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Ankylosing spondylitis (AS) can be difficult to diagnose because the condition develops slowly and there's no definitive test.

The first thing you should do if you think you have AS is to see your GP. They'll ask about your symptoms, including:

  • what symptoms you're experiencing
  • when they started
  • how long you've had them

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

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 a main symptom of the condition.

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

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

These may include:

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's not entirely reliable as not everyone with the condition has this gene and some people have the gene without ever developing AS.

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 inflammation of the sacroiliac joints (sacroiliitis) and you have at least 1 of the following:

  • at least 3 months of lower back pain that gets better with exercise and doesn't improve with rest
  • limited movement in your lower back (lumbar spine)
  • limited chest expansion compared with what is expected for your age and sex

If you have all 3 of these features but do not have sacroiliitis – or if you only have sacroiliitis – you'll be diagnosed with probable ankylosing spondylitis.