You should see your GP if you think you may have ankylosing spondylitis (AS). There is no single test to diagnose the condition, but your GP will ask you about your symptoms.
The back pain that is 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.
If your GP thinks you could have ankylosing spondylitis, they may perform some blood tests, including:
a full blood count (FBC), which measures all of the different types of blood cells in the sample, and can help to determine whether there are fewer red blood cells (cells that transport oxygen), which may indicate anaemia
erythrocyte sedimentation rate (ESR) - a sample of blood is put into a test tube and the length of time that the red blood cells take to sink to the bottom of the tube is measured
C-reactive protein (CRP) - a blood sample is measured to see how much CRP (a protein that is produced by the liver) it contains
The ESR and CRP tests provide a measure of how much inflammation (swelling) is in your body. Inflammation in your spine and joints is one of the main symptoms of ankylosing spondylitis.
If your GP thinks you may have ankylosing spondylitis, they will refer you to a rheumatologist for further tests. A rheumatologist specialises in conditions that affect the muscles and joints.
Your rheumatologist will carry out some imaging tests to examine the appearance of your spine and pelvis. Some possible tests are described below.
An X-ray uses short bursts of high-energy radiation to create images of hard substances in your body, such as bones. X-rays 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
See the Health A-Z topic about X-rays for more information.
A magnetic resonance imaging (MRI) scan creates an image of the inside of your body using a strong magnetic field and radio waves.
A MRI scan may highlight changes in your sacroiliac joints (at the base of your spine) that might not show up on an X-ray.
See the Health A-Z topic about MRI scans for more information.
An ultrasound scan uses sound waves to examine the inside of your body, in the same way they are used to view a baby inside the womb (uterus).
An ultrasound scan can pick up inflammation of the tissues (tendons and ligaments) that are attached to your bones.
See the Health A-Z topic about Ultrasound scans for more information.
Confirming ankylosing spondylitis
The imaging procedures described above can be used to highlight the extent of any spinal inflammation and ankylosis (fusing of the spine) that you may have.
However, as ankylosing spondylitis often takes a long time to develop, any damage to your spine may not yet be visible. This is why the condition is often difficult to diagnose. In many cases, confirming a diagnosis is a long process that can often take several years.
The criteria used to confirm the diagnosis are described below.
A definite diagnosis of ankylosing spondylitis can be confirmed if sacroiliitis (inflammation of the sacroiliac joints) is apparent on an X-ray, 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.
Inflammation is the body's response to infection, irritation or injury, which causes redness, swelling, pain and sometimes a feeling of heat in the affected area.
Joints are the connection point between two bones that allow movement.
Ligaments are made of fibrous tissue. They connect bone to bone, providing support while allowing flexibility and movement.
The joints either side at the base of the spine.
A tendon is a white fibrous cord that joins bones to muscle. It allows the bone to move when the muscle contracts.