Psoriatic arthritis (PsA) is a chronic inflammatory disease that causes joint pain, swelling, and stiffness. Up to 30 percent of people with psoriasis may develop PsA as well. PsA is diagnosed using a range of clinical tools, including medical history, physical examination, blood tests, imaging studies, and in some cases, a biopsy of synovial fluid from inflamed joints.
Although no single test or guideline can confirm a diagnosis of psoriatic arthritis, these tests allow dermatologists and rheumatologists to assess the presence of arthritis, the type and severity of the disease, and the appropriate treatment.
Skin psoriasis preceded a psoriatic arthritis diagnosis in nearly 80 percent of people with the joint disease, according to a 2014 study. Because symptoms of psoriatic arthritis mimic those of other inflammatory arthritic diseases — such as rheumatoid arthritis (RA), osteoarthritis, ankylosing spondylitis (AS), and gout — diagnosis can be delayed up to 12 years after the onset of psoriasis.
Dermatologists and rheumatologists typically diagnose and treat psoriatic arthritis. They start by taking a medical history that identifies any family members with psoriasis or autoimmune diseases. Approximately 40 percent of people with PsA have at least one close family member with psoriasis or psoriatic arthritis.
A physical exam will follow to check the skin, nails, and joints for possible symptoms of psoriatic arthritis or psoriasis. Your doctor may ask about when symptoms first started, how often they appear, the severity of symptoms, and any factors that make them better or worse.
Having active psoriasis can make it easier to diagnose psoriatic arthritis. Nail changes (pitting, crumbling, or ridging of nails, including separation from the nail bed) are an early sign of PsA. Your doctor will also look for swollen fingers or toes (dactylitis or “sausage digits”), enthesitis (pain and swelling of tendons and ligaments where they connect to bone), and other hallmark symptoms of psoriatic arthritis.
The physician will also evaluate the condition of your joints by lightly pressing on areas to check for pain, tenderness, swelling, and warmth. You may also be asked to perform simple physical activities that demonstrate range of motion, stiffness, and overall mobility.
In the absence of specific PsA markers, a range of blood and imaging tests are used to detect inflammation, rule out other types of arthritis, and evaluate bone damage. Several types of blood tests may be ordered to measure levels of inflammation.
This blood test is routinely used to diagnose or rule out rheumatoid arthritis. It measures levels of a protein that causes the immune system to mistakenly attack healthy tissues. People with PsA are typically RF-negative, so a positive rheumatoid factor test would suggest rheumatoid arthritis instead. However low levels of RF may also be present in the blood, indicating either RA or — rarely — the presence of both conditions.
A CRP test measures the amount of C-reactive protein made by the liver that’s released into the bloodstream. High CRP levels indicate inflammation. A CRP test does not confirm a diagnosis, but it provides more information to make one — in conjunction with other blood tests and imaging studies.
An ESR test detects levels of inflammation by measuring the rate at which red blood cells (erythrocytes) fall or settle in a tall, vertical tube. The more red blood cells appear at the bottom of the tube (sedimentation) in one hour, the higher the inflammation level. The presence of CRP and other antibodies in the blood cause these cells to settle faster. Like the CRP test, ESR alone doesn’t diagnose a specific illness.
Anti-CCP is another type of protein that attacks healthy tissues in the body. An anti-CCP test is primarily used to diagnose or rule out RA. In one study, 17.5 percent of people with psoriatic arthritis tested positive for anti-CCP, which may indicate active disease in the body.
An emerging biomarker of joint damage in both RA and PsA, 14-3-3 eta is a protein found in the central nervous system and synovial joint tissue. Studies confirm that some people with PsA test positive for 14-3-3. This test is usually performed to rule out RA in combination with tests for RF and CCP markers.
HLA-B27 is a gene associated with several rheumatic diseases. A blood test for this marker is sometimes used to diagnose PsA in people with a family history of psoriatic disease. Testing positive suggests a higher risk for developing spinal involvement with PsA or ankylosing spondylitis (AS), a related condition that affects joints in the spine. More than 50 percent of people with an inflamed spine test positive for the gene. The gene doesn’t cause the disease, but may predispose those with PsA to future spinal involvement.
Diagnostic images are frequently used to confirm a psoriatic arthritis diagnosis. X-rays detect joint damage while magnetic resonance imaging (MRI), ultrasound, or CT scans provide a closer look at soft tissue in the joints.
Standard X-rays are most effective at diagnosing later-stage PsA. They can detect severe bone changes — such as the “pencil-in-cup” phenomenon — that distinguish PsA from other rheumatic diseases. This classic symptom occurs when one end of a bone has eroded to a pencil-point shape at the joint. It’s typically a sign of severe joint damage that could dictate more aggressive treatment.
Ultrasound uses sound waves to capture images inside the body. It has proven effective in detecting enthesitis, a hallmark symptom of PsA, even before a person experiences pain or tenderness where tendons attach to bone. This imaging technique is also helpful in differentiating synovial inflammation from other forms of arthritis.
Magnetic resonance imaging uses large magnets and radio waves to create images of organs and structures inside the body, such as soft tissues that may be damaged by enthesitis. Studies have shown MRIs are effective in detecting cases of active PsA.
To further explore a diagnosis of psoriatic arthritis, your doctor may want to study a sample of synovial fluid from an affected joint or take a skin sample to confirm the presence of psoriasis. Psoriasis on the skin is a strong indicator that arthritic symptoms could be PsA.
Arthrocentesis is an office procedure used to rule out some forms of arthritis during the diagnostic process. Synovial fluid in the joints is collected with a needle and syringe, and studied for the presence of immune cells (including white blood cells) that cause inflammation. The fluid may also be tested for serum uric acid, which indicates the presence of gout. People with psoriatic disease are at high risk for gout, which is caused by a buildup of uric acid crystals in foot joints.
To confirm the presence of psoriasis and rule out other skin conditions, such as eczema, your doctor may perform a "punch" biopsy. A pencil-shaped device is used to remove a small tissue sample for examination under a microscope. The incision is then closed with a couple of stitches.
The Classification Criteria for Psoriatic Arthritis (CASPAR) are often used to define PsA in clinical trials. Many rheumatologists also rely on the criteria to make an accurate and conclusive diagnosis of PsA. The criteria require that a person already have some form of inflammatory arthritis and at least three points from the following list:
PsA can strike at any age, but it occurs most frequently between the ages of 30 and 50 in adults and 11 and 12 years of age in children. Symptoms typically appear 10 years after onset of psoriasis. PsA affects men and women equally.
Although there is currently no cure for PsA, a growing range of treatments can help prevent the disease from spreading and destroying joints. Early diagnosis of PsA is important because permanent joint damage can occur within the first two years after onset, with the number of affected joints increasing over time. Studies support early diagnosis and treatment to improve long-term outcomes.