Blood Tests to Diagnose Axial Spondyloarthritis
Diagnosing AxSpA spectrum conditions can be challenging. A variety of symptoms may be present, with or without visible changes on imaging tests. It’s important to have a thorough diagnostic process that includes a variety of things, including a medical history, physical examination, and if need be, various tests.
Tests can be helpful in confirming a diagnosis or providing more information about a condition. It is possible to have inconclusive results from tests and still be diagnosed with an AxSpA spectrum condition based on a medical history and physical examination. While tests can provide more information, they are not the only thing a diagnosis should be based upon.
Blood tests are used in the diagnostic process of AxSpA spectrum conditions, but should not be used as a singular evaluation for AxSpA spectrum conditions. Additionally, blood tests may be ordered to rule out other conditions that can cause joint pain such as rheumatoid arthritis, gout or lupus. Prior to prescribing medications for AxSpA, physicians may order blood tests to rule out infections including hepatitis and tuberculosis.
What are the blood tests for axial spondyloarthritis?
When blood tests are ordered to aid in diagnosing an AxSpA spectrum condition, the doctor will be looking for certain markers. Some of these markers can include genetic markers or markers for inflammation like certain proteins. The presence or absence of these markers does not necessarily mean you do or do not definitively have an AxSpA spectrum condition, but it can aid in confirming or helping to rule out a diagnosis.1,2
Genetic markers in blood tests
Many AxSpA spectrum conditions, particularly ankylosing spondylitis (AS), are associated with a specific human leukocyte antigen (HLA) gene called HLA-B27.1,3 HLAs help the immune system figure out which cells are normal and healthy, and which cells are harmful. There are nearly 30 genes that have been found to be associated with AS and AxSpA spectrum conditions, but HLA-B27 is by far the most common.1,3 More than 90 percent of people with AxSpA spectrum conditions have the HLA-B27 gene.1
Merely having the gene does not mean you definitely have or will develop a condition on the AxSpA spectrum, though. It just means you’re at higher risk than someone who does not have the gene. Conversely, not having the gene doesn’t mean you don’t have or can’t develop a condition on the AxSpA spectrum. Approximately 10 percent of people with AxSpA spectrum conditions have a negative blood test for the gene HLA-B27.1
Inflammation markers in blood tests
When part of your body is inflamed, extra proteins are released. These proteins are released from the part of the body that is inflamed, and then go into the bloodstream. Given that AxSpA spectrum conditions are inflammatory conditions, it makes sense to check for inflammation markers. Simply having inflammation markers on a blood test does not confirm or deny an AxSpA spectrum diagnosis. In approximately half of all AxSpA spectrum diagnoses, inflammation markers are within the normal range.1
Some of the inflammation markers tested for include erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and plasma viscosity (PV).
The ESR test, also called sed rate, measures how far red blood cells fall in a test tube in one hour.4 The farther they have fallen, the higher the inflammatory response in the body.4 The test does not diagnose what is causing the inflammation, it just provides an answer as to whether there is inflammation and how much.
When there is inflammation in the body, increased levels of C-reactive protein (CRP) occur. The CRP test measures the level of CRP in the blood. The high-sensitivity CRP test, or hs-CRP test, can also measure your risk of developing coronary artery disease, or narrowing of the arteries of your heart.5
In a plasma viscosity (PV) blood test, it measures many of the same things as an ESR test. It tests the thickness of the blood, in plainest terms. This can indirectly detect inflammation.6 It is not used as often as the other two tests.
When evaluating you for an AxSpA spectrum condition, your doctor might order one or all of these blood tests. They should be used in conjunction with other evaluation measures to provide as much information as possible for a possible diagnosis.