Let's Talk About Ankylosing Spondylitis Tests and Diagnosis

Know anyone with back problems? Just about everyone, right? That’s one reason why AS can be difficult to diagnose. We’re here to help speed up the process.

by Stephanie Wood Health Writer

First, the good news: An ankylosing spondylitis (AS) diagnosis doesn’t require a lot of invasive tests or drinking chalky liquids—yay! The not-so-good: It can literally take a decade to confirm what’s going on. Studies show the average delay before getting a diagnosis of AS is anywhere from eight to 14 years. It's doesn't help that the number-one symptom—back pain—is so darn common. Also not helpful: AS can mimic other chronic illnesses, which must be ruled out. We’re going to walk you through the tests and imaging work required to get the answers and the treatment you’ll need to feel better.

AS Tests and Diagnosis

Our Pro Panel

We went to some of the nation’s top experts on AS to bring you the most up-to-date information possible.

Anca Askanase, M.D. headshot.

Anca Askanase, M.D.

Rheumatologist, Director of Rheumatology Clinical Trials

Columbia University Medical Center

New York City

Howard Blumstein, M.D. headshot.

Howard Blumstein, M.D.

Rheumatologist, Clinical Professor of Medicine

Stony Brook University

Smithtown, NY

Jonathan Greer, M.D. headshot.

Jonathan Greer, M.D.

Rheumatologist, Assistant Clinical Professor of Medicine

University of Miami

Palm Beach, FL

Ankylosing Spondylitis Tests and Diagnosis
Frequently Asked Questions
What should I do if my X-rays are negative?

If you have symptoms of AS but there is no sign of damage on an X-ray, don’t give up! Request that your rheumatologist do an MRI, which can show tissue inflammation that may confirm the diagnosis. If he or she balks, get another opinion from a rheumatologist with specific expertise in AS. The Spondylitis Association of America has a patient-recommended directory of rheumatologists to help you find one.

What is my prognosis?

There is no cure for AS, but there are many effective ways to manage it including exercise, NSAIDs, and biologic medications. Many people have only mild to moderate symptoms that never progress to the ankylosis (bone-fusing) stage. In general, men, African Americans, and people who are diagnosed at younger ages tend to have more severe disease activity.

Will my child have AS?

We know there is a strong hereditary link, with the vast majority of Caucasian people with AS testing positive for the HLA-B27 genetic marker. Still, the chances of you passing it on to your child are relatively low. There is about a 50% chance that the child of an HLA-B27-positive parent with AS will inherit the gene, but only a very small percentage of those offspring will develop the disorder.

Will lifestyle changes slow my disease progression?

Yes! There are things you can do to help keep AS at bay. Exercising regularly is the most critical lifestyle habit to keep your spine and other joints flexible. There’s not a lot of research about the role of specific foods or diets, but we do know obese patients have more severe symptoms, less physical function, and lower response rates to some biologics, so keeping your weight in check is important. And, smoking is associated with higher disease activity and radiographic progression. So no ifs, ands, or butts: Talk to your doctor about quitting cigarettes today.

What Is Ankylosing Spondylitis, Again?

Let’s refresh: Ankylosing spondylitis (pronounced ank-eye-low-sing spon-dill-eye-tiss) is a form of inflammatory arthritis that usually first affects the lower back, especially the sacroiliac joint that connects your spine to your pelvis. From there, AS can travel up the vertebrae, and other joints can become inflamed as well, including the shoulders, hips, ribs, heels, and small joints of the hands and feet.

But unlike acute back pain, which typically resolves within weeks, AS symptoms last for at least three months at a time. In the worst-case scenario, this chronic inflammation can lead to ankylosis—new bone formation in the spine—causing sections of the spine to fuse in a fixed, immobile position. (This process takes years, and new treatments are hugely effective at preventing it, so don't stress too much about this possibility yet.)

Other must-knows: AS is a type of autoimmune disorder that can have a body-wide impact. In fact, people with AS have a higher risk for certain inflammatory eye conditions as well as inflammatory bowel disease (IBD), particularly Crohn's. Sometimes the heart and lungs can also be affected. So yes, AS is a serious condition, and while there's no cure, it is very treatable, especially when diagnosed early. Here's how to make that happen.

