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.

If your results do not show inflammation, this may not rule out AS, and you might need more tests.

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:

• an X-ray
• a MRI scan
• an ultrasound scan

Genetic testing

A genetic blood test may sometimes be carried out to see if you carry the HLA-B27 gene variant, 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 variant and some people have the gene variant without ever developing AS.

Ankylosing spondylitis is a type of axial spondyloarthritis where inflammation of the sacroiliac joints can be seen on an X-ray.

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

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 an X-ray cannot confirm AS, you'll usually be offered an MRI scan.

If the MRI scan shows inflammation of the sacroiliac joints you'll be diagnosed with non-radiographic axial spondyloarthritis. This is another type of axial spondyloarthritis.

Sometimes inflammation does not show up on an X-ray or an MRI scan. In this case you might be diagnosed with non-radiographic axial spondyloarthritis if you have the HLA-B27 gene variant and have symptoms of the condition.