Ankylosing Spondylitis Testing: How Healthcare Providers Make the Diagnosis

Ankylosing spondylitis (AS) is an autoimmune condition that mainly causes pain in the lower back and buttocks.  

The diagnosis of AS is often delayed for a number of reasons. AS develops slowly, so it may be months before symptoms are bothersome enough to see a healthcare provider. And since AS is rare, but back pain is common, it’s not typically the first thing that comes to mind. The testing is not straightforward, either.

But the more you understand about AS, the more likely you are to get the care that you need. 

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What is ankylosing spondylitis?

Ankylosing spondylitis (AS) is a rare type of arthritis that causes inflammation in the spine and joints, as well as other parts of the body. You might also hear it called spondyloarthropathy or spondyloarthritis. It affects about 0.5% of adults in the United States. It’s an autoimmune condition, which means that the immune system (the body’s army) gets confused and causes too much inflammation.  

Experts aren’t exactly sure what causes AS, but they know that genes and environment play a role. In severe cases, it can cause stiffness and loss of flexibility in the spine, changes in posture, and chronic pain. Fortunately, AS is treatable.

When should I be tested for ankylosing spondylitis?

It’s a good idea to talk with your healthcare provider about testing for AS if you have symptoms of inflammatory back pain. Inflammatory back pain may affect the neck, mid-back, low back, and/or buttocks (SI joints). Typically, inflammatory back pain: 

• Starts before the age of 45 and develops over the course of months to years

• Wakes you up in the middle of the night

• Is worst in the morning

• Is associated with significant stiffness and decreased flexibility in the spine

• Gets better when you’re up and moving, and worse with rest

• Gets better with nonsteroidal anti-inflammatory drugs (NSAIDs) like Advil or Aleve

If your back pain has some but not all of these qualities, it could still be inflammatory back pain. Let your healthcare provider know what’s going on so they can help you figure out if you need more testing. 

It’s also a good idea to talk with your healthcare provider about AS if you have low back pain and any of the following:

• Painful, swollen joints (inflammatory arthritis)

• Psoriasis or psoriatic arthritis

• Inflammatory bowel disease (Crohn’s disease or ulcerative colitis)

• Iritis (uveitis)

• One finger or toe that’s swollen and painful like a sausage (dactylitis)

• Pain at the back of the heel (enthesitis)

How is ankylosing spondylitis diagnosed?

To diagnose AS, healthcare providers use clues from your:

• Symptoms: Healthcare providers will look at signs like the ones listed above.

• Personal medical history: Certain medical conditions like uveitis increase the likelihood of having AS.

• Family medical history: A first-degree relative with AS increases the likelihood of having the condition. 

• Physical examination: Your healthcare provider will look for swollen, tender joints. They’ll also look for decreased flexibility in the spine. However, sometimes the physical exam yields normal results.

• Tests: Your healthcare provider will order some tests, such as blood and imaging tests. We go into detail about these tests below.

The following tests can be helpful when trying to diagnose AS. But it’s important to remember that none of these confirm or rule out the diagnosis completely.

Blood tests

There are two types of blood tests that can help diagnose AS:

• Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP): When elevated, these tests point to inflammation in the body. AS can increase ESR and CRP, but so can a lot of other things (like infection). So, elevated ESR and CRP values don’t automatically mean it’s AS. Some people with AS have normal ESR and CRP values, so negative tests don’t rule it out. 

• HLA-B27: This is a gene that increases the risk of developing AS. But, a positive HLA-B27 test doesn’t automatically mean you have AS either. It’s found in up to 8% of people in the United States, and only about 5% to 10% of those people will get AS one day. It’s also possible to have AS without the HLA-B27 gene, so a negative test doesn’t rule it out. 

Imaging tests

There are a few different imaging tests that can point toward AS. These include:

• X-rays of the SI joints or spine: X-rays may show inflammation of the SI joints (sacroiliitis), or abnormal bridges of bone in the spine (syndesmophytes). 

• MRI of the SI joints or spine: If X-rays are normal, but suspicion is still high for AS, your healthcare provider might order an MRI to take a closer look. These can give more details about the bone and surrounding tissues.

Abnormal X-rays and MRIs increase the chance of AS, but aren’t enough to diagnose it. That’s because other conditions can cause similar abnormal findings. And some people with AS have normal X-rays and MRIs. This is called non-radiographic axial spondyloarthritis.

What kind of doctor should you see for ankylosing spondylitis?

Your primary care provider may order tests to help diagnose AS. But a rheumatologist (specialist in autoimmune and joint diseases) typically confirms the diagnosis and treats the condition. 

Is ankylosing spondylitis a serious condition?

Sometimes. AS is different for every person who has it, and some people experience more severe symptoms than others. 

Research shows that when the diagnosis of AS is delayed, symptoms can be worse, and treatments might not work as quickly or effectively as they do in people who receive an early diagnosis. So if you’re worried about AS, let your healthcare provider know.

Is ankylosing spondylitis curable?

Not yet. There’s currently no cure for AS, but doctors and scientists are working hard to find one. The good news? AS is now a very treatable condition. There’s a lot that can be done to improve your symptoms and quality of life.

Why trust our experts?

Written by:

Samantha C. Shapiro, MD

Samantha Shapiro, MD, is a board-certified rheumatologist and internist with expertise in autoimmune and inflammatory conditions. She founded the division of rheumatology at Dell Medical School at The University of Texas at Austin.

Edited by:

Katie E. Golden, MD

Katie E. Golden, MD, is a board-certified emergency medicine physician and a medical editor at GoodRx.

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