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How Is AS Diagnosed?

Too often, AS gets diagnosed by trial and error. But that's because it's not necessarily the first thing that comes to a doctor's mind (let alone the person having the symptoms). After all, a lot of people...8 in 10 Americans a lot...experience back pain at some point in their lives, and when the complaint is coming from a young, active person, it may not raise much suspicion in a primary-care doctor.

Further complicating the situation is the fact that the peripheral joint pain that sometimes occurs with AS (such as in the shoulders, knees, or heels) is also a symptom of other inflammatory illnesses. And in some patients, especially women, that peripheral pain is their chief complaint—not their back. Because of that, men tend to be diagnosed with AS sooner, while women are more often misdiagnosed with fibromyalgia or even psychosomatic—“it’s all in your head”—disorders. As a result, women who are finally diagnosed may have more advanced stages of the disease when they begin treatment.

To make sure a delay in diagnosis doesn't happen to you, if you have joint/back pain that doesn't go away, it's important to get to a rheumatologist sooner rather than later in the process. A rheumatologist is a medical doctor who treats musculoskeletal disease and systemic autoimmune conditions that affect the joints, muscles, and bones. Ask your primary-care physician for a referral or check out the Spondylitis Association of America’s patient-recommended directory of rheumatologists.

As with most other autoimmune conditions, there is no one test that can clinch an AS diagnosis in the early stages. A thorough physical exam, imaging tests including X-rays and possibly an MRI (magnetic resonance imaging), your individual as well as family medical histories, and blood work will all be used in making a diagnosis.

The First Questions Your Doctor Will Likely Ask Are:

  • At what age did your symptoms start? Were they before age 40?

  • Have your symptoms lasted for three months or longer?

  • Are your symptoms worse in the morning when you first wake up? Do they last at least 30 minutes?

  • Do your symptoms also get worse with inactivity, such as when you’ve been working at a desk for a while?

  • Do your symptoms get better with exercise?

  • Do you have pain in other areas in addition to the lower back and hips, such as the mid- and upper back, neck, shoulders, ribcage? How about in your knees, feet, and ankles?

Your doctor will also want to know about:

  • A family history of relatives with AS, or AS-like back pain

  • Family history of IBS (such as Crohn’s disease or ulcerative colitis)

  • Any history of eye inflammation

  • Any history of gastrointestinal issues

  • How much fatigue you’ve been experiencing

The Physical Exam

Be prepared for more than blood pressure cuffs and stethoscopes. Your doctor will do a number of tests to look for inflammation and check your range of motion in your spine and other joints. This will likely include:

  • Physically feeling for tenderness by applying pressure (called touching) along the back, pelvic bones, sacroiliac joints, chest, and heels

  • Asking you to move in different directions, including bending forward and touching your toes without bending the knees, to see how flexible your spine is

  • Having you stand with your back flat against the wall to measure how flexible your spine and neck are in different areas

  • Asking you to breathe deeply to check for stiffness in the ribs and see how far your chest can expand when you exhale

Blood Tests for AS

Your doctor will order blood work to check for certain genetic and inflammatory markers that can help give him a clearer picture of your risk for AS. They are:

  • HLA-B27 gene: This gene is a strong indicator of AS in Caucasians of European descent. About 90% of people with AS from this group will have it. About 60% of AS patients from other ethnic groups have the gene. This genetic marker is not required for a diagnosis but having it is one more indicator to add to the evidence. Testing positive for HLA-B27 may also make it easier to get further testing, such as an MRI when an X-ray is inconclusive. Most patients who are positive for HLA-B27 will not have AS. It's a test with high sensitivity (catches everyone who might have AS) but low specificity (just because you have HLA-B27 does not mean you will have AS).

  • Erythrocyte sedimentation rate (ESR): ESR measure the rate at which red blood cells settle out of your blood; it tends to be higher when there's inflammation in the body.

