Psoriatic arthritis (PsA) is an inflammatory disease that affects roughly one-third of all people who have the skin condition called psoriasis. Doctors typically make a preliminary diagnosis of PsA based on a person’s medical history and a physical exam — that is, they can do so if skin symptoms consistent with psoriasis are visible. Otherwise, providers may need additional imaging (X-ray, MRI, or ultrasound) and blood work to confirm a PsA diagnosis — and to rule out other potential causes.

This article will explore a few common blood tests doctors use to look for PsA. Read on to see what these tests involve and how they can help in diagnosing the condition.

Blood Tests for Psoriatic Arthritis

Diagnosing PsA can be complicated. In some cases, a doctor can make an initial diagnosis based on a person’s medical history and a physical exam. For example, if you already have the skin symptoms associated with psoriasis and begin experiencing joint pain, swelling and stiffness, or other symptoms like uveitis (red, painful eyes), your odds of having PsA are relatively high.

But identifying PsA isn’t always simple. Although PsA usually starts about a decade after psoriasis, it appears before the skin condition 10 percent to 15 percent of the time, according to the Arthritis Foundation. In those cases, doctors need to perform blood tests to rule out other types of arthritis and inflammatory conditions before making a PsA diagnosis. A few of the most common labs for PsA are described below.

1. Rheumatoid Factor Blood Test

Rheumatoid factor (RF) is a type of protein known as an autoantibody. Because RF targets the body’s own healthy tissues, elevated RF levels in the blood often indicate the presence of an autoimmune disease.

As its name suggests, an RF blood test is often used to screen for rheumatoid arthritis (RA). (Rheumatoid arthritis affects only the joints, whereas PsA affects both the joints and skin.) Lab technicians will assess the levels of RF in your blood sample.

What Does a Positive RF Blood Test Result Mean?

Positive RF test results indicate that your body is producing more RF antibodies than it normally would. Generally, the normal range for RF ranges from zero to 15 international units per milliliter of blood. Mild elevation can appear in 4 percent of healthy people, but significantly higher RF results may indicate unusual autoimmune activity.

Doctors frequently order RF blood tests to confirm rheumatoid arthritis. However, the test can also indicate other RF-elevating diseases, including cancer, chronic infections (hepatitis B or C), or other autoinflammatory diseases such as Sjӧgren’s syndrome, systemic lupus erythematosus (the most common form of lupus), and sarcoidosis. That said, the test isn’t perfect — about 1 in 20 people without RA still receive positive RF results. Some may have one of the diseases mentioned above, while others may be healthy and simply have a high RF.

If your doctor suspects you have PsA, they’ll order an RF test to rule out other conditions as the cause of your symptoms. People with PsA don’t usually have elevated RF levels.

2. Anti-Cyclic Citrullinated Peptide Blood Test

Like the RF test, an anti-cyclic citrullinated peptide (anti-CCP) blood test can rule out RA. Another autoantibody produced by the immune system, anti-CCP is very commonly found in people with RA. Generally speaking, a test result is considered positive if the anti-CCP concentration is at least 20 units per milliliter.

What Do Your Anti-CCP Test Results Mean?

Testing for anti-CCP may not catch all cases of RA, but those who get a positive test result are very likely to have the condition. Researchers suggest that the test is over 90 percent specific. In other words, fewer than 1 in 10 people who receive positive results on their anti-CCP tests don’t have RA. Still, it’s not a particularly sensitive test. Data indicates that up to one-third of people have RA despite receiving negative anti-CCP test results.

If your anti-CCP test result comes back positive, it’s more likely that your symptoms are caused by RA, not PsA. If it comes back negative, your symptoms are more likely due to PsA.

3. Human Leukocyte Antigen B27 Blood Test

Unlike RF and anti-CCP blood tests, human leukocyte antigen B27 (HLA-B27) blood tests are done to look for the presence of spondylitis (inflammatory back pain) with PsA rather than to rule out PsA. These tests detect HLA-B27, a protein that lives on white blood cells. Human leukocyte antigens help the immune system differentiate between its own cells and foreign material.

What Can HLA-B27 Put You at Risk For?

The leukocyte antigen B27 is relatively rare — it’s found in only about 6 percent of the U.S. population. Those who do have it tend to be more likely to develop autoimmune conditions, such as ankylosing spondylitis, reactive arthritis, anterior uveitis, and PsA. Inflammatory bowel disease can also be linked to the presence of HLA-B27.

A positive HLA-B27 test result alone isn’t a firm confirmation of PsA with back and spine involvement, but it can strengthen a diagnosis of PsA with spondylitis.

4. Sedimentation Rate Blood Test

Erythrocyte sedimentation rate (ESR) is a common blood test that assesses inflammatory activity in the body and can indicate the presence of an autoimmune condition.

During this test, your blood sample is poured into a long, thin tube. Over the course of an hour, red blood cells (erythrocytes) gradually descend to the bottom of the tube. However, in samples from people with inflammation, cells (known as “sediment”) tend to fall faster because inflammation often causes them to clump. At the end of the hour, lab technicians will measure the descent of the blood cells. The farther the cells fell, the more the test suggests an autoimmune condition.

According to Mayo Clinic, a healthy sedimentation rate generally ranges from zero to 22 millimeters per hour for males and zero to 29 millimeters an hour for females. Around 40 percent of people with PsA have elevated ESR rates. However, a high ESR is very nonspecific and can be caused by a wide variety of conditions, including RA, giant cell arteritis, and polymyalgia rheumatica.

It’s also important to note that some unrelated factors — such as older age, pregnancy, kidney problems, cancer, infection, anemia, and thyroid disease — can skew ESR results. Doctors will likely ask if you have any conditions that could interfere with the test before ordering it and analyzing your results.

5. C-Reactive Protein Blood Test

As its name suggests, this blood test measures the concentration of C-reactive protein (CRP) in your blood. Although healthy people will have some amount of the protein in their blood, the liver produces CRP in response to inflammation. People with autoimmune or inflammatory conditions often have elevated CRP levels.

What Does a Positive CRP Test Result Mean?

CRP can be elevated for many reasons, but doctors can use CRP testing to support a diagnosis of PsA. Medical professionals also use this test to determine the severity of psoriasis and how well current treatments are working. If CRP levels fall, for example, those results may be taken as a sign that inflammation is receding and the condition is improving.

Testing for CRP alone can’t provide a final verdict on PsA. Because the test indicates the presence of inflammation, it could signal any number of autoimmune or inflammatory diagnoses. Or it might not indicate an underlying condition at all — factors such as obesity, smoking, and lack of exercise can all elevate CRP levels.

Once your doctor has conducted a physical exam, reviewed your medical history, and assessed the results of your blood tests for PsA, they’ll determine a diagnosis. If you’re diagnosed with PsA, your doctor will work with you to help manage your symptoms and avoid long-term joint damage.

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