Diagnosing Ankylosing Spondylitis: 4 Things Doctors Look At

Part of diagnosing AS is ruling out other similar conditions.

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As with many rheumatoid conditions, there is no single, definitive test to diagnosis ankylosing spondylitis (AS), a chronic arthritis caused by inflammation of the spine. If someone is experiencing symptoms of AS, such as lower back pain and stiffness that’s worse in the morning, a rheumatologist will use a variety of exams to rule out similar conditions and get an accurate diagnosis.

“Once they develop these symptoms [of back pain], if there’s also accompanying complaints—like having loose stool, difficulty with eyesight, eye pain—the whole constellation might tip me off that they might have ankylosing spondylitis,” says Themistocles Protopsaltis, MD, spine surgeon at NYU Langone Hospital.   

Before making a diagnosis of AS, doctors consider four main exams: medical history, physical exam, imaging tests, and blood tests.

Medical History and Symptoms

The symptoms (and progression of symptoms) of AS can vary from person to person. The AS symptoms may also resemble symptoms of other types of arthritis, such as rheumatoid arthritis. Doctors will carefully consider your symptoms, and they’ll look for:

  • Pain and stiffness in the spine and back

  • Pain is “dull,” not localized or sharp

  • Pain and stiffness are worse in the morning or during inactivity

  • Possibly fatigue, fever, or loss of appetite caused by the immune response to inflammation

Doctors will also consider how the rest of your medical history matches up with your symptoms. They may look for a previous instance of eye inflammation or gastrointestinal problems (such as inflammatory bowel disease), as these are both common in people with AS, according to the Spondylitis Association of America.

Finally, doctors will want to know if you have a family history of AS. Since AS is genetic, it’s highly possible to have AS if a close family member has it as well.

Physical Exam

Doctors will check for evidence of inflammation, which would create tenderness around joints like the pelvic bones, sacroiliac joints, and rib cage. (The sacroiliac joints are where the spine connects with the pelvic bones.)

Additionally, doctors will want to know how long pain has occurred. The pain must be chronic—lasting more than three months—in order to be considered AS. The pain can be persistent or come and go throughout the day, but pain and stiffness caused by AS is typically worse in the morning and during inactivity.

Imaging Tests

If inflammation in the spine has occurred, it may appear on imagining tests, such as an X-ray or magnetic resonance imaging (MRI) scan. At earlier stages of the disease, an MRI might demonstrate changes in the spine or sacroiliac joints.

An X-ray may help in more advanced stages of AS. “When we look at an X-ray of the spine, we could see … ankylosis, which means ‘bridging bone,’ where there used to be a mobile disc,” says Dr. Protopsaltis. “The disc becomes calcified on the outer rim.”

On an X-ray, ankylosis, also called spinal fusion, takes on a trademark appearance which doctors refer to as “bamboo spine.” Instead of the typical individual vertebrae of a healthy spine, a spine with ankylosis “looks like one big, sweeping sheet of bone,” like a bamboo stalk, says Dr. Protopsaltis.

“The other place to look is the sacroiliac joints,” or where the spine meets the pelvis, says Dr. Protopsaltis. “An X-ray of the pelvis is part of the diagnostic criteria to look for inflammation.”

Blood Tests

Blood tests are useful for detecting genetic markers of AS and finding inflammation in the body.

To test for inflammation, doctors may use a C-reactive protein test (CRP), an erythrocyte sedimentation rate test (ESR), or a plasma viscosity test (PV). These tests may reveal abnormal levels of inflammation in someone with AS. However, only 30 to 40 percent of people with AS have enough inflammation for a blood test to detect, so normal results do not necessarily mean you don’t have AS.

Gene testing for AS can be useful but is not absolute. About 90 percent of caucasians with AS and 50 percent of African Americans with AS have the HLA-B27 gene, according to Spondylitis Association of America. However, there are people who have AS who do not have the HLA-B27 gene, and there are many people who have the gene but do not have AS.

The earlier you get diagnosed for AS, the earlier you can begin treatment. While there’s no cure, treatment for AS can help relieve pain and stiffness, lower inflammation, and reduce the risk of complications.

Maintaining a good relationship with your doctors, especially your rheumatologist, is crucial for treating and managing symptoms of AS.  “[Your rheumatologist] can serve as a gatekeeper, letting you know when medicines aren’t working, and when surgery might be necessary, and they can modify the regimen to make sure that your function is optimized and your symptoms are at bay,” says Dr. Protopsaltis.