Psoriatic arthritis (PsA) is an inflammatory disease that affects roughly one-third of all people who have the skin condition psoriasis. Doctors can 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 the person already exhibits skin symptoms consistent with psoriasis. However, if skin symptoms are absent, providers may need additional imaging (X-ray) and blood tests to confirm their diagnosis — and to rule out other potential causes.
This article will explore a few common blood tests doctors use to confirm a PsA diagnosis. Read on to see what these tests involve and how they can be used to diagnose the condition.
Diagnosing psoriatic arthritis can be a complicated process. In some cases, a doctor can make an initial diagnosis based on medical history and a physical exam. If a person already has the skin symptoms associated with psoriasis and begins experiencing joint pain, the odds that they have PsA are relatively high. But identifying psoriatic arthritis isn’t always simple. And while PsA usually starts about a decade after psoriasis, it has been found to appear before the skin condition in about 15 percent of cases. When this occurs, doctors will need to run blood tests to rule out other types of arthritis and inflammatory conditions before making a psoriatic arthritis diagnosis.
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When you attend your diagnostic appointment, your rheumatologist will likely draw some blood and conduct routine tests to make (or confirm) a PsA diagnosis. A few of the most common blood-based screenings are listed below.
Rheumatoid factor (RF) is a type of protein known as an autoantibody. Because RF targets the body’s own healthy tissues, elevated levels of the substance in the blood often indicate the presence of an autoimmune disease.
As its name suggests, a rheumatoid factor blood test is often used to screen for rheumatoid arthritis (RA). (While related, RA affects the joints alone, while PsA affects both the joints and skin.) During this test, your doctor will draw a small sample of blood — usually from a vein in your arm — and send it off for laboratory testing. Lab technicians will assess the levels of rheumatoid factor in your blood.
Positive RF test results indicate that your body is producing more rheumatoid factor antibodies than it normally would. Generally, the normal range for RF is between 0 IU/mL and 15 IU/mL. Mild elevation in the upper teens and 20s isn’t usually a cause for concern. However, significantly higher rheumatoid factor results may indicate unusual autoimmune activity.
While doctors frequently order rheumatoid factor blood tests to confirm RA, the test can also indicate other RF-elevating diseases, such as cancer, Sjogren’s syndrome, systemic lupus erythematosus (SLE), and sarcoidosis. That said, the test isn’t perfect. According to the The United Kingdom’s National Health Service, about one 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 rheumatoid factor.
If your doctor suspects you have psoriatic arthritis, they will order an RF test to rule out other conditions as the cause of your symptoms. People with PsA usually do not have elevated RF levels.
Like the RF test, an anti-cyclic citrullinated peptide (anti-CCP) blood test is used to rule out the presence of RA. Another autoantibody produced by the immune system, anti-CCP is very commonly found in people with rheumatoid arthritis. Generally speaking, a test result is considered to be positive if a person’s anti-CCP concentration is 20 u/mL or more.
Testing for anti-CCP may not catch all cases of rheumatoid arthritis, but those who receive 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 one in 10 people who receive positive results on their anti-CCP tests do not have rheumatoid arthritis. That said, it’s not a particularly sensitive test. Data indicates that up to one-third of people have RA, despite receiving a negative anti-CCP test.
If your anti-CCP test comes back positive, it’s more likely that your symptoms are caused by RA, not PsA. If it comes back negative, it’s more likely that your symptoms are due to PsA.
Unlike RF and anti-CCP blood tests, HLA-B27 blood tests are conducted to indicate the presence of spondylitis (inflammatory back pain) with PsA rather than to rule it out. These tests are designed to detect human leukocyte antigen B27 (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.
The leukocyte antigen B27 is a relatively rare — it is found in only 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, uveitis, inflammatory bowel disease, and psoriatic arthritis.
While a positive HLA-B27 test alone isn’t a firm confirmation that a person has psoriatic arthritis with spondylitic involvement, it can be used to strengthen a diagnosis of psoriatic arthritis with spondylitis.
Like RF and anti-CCP tests, erythrocyte sedimentation rate (ESR) blood tests can be used to assess the inflammatory activity in a person’s body and confirm the presence of certain autoimmune conditions.
During this test, your blood sample will be poured into a long, thin tube. Over the course of an hour, the red blood cells (erythrocytes) in the sample will gradually descend to the bottom of the tube. However, samples from people with inflammation, cells (known as “sediment”) tend to fall faster. This is because inflammation often causes blood cells to clump. At the end of the hour, lab technicians will measure the descent of the blood cells. The further the cells have fallen, the more suggestive the test is of an autoimmune condition.
According to the Mayo Clinic, a healthy sedimentation rate generally ranges from 0 to 22 mm/hour for males and 0 to 29 mm/hour for females. Around 40 percent of people with psoriatic arthritis exhibit elevated ESR rates. That said, a high ESR is very non-specific and can be caused by a wide variety of different conditions including RA, giant cell arteritis, polymyalgia rheumatica, and many others.
It is also important to note that some unrelated factors — such as older age, pregnancy, kidney problems, cancer, infection, anemia, and thyroid disease — can skew a person’s ESR results. Doctors will likely ask you if you have any conditions that could interfere with the test before ordering it and analyzing your results.
As its name suggests, this blood test measures the concentration of C-reactive protein (CRP) in your blood. While 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 exhibit elevated CRP levels.
While CRP can be elevated for many different reasons, doctors can use CRP testing to support a diagnosis of psoriatic arthritis. Medical professionals will also use this test to assess the severity of a person’s psoriasis and how well their current treatments are working. If a person’s CRP levels fall, for example, those results may be taken as a sign that a person’s inflammation is receding and their condition improving.
Testing for CRP alone can’t provide a definitive verdict on psoriatic arthritis. 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 a lack of exercise can all elevate a person’s CRP levels.
Once your doctor has conducted a physical exam, reviewed your medical history, and assessed the results of your blood tests, they will determine a diagnosis. If you are diagnosed with psoriatic arthritis, your doctor will work with you to help you manage your symptoms and avoid long-term joint damage.
Managing psoriatic arthritis can be challenging, especially if you don’t know anyone who shares your diagnosis. Navigating treatment or addressing new complications can be stressful and isolating — but it doesn’t need to be. On MyPsoriasisTeam, you can connect with more than 103,000 team members who know what it’s like to stand in your shoes. Join to build friendships, ask for advice, and share your tips with others.
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