Diagnosing spondylitis usually requires a physical examination, a review of family and medical history, and imaging tests. Still, blood tests that assess inflammation and other risk factors can be important for establishing whether spondylitis is the most likely cause of chronic pain in the spine and sacroiliac joints (hips).
Spondylitis is also known as spondyloarthritis and axial spondylitis. Axial spondylitis may either be radiographic (visible on an X-ray, also known as ankylosing spondylitis, or AS) or nonradiographic (not visible on an X-ray).
If you’ve struggled to get a definitive diagnosis for spondylitis, it may help to better understand the roles of blood tests for inflammation, what different results mean, and how doctors may use them to confirm or rule out spondylitis.
Biomarkers are biological signs that can be objectively measured. Some biomarkers, such as blood pressure, can be measured with relatively simple tools that are part of a physical exam. Other biomarkers require more sophisticated analyses of blood chemistry or genetic markers. Biomarkers can provide valuable information for diagnosing diseases and developing treatments.
Diagnosing spondylitis can be difficult. An early diagnosis of spondylitis is associated with better outcomes. Symptoms can mimic other types of lower back pain, and spondylitis diagnoses are commonly delayed by five or 10 years. Some back pain is mechanical, caused by physical stress on joints, spine, or soft tissue. Spondylitis, however, is an autoimmune disease caused by inflammation, triggered by dysfunction in the immune system. Doctors treat mechanical and inflammatory back pain very differently, and having an accurate diagnosis is essential.
Although biomarkers in blood can help narrow down a diagnosis of spondylitis, there are currently no definitive blood tests for spondylitis. Nonetheless, some specific blood tests can point to inflammation that is associated with spondylitis and other inflammatory diseases, such as rheumatoid arthritis. The earlier a person receives an accurate spondylitis diagnosis, the sooner they can start to manage their symptoms and slow their disease progression.
There are three main blood tests that your health care team may recommend to assess biomarkers related to inflammation that may point to spondylitis. These tests include:
ESR is a blood test used to assess inflammation that may be related to autoimmune disease. People with inflammatory diseases like spondylitis produce proteins that are abnormal and cause erythrocytes (red blood cells) to clump together and sink in a test tube.
An ESR blood test measures how fast red blood cells sink in a blood sample. Red blood cells that have clumped together due to inflammation are denser and heavier than normal red blood cells. They’ll have a higher sedimentation rate and sink faster. This is known as sedimentation rate, or “sed rate” for short.
The normal sedimentation rate is 20 millimeters per hour, but the rate may be 150 millimeters per hour or higher in people with inflammation. Researchers have linked faster sedimentation rates to higher rates of inflammation, which may indicate higher disease activity.
ESR blood test results are only one indicator of inflammation, and these results can fluctuate over time. ESR tests show elevated rates in approximately 40 percent to 50 percent of cases of AS due to these fluctuations.
CRP is a protein that forms primarily in the liver and can increase as much as 1,000-fold during periods of infection or inflammation. Measuring CRP levels in the blood can help determine whether a person has chronic or acute inflammation.
The normal level for most healthy people is about 0.08 milligrams per deciliter. Higher numbers indicate more inflammation. Having a CRP level greater than 7.9 milligrams per liter is one criterion for a person to be eligible for biologic drugs. Imaging test results can also determine a person’s eligibility for these types of drugs.
CRP blood tests detect elevated CRP levels in only 40 percent to 50 percent of people with AS because inflammation fluctuates in people with the condition. A single blood test generally can’t detect high CRP levels, which is why some researchers recommend that people who show symptoms of spondylitis receive multiple CRP blood tests. Elevated CRP levels in people with spondylitis may indicate their disease activity is high and that they might benefit from biologic treatment.
Rheumatologists can also look at CRP levels to help predict treatment response. For instance, studies have found the biologic drug secukinumab (sold as Cosentryx) can be particularly effective for people with AS who have elevated levels of CRP.
An HLA-B27 blood test is a type of genetic testing rather than a test for inflammation. HLA-B27 is a gene associated with spondylitis and some other inflammatory conditions. Although people can have the gene and no sign of spondylitis, the presence of HLA-B27 alongside symptoms of spondylitis can strongly indicate a person has the condition.
In the United States, about 6 percent to 8 percent of people are estimated to have the HLA-B27 variant, and most of them never develop inflammatory conditions.
Researchers believe the presence of HLA-B27 contributes about 30 percent of inherited risk for AS. Approximately 80 percent to 90 percent of people with AS test positive for the HLA-B27 gene.
Although HLA-B27 is particularly linked to a diagnosis of AS, it’s also associated with other types of spondyloarthritis to a lesser degree. A doctor will recommend a blood test for HLA-B27 for someone they suspect has AS. However, like other blood tests for spondylitis, an HLA-B27 test is not definitive for a diagnosis. It is still possible to have AS and have a negative test result for HLA-B27.
People who test positive for HLA-B27 tend to develop AS earlier and have more AS in their family history. They also develop uveitis (a type of eye inflammation) at a higher rate, and have lower rates of psoriasis and inflammatory bowel disease than other people with AS.
Researchers are working to discover new biomarkers that can help clinicians diagnose spondylitis with more confidence in the early stages.
Some potential biomarkers may be found in cytokines and other inflammatory proteins present in higher levels in the blood of people with spondylitis. Other clues for new biomarkers may be found in antibodies (immune proteins) or substances that affect bones, cartilage, or soft tissue in people with spondylitis.
With new biomarkers, researchers hope to develop tools that improve diagnosis, treatment, and outcomes for people with spondylitis.
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