Ankylosing spondylitis - Diagnosis 

Diagnosing ankylosing spondylitis 

You should see your GP if you think you have ankylosing spondylitis (AS). There is no single blood test to diagnose the condition, but your GP will ask about your symptoms.

Back pain associated with ankylosing spondylitis is quite distinctive. For example, it usually gets worse with rest and may wake you up during the second half of the night.

Blood tests

If your GP suspects ankylosing spondylitis, they may perform blood tests, including:

  • a full blood count (FBC)
  • erythrocyte sedimentation rate (ESR)
  • C-reactive protein (CRP) 

The ESR and CRP tests provide a measure of how much swelling is in your body. Inflammation in your spine and joints is one of the main symptoms of ankylosing spondylitis.

If your results indicate inflammation, you will be referred to a rheumatologist for further tests. A rheumatologist specialises 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. These are described below.  


An X-ray of your lower back can show severe signs of ankylosing spondylitis, such as:

  • damage to the joints at the base of your spine (the sacroiliac joints)
  • new bone forming between the vertebrae (bones) in your spine

MRI scan

magnetic resonance imaging (MRI) scan may highlight changes in your sacroiliac joints (at the base of your spine) 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.

Confirming ankylosing spondylitis

Imaging tests can highlight the extent of spinal inflammation and ankylosis (fusing of the spine).

However, as the condition progresses, damage to your spine may not yet be visible. This is why diagnosis is often difficult. In many cases, confirming a diagnosis is a long process that can take years.

A definite diagnosis of ankylosing spondylitis can usually be confirmed if an X-ray shows sacroiliitis (inflammation of the sacroiliac joints), and you have one of the following:

  • at least three months of lower back pain that gets better with exercise and worse 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’.
Increasingly, MRI scans are being used to detect ankylosing spondylitis early.

Page last reviewed: 25/06/2012

Next review due: 25/06/2014


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MRI scan

An MRI scan uses strong magnetic fields and radio waves to produce detailed images of the inside of the body