and joint and bone erosion. 
• X-ray: An X-ray is a radiographic imaging tool that takes detailed photos of your joints. It may be hard for an X-ray to detect any joint changes in the early stages. An X-ray can show signs of damage to the bones in the later stages.

Blood Tests

A 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:

• Anti-cyclic citrullinated peptide antibodies (Anti-CCP): This test measures the amount of cyclic citrullinated peptide (CCP) antibodies. This is another protein that attacks healthy joint cells. Research has shown 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. 
• C-reactive protein (CRP): The liver produces higher amounts of CRP when there's inflammation in the 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 like a common cold or diabetes. A result greater than 1.0 mg/dL can indicate infection, autoimmune disorder, or cancer.
• Erythrocyte sedimentation rate (ESR): This blood test checks for inflammation by examining how quickly red blood cells sink into your blood. The faster the ESR, the higher the amount of inflammation in the body.
• Human leukocyte antigen B27 (HLA-B27): This protein is found in white blood cells and is a genetic marker for PsA and other autoimmune disorders. This protein normally helps the immune system understand the difference between healthy and harmful cells. An abnormal test result for HLA-B27 can indicate an autoimmune disorder.
• Rheumatoid factor (RF): PsA and rheumatoid arthritis (RA) often share symptoms. A care team will use an RF test to rule out RA. This test measures the amount of RF (antibodies) in your immune system that attack healthy cells. A negative RF test means you have little or no RF, and a positive RF test indicates a high level of RF. It's normal for people with RA to have RF in their blood but much less so for those with PsA.

A care team may order a variety of blood tests to help confirm a PsA diagnosis. You may also be asked to come in for routine blood tests to check how you are responding to treatment. 

Diagnostic Criteria 

PsA was initially hard to diagnose because there were no standard criteria that everyone agreed upon. A global group of rheumatologists convened to publish the CASPAR criteria in 2006. This is the most commonly used set of diagnostic criteria for PsA.

These criteria have been used for diagnosis. They were actually developed as a case definition for inclusion in clinical trials, 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.

A Quick Review

A care team consisting of a primary healthcare provider, rheumatologist, and dermatologist will help diagnose PsA. These providers can help determine a diagnosis in several different ways, involving a physical exam, a person's medical history, and testing.

Testing can involve imaging tests like X-rays and ultrasounds, as well as blood tests that indicate the presence or levels of certain proteins or cells. You'll ultimately want to see a healthcare provider if you experience symptoms of PsA to confirm a diagnosis.