Diagnosing Ankylosing Spondylitis

There is no single test, so a good medical examination is needed

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The diagnosis of ankylosing spondylitis requires good detective work on the part of your doctor, as there is no gold standard test. Rather a doctor must consider a person's symptoms, physical examination, and medical tests like x-rays and blood work to make the diagnosis—similar to putting together the pieces of a puzzle.

A Medical History

A person with AS often first sees their primary care doctor or family doctor with a complaint about pain, usually in the back or upper buttock. To sort out the cause of this pain, a doctor may ask the following questions:

  • Are you younger than age 40?
  • Has the pain persisted for three or more months?
  • Does the pain improve with exercise?
  • Does the pain worsen with rest, especially noticeable at night?
  • Does the pain significantly improve after taking an ​NSAID? (for example, ibuprofen)

Answering yes to the majority of these questions will prompt your doctor to consider a diagnosis of an inflammatory arthritis (like ankylosing spondylitis), as opposed to the more common mechanical back pain (for example, muscle strain or degenerative disc disease).

Your doctor will also inquire about whole-body symptoms like fatigue or malaise, as this can further support a diagnosis of ankylosing spondylitis. This is because whole-body symptoms are potential clues that something abnormal is going on with your immune system. In anklyosing spondylitis, your immune system attacks healthy joints (especially the spinal and sacroiliac joints).

A family history of ankylosing spondylitis, or a personal history of gastrointestinal infections, inflammatory bowel disease, uveitis (inflammation of the colored part of your eye), or psoriasis are additional diagnostic clues.

If your primary care doctor or family doctor suspects ankylosing spondylitis or other immune-system mediated process, he or she will refer you to a rheumatologist—a doctor who specializes in joint and autoimmune diseases.

Physical Examination

During a physical examination for ankylosing spondylitis, your doctor will examine your spine, hip, and sacroiliac joints by pressing on them to check for tenderness. By moving your joints, your doctor can assess range of motion and flexibility. Your doctor will also likely examine the small joints of your hands and feet, as well as other joints like your elbows, knees, and shoulders, to determine if there is swelling, warmth, or fluid present.

Checking your heels for enthesitis, listening to your lungs, and performing a skin and scalp check (looking for psoriasis) are also common parts of a physical examination when a person is suspected of having ankylosing spondylitis.

Do not be surprised either if your doctor refers you to an eye doctor (an ophthalmologist) if you are having any eye pain or redness and/or blurry vision, as uveitis is a common complication of anklyosing spondylitis.

Imaging Tests

X-rays and sometimes magnetic resonance imaging (MRI) of the sacroiliac joints (and sometimes the spine) are generally used to confirm a diagnosis of ankylosing spondylitis. The problem with imaging tests is that inflammation can take years to show up, although MRI may detect it earlier than x-rays.

Imaging tests are also used to monitor a person's disease progression, after they have been diagnosed with ankylosing spondylitis.

HLA-B27 Blood Tests

The main blood test for diagnosing anklyosing spondylitis is the HLA-B27 test. HLA-B27 is a type of protein found on white blood cells (your infection fighting cells). When present, it can cause your white blood cells to attack healthy cells in your body, like those within your spine, neck, or eyes.

You may be surprised to learn that the protein HLA-B27 is present in about 95 percent of Caucasians with anklyosing spondylitis. That being said, of those who are positive for the HLA-B27 gene, only approximately 5 percent have ankylosing spondylitis.

In African-Americans with AS, the HLA-B27 gene is not nearly as common—so is not as good of a diagnostic test.

So if you are having symptoms suggestive of AS, your doctor may test you for the HLA-B27 protein to help confirm a diagnosis, especially if there is some uncertainty going on (maybe there are some confusing parts of your physical examination or history). But again, the presence of HLA-B27 is not a slam dunk "yes you have AS" test—all the pieces have to fit together. A positive test with no symptoms or signs of an autoimmune disease does not mean much. Conversely, some people (including Caucasians, albeit rarely) have AS, but test negative for the gene.

All in all, the interpretation of the HLA-B27 tests is tricky, and therefore requires careful analysis by an experienced doctor.

Other Blood Tests

Two other blood tests your doctor may order if he or she suspects ankylosing spondylitis include:

One or both of these markers may be elevated when there is active inflammation in the body.

That being said, CRP and ESR are nonspecific blood tests, meaning they can be elevated in a number of different health conditions besides anklyosing spondylitis. These include:

In addition, ESR and CRP can be normal, and a person can still have ankylosing spondylitis (especially if their disease is not flaring at the time of the blood draw).

A Word From Verywell

It is important to not get too bogged down on the details here. The big picture is that a diagnosis of ankylosing spondylitis requires a thoughtful analysis of many factors by your doctor, with the crux lying in a good medical history and physical examination. Blood tests and x-rays can then strengthen the diagnosis.

Be sure to ask your doctor about any questions pertaining to your diagnosis or personal test results. Taking an active role in your health can only help you.

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