  • C-reactive protein levels: Like ESR, your CRP levels tend to go up when you're battling inflammation.

  • Rheumatoid factor: This test looks for an antibody associated with rheumatoid arthritis. If you test positive for RF, you probably do not have ankylosing spondylitis.

  • Anti-nuclear antibodies: These occur in cases of lupus, another inflammatory condition which has similar symptoms to AS. Test positive, and AS can usually be ruled out.

Imaging Tests for AS

There are two types of imaging tests that are used to check for erosion in the SI (sacroiliac) joints—the hallmark of an AS diagnosis. Without this visible proof, you can’t technically have AS (yet). Unfortunately, both tests have their limitations. Here’s why:

  • X-rays: A conventional X-ray is the first-line imaging test. Your doctor will look for widening of the SI joints, bone erosion, and how white the bone appears. (Whitening occurs because bone cells that are being stimulated by inflammation produce more calcium in response.) When AS is more advanced, the space between the SI joints is also no longer visible because they have become fused together. The catch—and it’s a big one—is that is can take years for any damage from AS to show up on an X-ray, so signs of early disease are often not visible, even if you have symptoms.

  • Magnetic resonance imaging (MRI): This imaging test is much better at seeing the soft tissues in the bone and surrounding the joints, so it can pick up inflammation even if actual damage hasn’t occurred yet. For instance, your doctor may see swelling in the bone marrow of the SI joints. This better image comes with a much higher price tag, however, so insurers may balk at the request. (That’s why having the HLA-B27 gene can come in handy—it may convince your insurer to cover the cost since the odds are greater that you could have AS.) Another caveat is that MRIs are also more difficult to interpret, so inflammation can be missed if the test is done and read by a person without AS experience.

Classifying Your Diagnosis

To confirm a diagnosis of AS, your doctor will need to confirm at least one clinical symptom and one sign on imaging. The clinical criteria include:

  • Lower back pain that persists for at least three months

  • Limited motion of the lumbar (lower) spine

  • Limited chest expansion (meaning it's hard or painful for you take a full, deep breath)

To meet the imaging criterion, your doctor will review your X-ray or MRI for visible signs of damage and/or inflammation. Depending on your test results, you’ll be put into one of two categories:

  • Radiographic AS, which means the pictures found evidence of AS

  • Non-radiographic AS, which means the pictures revealed no damage even though you may have other symptoms.

Technically, if you don’t have definitive damage apparent on an imaging test, you can’t be diagnosed with AS. This happens a lot in the early stages, since people with AS can have symptoms for years before bone erosion or fusion shows up on an imaging test. In fact, many people (especially women) stay in this non-radiographic stage and never progress further. Studies have found rates of progression in AS ranging as low as 5% to as much as 30% over a period of two to 30 years.

Just because an imaging test doesn’t deliver visible evidence of AS doesn’t mean you don’t need treatment for your symptoms. There is some evidence—although it’s not yet conclusive—that biologics, the newest class of medications for AS, may help prevent further damage to the spine. To help people in the early stages of the disease get treatment, the Assessment of Spondyloarthritis International Society (ASAS) has added criteria that can help confirm an early diagnosis without the traditional radiographic evidence for patients who have at least three months of back pain that began before age 45.

Those criteria are:

  • An MRI that shows active inflammation

  • A blood test that shows the presence of the HLA-B27 gene

  • Having at least two AS clinical features, including:

  • Inflammatory back pain

  • Arthritis

  • Heel pain

  • Eye swelling with pain, blurred vision (uveitis)

  • Toe or finger inflammation (dactylitis, which is often called “sausage digits” because swollen fingers look like sausages)

  • Psoriasis

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

  • A good response to NSAIDs

  • Family history of AS

  • Elevated C-reactive protein levels

If you think you might have AS, talk to your doctor. These tests are designed to get to the bottom of your symptoms so you can get the treatment you need. Help is on the way.

Stephanie Wood
Meet Our Writer
Stephanie Wood

Stephanie Wood is a award-winning freelance writer and former magazine editor specializing in health, nutrition, wellness, and parenting.