How Doctors Diagnose Ankylosing Spondylitis

Medically Reviewed by Tyler Wheeler, MD on September 24, 2023
3 min read

Ankylosing spondylitis (AS) can be tough to diagnose for several reasons.

Its symptoms, like back pain and fatigue, are similar to those of many other conditions. It tends to affect young people, who aren't likely to suspect they have a type of arthritis.

It may be harder for doctors to diagnose women with AS, which is five times more common in men. Women often have different symptoms than men, too. They may first notice pain in their necks, rather than their backs.

Also, there’s no single test for ankylosing spondylitis. Your doctor must rely on other methods to diagnose your condition.

If your primary doctor suspects you have AS, they might refer you to a rheumatologist. These doctors specialize in diagnosing and treating inflammatory diseases, like AS.

The doctor you see for your diagnosis will usually start with a physical exam. They may ask you to bend in different directions to see how well you move and where you have pain. They might push on parts of your body, move your legs around, or ask you to take deep breaths.

Your doctor will also ask you about:

  • When your pain began
  • How long it usually lasts
  • What time of day it tends to happen
  • Whether it gets better with exercise
  • If you have a family history of AS
  • Any other symptoms you may have

Your doctor may do imaging tests, like an X-ray or MRI.

An X-ray helps your doctor see changes in your bones and joints. But if your AS is in its early stages, this test may not pick up signs yet.

An MRI can capture a better picture of your bones and soft tissues. It can help your doctor find signs of AS more easily. But it’s a lot more expensive than an X-ray.

Your doctor might test your blood for the gene HLA-B27. Many people with AS have this gene. But not everyone who has it gets the condition.

Your doctor may also check your blood for signs of inflammation, which could be caused by AS. A high erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) can signal inflammation.

But not everyone with AS has these blood markers. And, they could result from other conditions like anemia or infection.

Because imaging and blood tests often fail to identify early symptoms, AS can sometimes take years to confirm.

Your doctor can usually make a diagnosis if an imaging test finds issues related to AS and if you have at least one of these signs:

  • Reduced ability to move your lower back
  • When you breathe, your chest doesn't expand as much as doctors would expect for someone of your age and sex
  • 3 or more months of lower back pain that gets better when you move around

If an imaging test doesn't show signs of AS, but you have all three of these symptoms, your doctor may diagnose you with probable ankylosing spondylitis.