Ankylosing spondylitis (AS) is challenging to diagnose because the symptoms tend to develop slowly, and there's no one, definitive test for it. The main symptom of AS is back pain, which is a common complaint that can have many different causes. Because it's so hard to diagnose, the average time from when people first experience symptoms to when they're diagnosed is 8 to 11 years. If you have AS, getting diagnosed early is important for a better outcome.
While there are no definitive tests that can tell you if you have ankylosing spondylitis, there are some tests that are helpful, including:
Physical examination
Your doctor will probably start with a physical exam and ask you about your symptoms. They may suspect ankylosing spondylitis if you have back pain that meets four of the following criteria:
- Started before the age of 35
- Gets gradually worse
- Lasts for at least three months
- Improves with exercise
- Is accompanied by morning stiffness that lasts for at least an hour
Doctors will also ask you about your family history and any other symptoms that could indicate a condition besides AS. As part of the physical exam, your doctor will check to see how stiff your back is, if you have any other tender areas, and if you can expand your lungs normally.
Imaging Studies
Imaging studies can help your doctor determine if you have AS. These include:
X-ray
An X-ray is probably the first imaging test your doctor will order if they suspect you have AS. Changes in the sacroiliac joint, which is located at the base of your spine, are the most definitive signs of AS. However, these signs often don't show up on X-ray until several years after you start having pain.
CT scan
If nothing shows up on the X-ray but your doctor suspects you have AS, they may order a computerized tomography (CT) scan. This tests uses X-rays and computer technology to give a more detailed picture.
MRI
Magnetic resonance imaging (MRI) uses a large magnet and computer technology to provide detailed pictures of the inside of your body. This may give you an earlier diagnosis than is possible with an X-ray.
Ultrasound
Ultrasounds use sound waves to create images. These are being studied as a way to diagnose AS earlier.
Blood Tests
Your doctor may perform blood tests to check for inflammation, as well as other conditions that could be causing your symptoms. They may also do a genetic test to see if you have the gene that is associated with AS: HLA-B27.
