Understanding Your Test Results

Reviewed by: HU Medical Review Board | Last reviewed: December 2022

Unfortunately, there is no single test to diagnose axial spondyloarthritis (axSpA). If your doctor thinks you might have axSpA, they will order blood and imaging tests. Your doctor will also do a physical exam and ask for a detailed medical history.1

Results of these tests can be complicated and may feel overwhelming. But the more you know about them, the more you can advocate for your own health.

Blood tests

Blood tests used to diagnose and monitor axSpA look for different markers that have been linked to axSpA.

HLA-B27 antigen test

The HLA-B27 antigen test looks for a protein called HLA-B27 on the surface of your cells. This protein is an example of a genetic marker. If you test positive for this marker, you may have an autoimmune disease.1,2

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In autoimmune disease, the body’s immune system cannot tell the difference between healthy cells and invaders like viruses, fungi, or bacteria. Because it cannot tell the difference, the body begins to attack and damage healthy cells.2

Radiographic axSpA (r-axSpA), also known as ankylosing spondylitis, is one of the autoimmune conditions linked to HLA-B27. Other autoimmune conditions linked to HLA-B27 include:1,2

  • Reactive arthritis
  • Psoriatic arthritis
  • Inflammatory bowel disease
  • Uveitis

It is important to remember that having the HLA-B27 genetic marker does not necessarily mean you have axSpA. Many people with the marker do not develop an autoimmune disease. The test is only used to help diagnose axSpA as part of a larger evaluation.1,2

A negative HLA-B27 test means that this genetic marker was not found. However, you can still have axSpA without the HLA-B27 marker. Your doctor will use other tests and a review of your medical history to determine whether you have axSpA.1,2

C-reactive protein (CRP)

The CRP test looks at the amount of CRP in your blood. CRP is a type of protein that rises during inflammation. A high level of CRP is considered a positive test.3

A positive CRP result may mean you have axSpA or another inflammatory condition. A positive CRP result is used to help diagnose axSpA when combined with other tests and exams.1,3

Erythrocyte sedimentation rate (ESR)

The ESR test measures the rate at which red blood cells settle in a tube over time. If they settle quickly, that is considered a positive ESR test. It indicates that you have inflammation in your body. But it does not mean that you have axSpA. You could have something else causing inflammation.1,4

Complete blood count (CBC)

The CBC test measures the number of red and white blood cells and platelets in your blood. A high number of white blood cells can occur when there is an infection or inflammation. A low number of red blood cells (anemia) can sometimes also be found in people with chronic inflammatory conditions like axSpA.5

X-ray and MRI imaging tests

The hallmark sign of ankylosing spondylitis (AS) is inflammation or fusion of the vertebrae where the spine connects to the hips. This part of the body is known as the sacroiliac (SI) joints. Inflammation in this area is called sacroiliitis.1,6

The problem with using an X-ray to diagnose AS is that it can take 7 to 10 years for signs of spinal damage to become visible on X-ray. Magnetic resonance imaging (MRI) may reveal changes in the spine or SI joints at an earlier stage of the disease than X-ray can.6

MRI is better than X-ray at finding early signs of inflammation or swelling in the tissues around the SI joint. But even if you have no evidence of changes to your spine or hips on an imaging test, you may still have axSpA.6

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