It can take a long time for a person to receive an ankylosing spondylitis (AS) diagnosis. This may be because the symptoms start gradually, doctors may mistake them for another condition, and the diagnostic criteria are not well-defined.

AS is a form of inflammatory arthritis that affects primarily the spine. It is one type of spondyloarthritis (SpA). Over time, inflammation can make movement more difficult, and in severe cases, a person’s spine may fuse together.

To diagnose AS, a healthcare professional may ask about a person’s family medical history, perform a physical examination, and order imaging tests, such as X-rays and MRI scans, and blood tests.

This article discusses the diagnostic criteria for AS, what diagnostic methods doctors use, what questions a person should ask a doctor, and more.

A doctor reading the diagnostic criteria for ankylosing spondylitis.Share on Pinterest
Javier Díez/Stocksy

Diagnostic criteria for AS are still not well-defined. Healthcare professionals have created diagnostic models for AS that have evolved over the years.

The Assessment of SpondyloArthritis International Society (ASAS) created the ASAS criteria in 2009.

According to these criteria, a healthcare professional can diagnose AS if a person under the age of 45 years who experiences lower back pain or stiffness for 3 or more months also meets one of the following requirements:

  • They have a radiological diagnosis of sacroiliitis, which includes:
    • MRI evidence of inflammation that is active or acute
    • sacroiliitis that is higher than grade 2 on both sides, or grade 3 or 4 on one side
  • They have the HLA-B27 gene and two or more clinical features of SpA.

Clinical features of SpA can include one or more of the following:

The ASAS also introduced the term “non-radiographic axial SpA,” which included the cases of those who had symptoms of inflammatory back pain but did not have a definite X-ray evidence of sacroiliitis. Sacroiliitis is inflammation of the sacroiliac joints, which is where the lower spine and the pelvis connect.

Older diagnostic criteria include:

Rome criteria

Researchers developed the first criteria for diagnosis of AS in 1961.

Those diagnostic criteria included:

  • lower back pain that lasts for 3 months or longer and improves with exercise but not with rest
  • limited lumbar mobility
  • limited chest expansion
  • pain and stiffness in the middle of the spine
  • a history of uveitis
  • grade 2 sacroiliitis on both sides

Doctors could also diagnose AS using any clinical criteria combined with radiographic evidence, such as X-rays and other imaging tests.

According to the Spondyloarthritis Research and Treatment Network, healthcare professionals made no corrections based on a person’s age or sex for spine and chest mobility.

New York criteria

In 1966, researchers proposed a newer set of diagnostic criteria, known as the New York criteria.

Clinical criteria to the New York diagnostic criteria included:

  • limited chest expansion
  • limited mobility in the lumbar spine
  • pain in the lumbar spine or the dorsolumbar junction

In addition, diagnosis required X-ray evidence.

Diagnosis under the New York criteria required a person to fit into one of several potential categories, such as strong radiographic evidence combined with at least one clinical criterion.

Modified New York criteria

In 1984, researchers introduced the modified New York diagnostic criteria.

The modified version included clinical criteria of:

  • lower back pain that lasts for 3 months or longer and improves with exercise but not with rest
  • limited range of motion in the lumbar spine
  • grade 2 sacroiliitis on both sides, or grade 3 or 4 sacroiliitis on one side
  • limited chest expansion, compared with the average and adjusted to age and sex

In addition, the modified New York diagnostic criteria included radiographic evidence to support diagnosis.

Diagnosis of AS typically starts with a review of a person’s personal and family medical history and the symptoms they are experiencing.

A review of symptoms will often include questions about:

  • how long they have lasted
  • what triggered the pain
  • what helps alleviate the pain

A doctor may also ask the following:

  • How long have the symptoms been present?
  • What treatments has a person already tried?
  • What makes a person feel better?
  • What makes them feel worse?

Reviewing symptoms can help the doctor rule out other conditions that may cause similar issues. Some distinctive symptoms they may look for include:

  • symptoms that present before the age of 40 years
  • pain that is primarily in the spine, above the pelvis
  • symptoms that improve with movement but not with rest
  • stiffness and pain that are often worse upon waking
  • pain that typically responds to NSAIDs

Additionally, the doctor will likely order testing such as:

  • blood tests to look for signs of inflammation
  • imaging tests, such as X-rays or MRI scans, to check for damage or inflammation
  • genetic testing for HLA-B27, which is present in approximately 90% of individuals with an AS diagnosis
  • a physical examination that may include inspecting joint mobility and chest expansion, in addition to other checks for motion and pain

Individuals who have unexplained lower back pain before the age of 40 years should consult a doctor, particularly if:

  • rest does not help, and stiffness upon waking is common
  • symptoms improve with exercise
  • other symptoms are present

A primary care doctor may not be able to diagnose AS if they are not familiar with the condition. They may refer a person to a rheumatologist for diagnosis.

A rheumatologist is a healthcare professional who specializes in musculoskeletal diseases and autoimmune conditions that affect a person’s joints, bones, and muscles.

AS may cause symptoms years before any radiographic evidence is present.

Some evidence suggests it can take 8–11 years for an individual to receive an AS diagnosis following the onset of symptoms.

During an initial consultation with a doctor, a person may wish to ask the following questions:

  • How much experience with different types of arthritis pain do you have?
  • Is there a specialist you can recommend?
  • What tests are necessary?
  • Based on the symptoms, do you suspect AS?

If the doctor is hesitant to refer a person to a rheumatologist, the person may wish to ask the following to get clarification:

  • Can you issue a referral to a rheumatologist?
  • Why do you think it is not AS?
  • Is it possible it is early stage AS?

Symptoms of AS are generally not specific to the condition. However, some features may help a doctor rule out other health issues.

Common symptoms of AS include:

Symptoms may be mild to severe and may not show the same in all people. They may also worsen at times, and they can go into periods of remission.

Treatment for AS typically aims to help relieve pain and other symptoms.

There are various treatment options available, such as:

Learn more

Learn more about treating AS here:

AS diagnostic criteria have changed over the years. Doctors do not have a single diagnostic test to make a formal AS diagnosis. Instead, they need to consider a detailed medical history, physical examination, blood tests, and imaging tests.

Once a person receives an AS diagnosis, there are several treatment options that may help alleviate their symptoms, such as pain and stiffness.

Learn more about AS here.