The leading rheumatology resource
for patients and physicians.

Commencez à lire en

Diseases > Ankylosing Spondylitis | AS > Diagnosis | Ankylosing Spondylitis | AS

What is AS?

Ankylosing spondylitis is also known as a spondyloarthropathy, or AS. It is one of 100 different different types of arthritis. Ankylosing spondylitis is in a family of related conditions called spondyloarthropathies. Other members of this family include psoriatic arthritis, reactive arthritis and arthritis associated with inflammatory bowel disease.

What causes AS?

Ankylosing spondylitis is a long-term (chronic) disease. It occurs when the body’s immune system begins to attack its own joints. The reason for this is not well understood. The attack causes inflammation of the joints. The word inflammation comes from the Latin word inflammare which means to light on fire. In people with AS, inflammation causes the joints to become warm, swollen, painful, and sometimes red. In other words, the joints can feel like they’re “on fire.”

With AS, the joints between bones in the spine and/or the joints between the spine and the pelvis (sacroiliac or “SI” joints) are usually the first targets of this immune attack. The immune system causes inflammation in the spine. The body then tries to repair itself by growing new bone. Unfortunately, this part of the body’s healing process doesn’t work well. In fact, it can lead to bone growth across a joint to connect two bones. If left untreated, a joint can end up being completely fused. This can cause considerable stiffness and pain.

Who gets AS?

Ankylosing spondylitis affects about one person in every thousand. That means in a city with a population of 100,000, you could expect about 1000 people to have AS. So you can see that AS isn’t that uncommon: you might know someone else with it. Some famous people who have or have had AS are Ed Sullivan, Motley Crue guitarist Mick Mars, and baseball player Rico Brogna. Ankylosing spondylitis is about three times more common in men than women.

Like many other diseases, it is thought that genetics has a role to play in who gets AS. About 9 out of 10 people with AS have a gene called HLA-B27. But, having the HLA-B27 gene doesn’t guarantee that you’ll get AS. Many people have this gene and don’t ever develop AS. Having the gene just means you have a higher chance of getting AS.

Think of it like this. Ankylosing spondylitis is like a fire burning in the joints of your spine. To make a fire you need wood and something to light it with. Let’s think of the wood like it’s your genes. You need the right type of wood (nice and dry) to light a fire. Researchers have found that you need the right type of genes to light the fire of AS. But that’s not the end of the story. You also need something to light the fire with. We’re not entirely sure what lights the fire of AS. It might be the chemicals from cigarette smoke or a virus in the environment. Several triggers might be needed to light the fire. We just don’t know and it’s like looking for a needle in a haystack.

How is AS diagnosed?

Ankylosing Spondylitis (AS) is best diagnosed by a specialist who is very familiar with the disease (rheumatologist). A careful and complete history will be taken followed by a thorough physical examination.  Based on this information, you will likely be sent for some tests (see below). The rheumatologist will then put all of this information together and form a likely diagnosis.

What tests are done to diagnose AS?

Blood tests to look for inflammation

This is a good starting point as AS is an inflammatory arthritis. One would expect these tests to be abnormal. Common tests include a Complete Blood Count (CBC), Erythrocyte Sedimentation Rate (ESR), and C-Reactive Protein (CRP)

Blood tests to look for a Genetic Marker

Over 90% of Caucasians with AS will be positive for a gene called HLA-B27. However, as HLA-B27 is also present in about 6% of this population you need to be careful interpreting the results of this test. A positive HLA-B27 test does not mean you have AS. You need to take the result of this test in context of the clinical scenario.


Plain x-rays of the pelvis, hips, spine, and other joints are often very useful in diagnosing AS. The challenge with x-rays is that they may look normal in patients with very early disease.

Magnetic Resonance Imaging (MRI)

Over the past 10 years MRI imaging has advanced significantly and is quickly becoming the most useful imaging for the diagnosis of AS. Not everyone needs an MRI though.  In some people the x-rays will be enough.

Bone Scan

Bone scans can sometimes be suggestive of AS but they are not useful to confirm a diagnosis. Bone scans can be abnormal for a number of reasons and can be misleading.

Read more – Symptoms of Ankylosing Spondylitis (AS)