Ankylosing spondylitis causes chronic joint pain, usually starting in the back and buttocks. Diagnosing ankylosing spondylitis usually involves a variety of tests.

No single test can diagnose ankylosing spondylitis (AS), but a range of questions and techniques can help confirm or rule out a diagnosis.

ankylosing spondylitis imagingShare on Pinterest
Leonid Eremeychuk/Getty Images

No single test can show that a person has AS. Doctors may order various tests, ask for a person’s medical history, and perform a physical exam to help them make a diagnosis.

Tests may include blood tests and imaging. Some tests may help doctors rule out other conditions with similar symptoms. Others may help monitor disease progression after diagnosis.

Who should get testing for AS?

Doctors may recommend testing for people with symptoms of AS, such as chronic back and stiffness. A doctor may also recommend testing for AS if a person has conditions that may occur with AS, such as:

  • Crohn’s disease
  • ulcerative colitis
  • psoriasis

A family history of AS can increase a person’s risk of developing it, so a doctor may recommend testing in people with a family history and AS symptoms.

Doctors use a range of methods to help make the diagnosis.

Genetic tests

One test is specific to AS. It is the HLA-B27 test, which can detect the HLA-B27 gene. A doctor will take a blood sample to carry out this test.

This genetic factor is present in many people with AS, and those with the gene may be more likely to develop the condition.

However, this is not always the case. Around 80% of people who inherit the genetic feature from a parent will not develop AS, and the risk also varies with ethnic background.

Blood tests for inflammation

A blood test can also detect signs of inflammation in the body. Those tests include:

  • Erythrocyte sedimentation rate (ESR) test: This test measures how fast a person’s red blood cells fall to the bottom of a test tube.
  • C-reactive protein (CRP) test: CRP is a protein produced by the liver that increases in response to inflammation in the body.

While these tests can spot common signs of inflammation in the body, they will not specify that the inflammation is due to AS.

Physical exam

During the physical exam, the doctor will usually check for signs of inflammation in the person’s body.

Typically, a doctor looks at the back, chest, pelvic bones, sacroiliac joints near the bottom of the spine, and the heels. They may press gently into these areas and ask about pain and tenderness.

The doctor can also check the range of motion of the spine. They may ask the person to do some mobility tests to see if there is any restriction of movement.

They may also assess how far the chest expands, how easy it is to breathe, and the range of motion of the cervical spine (neck).

Imaging and scans

After looking for signs in the blood, doctors will often use X-rays or MRIs.

X-rays may show if joints are damaged or fused. The X-ray may focus on the sacroiliac joints, where AS often appears.

An MRI scan or ultrasound can reveal changes that do not appear on an X-ray, such as inflammation without joint damage. Signs of inflammation include a widening or thinning space between bones or increased blood flow.

Imaging can help confirm the presence of AS and help people make treatment decisions. It can also help track changes over time.

No single test can diagnose AS, but a doctor will use findings from tests and the information a person gives about key symptoms to decide whether AS is likely to be present. They will consider factors such as:

  • swollen and painful joints, especially around the spine
  • fatigue
  • whether pain occurs on waking and how long it lasts

They may use a scoring system to assess levels of pain and disease activity.

A doctor may confirm that a person has the condition if they have inflammation of the sacroiliac joint and one or more of the following:

  • have had back pain for 3 months or longer that improves with exercise but not with rest
  • have limited movement in the lower back
  • have limited chest expansion for their age and sex

If a person has all three features but not inflammation of the sacroiliac joint, or if they only have inflammation of the sacroiliac joint, a doctor may diagnose probable AS.

Questions a doctor may ask

The doctor may ask the person about their symptoms and health history. A few specific signs and symptoms may point to the early signs of AS.

Is there a family history of AS?

If there is a history of AS in a person’s family, they are more likely to have it.

The doctor will still need to do thorough exams before making a diagnosis.

Is there unexplained pain that appears gradually?

AS often appears before the age of 45 years. Unexplained pain and stiffness are less common in younger people.

