Ankylosing spondylitis is diagnosed on the basis of your medical history, a physical examination, X-rays or other imaging studies, and possibly blood tests.
Ultimately, however, the diagnosis relies on your doctor's judgment, since there is no definitive test for ankylosing spondylitis.
As part of the physical exam, your doctor may:
A key characteristic of ankylosing spondylitis is the presence of sacroiliitis, or inflammation of the sacroiliac joint, where the spine and pelvis connect. (1) X-rays can sometimes show changes in the sacroiliac joint caused by ankylosing spondylitis, but such changes are often not visible in the early stages of the disease.
While there are no laboratory tests that definitively diagnose ankylosing spondylitis, your doctor may order blood tests to look for systemic inflammation and to evaluate for other inflammatory forms of arthritis, such as rheumatoid arthritis.
Some of the lab tests you may undergo include: (3)
Getting an early diagnosis can help prevent the serious progression that can occur in ankylosing spondylitis. If left untreated, AS can cause the spinal vertebrae to fuse together, leaving the spine inflexible and vulnerable to fractures. (4)
RELATED: 9 Facts About Ankylosing Spondylitis
As part of the process of diagnosing ankylosing spondylitis, your doctor will want to rule out mechanical back pain, which is not caused by inflammation and is treated differently from inflammatory back pain.
Mechanical back pain results from the disruption of the muscles and other structures of the back, including the vertebrae, spinal joints, spinal cord, spinal ligaments, and soft tissues. This disruption is usually caused by traumatic injury or a deformity in the back, but it can occur naturally over time due to gradual wear and tear from activities like exercising or lifting heavy objects. (5)
Distinguishing between the two types of back pain is important since treatment for mechanical back pain — which generally involves rest and the use of nondrug treatments — will not address the underlying causes of inflammatory back pain and could make it worse by allowing the inflammation to continue unabated. (6)
Inflammatory back pain can be distinguished from mechanical back pain in the following ways:
Although nonsteroidal anti-inflammatory drugs (NSAIDs) may be recommended for either type of back pain, the American College of Physicians recommends nondrug treatments as a first line of defense to treat mechanical lower-back pain. (9)
Your medical history and diagnostic test results will help your doctor rule out these other forms of arthritis.
With no definitive tests to identify ankylosing spondylitis, misdiagnosis is common. One study carried out by CreakyJoints, an online community for those living with arthritis, and ArthritisPower, a patient research registry, found that approximately 96 percent of participants ultimately diagnosed with ankylosing spondylitis had previously received at least one misdiagnosis. (16) Some of the more common misdiagnoses include:
Ankylosing spondylitis and fibromyalgia both cause pain and fatigue, but the underlying causes of these symptoms are very different: Ankylosing spondylitis is an inflammatory disease that damages the tissue it affects, while the pain of fibromyalgia is neurological in origin, and the condition does not cause inflammation or damage to tissue. (17)
Another major difference between the two: While the pain caused by ankylosing spondylitis is mainly confined to the spinal area, joints, and surrounding tissues, fibromyalgia pain can affect the whole body. (18)
However, the connection between fibromyalgia and ankylosing spondylitis is a little more complicated. Fibromyalgia can be triggered by a number of factors — among them the spinal issues associated with AS. (19) So while the presence of one of these conditions is not an indicator of the other, the two may sometimes coexist in the same person. (20)
Sciatica is a condition involving pain, tingling, or numbness in the lower back and legs due to compression of the sciatic nerve. Sciatica can be acute or chronic. (21) In some cases, the symptoms of sacroiliitis may mimic those of sciatica, potentially leading to a misdiagnosis. (22)
Ankylosing spondylitis can also be misdiagnosed as a psychosomatic disorder, in which psychological stress leads to physical symptoms. (23) This misdiagnosis may be more common among women than men. (16)
Another condition that closely resembles ankylosing spondylitis — and sometimes develops into it — is nonradiographic axial spondyloarthritis (nr-axSpA). Like ankylosing spondylitis, nr-axSpA causes inflammation in the spine and sacroiliac joints. (24)
The key difference between these two conditions is that tissue damage caused by ankylosing spondylitis is detectable on X-ray; this is not the case for nr-axSpA, which is visible only through an MRI scan. (24) And while nr-axSpA and AS are considered to fall on opposite ends of the same condition spectrum, not all diagnoses of nr-axSpA will progress to AS. Therefore, the two conditions are classified more generally as separate subtypes of axial spondyloarthritis. (24)
Seek urgent medical attention if your lower-back pain:
If you don’t need urgent medical attention and would like to collect a little more information on your own before seeing a doctor, you might want to take the CreakyJoints’ Painspot quiz. Users select the body location where they’re experiencing pain, then answer yes or no to a series of questions about their pain. The tool then suggests which conditions could be causing the pain. While these results should not be considered a definitive diagnosis, they can provide a useful starting point for discussion with your primary care physician or rheumatologist.
When seeking a diagnosis for back pain, it can also help to keep a symptom diary. Tracking symptoms over time allows you to see how they develop or change and how they respond to different pain management techniques. Your symptom diary may also help your doctor arrive at a diagnosis.
Additional reporting by Laura McArdle.