Diagnosing ankylosing spondylitis 

Ankylosing spondylitis (AS) can be difficult to diagnose because the condition develops slowly and there is no definitive test to confirm a diagnosis.

The first thing you should do if you think you have AS is to see your GP. They will ask about your symptoms, including what symptoms you are experiencing, when they started and how long you have had them.

Back pain associated with AS can be quite distinctive. For example, it usually doesn't improve with rest and may wake you up during the night.

Blood tests

If your GP suspects AS, they may arrange blood tests to check for signs of inflammation in your body. Inflammation in your spine and joints is one of the main symptoms of the condition.

If your results suggest you do have inflammation, you will be referred to a rheumatologist for further tests. A rheumatologist is a specialist in conditions that affect muscles and joints.

Further tests

Your rheumatologist will carry out imaging tests to examine the appearance of your spine and pelvis, as well as further blood tests. These are described below.


An X-ray of your lower back can show damage to the joints at the base of your spine (the sacroiliac joints) and new bone forming between the vertebrae (bones in your spine), which are common signs of advanced AS.

MRI scan

magnetic resonance imaging (MRI) scan may highlight changes in your sacroiliac joints that might not show up on an X-ray. It may also show any inflammation of ligaments in the spinal region.

Ultrasound scan

An ultrasound scan can pick up inflammation of the tissues (tendons and ligaments) attached to your bones.

Genetic testing

A genetic blood test may sometimes be carried out to see if you carry the HLA-B27 gene, which is found in most people with AS.

This can contribute towards a diagnosis of AS, but it is not entirely reliable as not everyone with the condition has this gene, and some people have the gene without ever developing AS.

Confirming ankylosing spondylitis

Although scans can sometimes show spinal inflammation and fusing of the spine (ankylosis), damage to the spine cannot always be picked up in the early stages of AS.

This is why diagnosis is often difficult. In many cases, confirming a diagnosis is a long process that can take years.

A diagnosis of AS can usually be confirmed if an X-ray shows sacroiliitis (inflammation of the sacroiliac joints) and you have at least one of the following:

  • at least three months of lower back pain that gets better with exercise and doesn't improve with rest
  • limited movement in your lumbar spine (lower back)
  • limited chest expansion compared to what is expected for your age and sex

If you have all three of these features but do not have sacroiliitis  or if you only have sacroiliitis  you will be diagnosed with "probable ankylosing spondylitis".

Page last reviewed: 26/06/2014

Next review due: 26/06/2016