A doctor may consider AS if a young person has pain, especially back pain and pain in the buttocks, that has no known cause.

For example, if the person has not had an injury or put undue strain on their back, they may have AS.

The pain often develops over several weeks or months. It may start as a slight stiffness and gradually grow into aching, stiff joints and reduced mobility in the spine.

A history of back pain lasting for 3 months or more may be a key factor in diagnosis.

Is pain worse when sleeping or prone?

With many conditions, chronic back pain improves when resting. The opposite is true with AS.

People with AS will experience more pain when resting. It can worsen during the night and be the most pronounced upon awakening. It may ease as the person moves around throughout the day.

Some people with AS have pain that comes and goes, but others have severe and persistent pain.

Does pain improve with physical motion?

The pain caused by AS is also different from other types of back pain because exercise makes it better, not worse.

Exercise and stretching can irritate people with back pain, but someone with back pain caused by AS typically feels relief after stretching or doing other forms of exercise.

Do NSAIDs treat the pain?

Over-the-counter drugs such as ibuprofen or prescription nonsteroidal anti-inflammatory drugs (NSAIDs) will ease the pain in the early stages of AS. They are usually not enough to deal with the disorder over time.

However, if a person’s symptoms initially get better with NSAIDs, they may have AS.

Is chest expansion reduced?

Some individuals with AS will also experience chest pain and stiffness. The ribcage may feel tight and not expand as much as the doctor would expect. This can make it hard to breathe and lead to an increase in lung infections.

Are there signs of inflammation?

Others may have stiff joints, pain in their ankles or heels, or inflammation in their wrists. Inflammation of the sacroiliac joints, which connect the base of the spine to the pelvis, is a sign of AS.

Some people may experience these symptoms, which can also be signs of inflammation:

  • a fever (not common)
  • tiredness
  • hot, red joints

Are there other symptoms?

Other symptoms associated with AS include:

  • inflammation of the eye and vision changes
  • abdominal pain and bowel changes
  • general tiredness
  • skin rashes, particularly on the soles of the feet
  • psoriasis, if a person has the psoriatic arthritis form of spondyloarthritis
  • low appetite and weight loss

Knowing if a person has any of these symptoms helps a doctor decide whether they have AS.

Doctors will often ask these questions to determine if they should continue to the next step of diagnosing AS, which is usually a physical exam.

While no single test can indicate a person has AS, a combination of specific test results and symptoms may lead a doctor to diagnose AS and recommend treatment.

In some cases, a doctor may rule out other causes of back pain and diagnose probable AS.

Treatment aims to reduce the symptoms, slow the progression of AS, and improve a person’s functionality.

Treatment plans will typically include:

  • NSAIDs to manage pain and inflammation
  • TNF-a inhibitors and other biologic drugs that can slow the progression of AS
  • physical therapy, exercise, and techniques to manage posture and improve the flexibility of the spine and other affected joints
  • breathing exercises to help expand the chest

A doctor may recommend low impact activities that provide a full-body workout, such as swimming and cycling.

Who treats ankylosing spondylitis?

Treatment for AS typically involves various health professionals. This can include:

  • a rheumatologist, a doctor specializing in arthritis
  • an orthopedic surgeon, a doctor specializing in treating diseases of the bones and joints
  • physical and occupational therapists, specialists who may help reduce pain and improve functionality

Depending on an individual’s symptoms, they may also receive treatment from other specialists, such as:

  • a dermatologist, a doctor specializing in the skin
  • a gastroenterologist, a doctor specializing in the digestive system
  • an ophthalmologist, a doctor specializing in eye health

A person may also work with mental health professionals to manage mental health symptoms or conditions that arise.

No single test can diagnose AS, but doctors typically look at various factors.

These include symptoms, personal and family medical history, and the results of imaging tests, blood tests, and possibly genetic tests.

Getting an early diagnosis can help a person access suitable treatment. This will increase their chances of managing the condition, maintaining a good quality of life, and preventing disease progression.