How Ankylosing Spondylitis Is Diagnosed
The diagnosis of ankylosing spondylitis (AS) requires good detective work on the part of your doctor, as there is no test that will guarantee a 100 percent confirmed diagnosis. Rather, a doctor must consider a person’s symptoms, physical examination, and medical tests like X-rays and blood work to make the correct diagnosis.
Since the diagnosis of AS is sometimes subtle, some people are not correctly diagnosed for many years.
How to diagnose ankylosing spondylitis
Illustration by Cindy Chung, Verywell.
A person with AS often first sees their primary care doctor or family doctor with a complaint about pain, usually in the back or upper buttock. To sort out the cause of this pain, a doctor may ask the following questions:1
Are you younger than age 40?
Has the pain persisted for three or more months?
Does the pain improve with exercise?
Does the pain worsen with rest, especially at night?
Does the pain significantly improve after taking an NSAID (for example, ibuprofen)?
Answering “yes” to the majority of these questions will prompt your doctor to consider a diagnosis of an inflammatory arthritis (like ankylosing spondylitis), as opposed to the more common mechanical back pain (for example, muscle strain or degenerative disc disease).
Your doctor will also inquire about whole-body symptoms like fatigue or malaise, as this can further support a diagnosis of ankylosing spondylitis. This is because whole-body symptoms are potential clues that something abnormal is going on with your immune system.
In ankylosing spondylitis, your immune system attacks healthy joints, especially the spinal and sacroiliac joints.
A family history of ankylosing spondylitis, or a personal history of gastrointestinal infections, inflammatory bowel disease, uveitis (inflammation of the colored part of your eye), or psoriasis are additional diagnostic clues.2
If your primary care doctor or family doctor suspects ankylosing spondylitis or other immune-system mediated process, he or she is likely to refer you to a rheumatologist—a doctor who specializes in joint and autoimmune diseases.
Diagnosis of the condition requires a full physical exam to assess whether AS or another condition is more likely.
During a physical examination for ankylosing spondylitis, your doctor will examine your spine, hip, and sacroiliac joints by pressing on them to check for tenderness.3 By moving your joints, your doctor can assess range of motion and flexibility.
Your doctor will also examine the small joints of your hands and feet, and other joints like your elbows, knees, and shoulders, to determine if there is swelling, warmth, or fluid present.
Checking your heels for enthesitis, listening to your lungs, and performing a skin and scalp check (looking for psoriasis) are also common parts of a physical examination when a person is suspected of having ankylosing spondylitis.
Do not be surprised if your doctor refers you to an eye doctor (an ophthalmologist) if you are having any eye pain or redness and/or blurry vision. Uveitis is a possible complication of ankylosing spondylitis.3
Do your best to answer questions your doctor asks—but also don’t hesitate to ask your own questions! You should know what he or she is testing, why, what results mean, and the implications of the results. Blood Tests
HLA-B27 Blood test
The main blood test for diagnosing ankylosing spondylitis is the HLA-B27 test.3 HLA-B27 is a specific genetic variation that makes a version of a protein found on most cells in your body, including white blood cells (your infection fighting cells). When present, it can cause your white blood cells to attack healthy cells in your body, like those within your spine, neck, or eyes.
HLA-B27 is present in a large percentage of people with a white racial background who have AS. That being said, of those who are positive for the HLA-B27 gene, only a small percent actually get ankylosing spondylitis. In African-Americans with AS, the HLA-B27 gene is not nearly as common, so it does not work as well as a diagnostic test.
If you are having symptoms suggestive of AS, your doctor may test you for the HLA-B27 gene to help confirm a diagnosis.
However, it is not a definitive test—all the pieces have to fit together. A positive test with no symptoms or signs of an autoimmune disease does not mean much. Conversely, some people have AS but test negative for the gene. Also, HLA-B27 is also present in a relatively high percentage in certain other kinds of inflammatory arthritis, like psoriatic arthritis.
Other Blood Tests
Two other blood tests your doctor may order if he or she suspects ankylosing spondylitis include:3
C-reactive protein (CRP)
Erythrocyte sedimentation rate (ESR)
One or both of these markers may be elevated when there is active inflammation in the body. However, CRP and ESR are nonspecific blood tests, meaning they can be elevated in a number of different health conditions besides ankylosing spondylitis, such as in autoimmune diseases or infection.
In addition, ESR and CRP are sometimes normal in a person with ankylosing spondylitis (especially if their disease is not flaring at the time of the blood draw). Depending on the situation, sometimes a physician will also order additional tests to help rule out other conditions.
X-rays and sometimes magnetic resonance imaging (MRI) of the sacroiliac joints—and sometimes the spine—are generally used to confirm a diagnosis of ankylosing spondylitis.
A problem with imaging tests is that inflammation can take years to show up. MRI is generally better at detecting earlier signs of disease.
Imaging tests are also used to monitor a person’s disease progression, after they have been diagnosed with ankylosing spondylitis.4
Ankylosing spondylitis can be considered a smaller subset of a larger group of conditions called axial spondyloarthritis.5 As part of the ankylosing spondylitis diagnosis, your clinician determines whether you have non-radiographic axial spondylitis or radiographic axial spondylitis.
The latter is considered basically the same as ankylosing spondylitis. This distinction is made primarily based on medical imaging (usually MRI).
In diagnosing AS, doctors have to eliminate other disease possibilities. This is because other diseases may display some or all of the symptoms and signs of AS, though they may have other symptoms and signs as well.
Since there isn’t a good single test for AS, it is very important that your doctor rule out other possibilities for your symptoms. This is crucial as these other conditions often require different treatments.
Some medical conditions that have some overlapping signs and symptoms of AS include:6
arthritis in response to an infection (reactive arthritis)
arthritis related to inflammatory bowel disease
mechanical back pain
fracture of a vertebra
Another possibility is back pain resulting from metastatic cancer. However, in this case, additional signs and symptoms are almost always present.
A diagnosis of ankylosing spondylitis requires a thoughtful analysis of many factors by your doctor. A good medical history and physical examination are key. Blood tests and X-rays can then strengthen the diagnosis. Sometimes, the correct diagnosis is only revealed over time, as more symptoms appear. Don’t hesitate to ask your doctor about any questions pertaining to your diagnosis or personal test results. Taking an active role in your health can only help you.