How is Ankylosing Spondylitis Diagnosed?

Reviewed by: HU Medical Review Board | Last reviewed: February 2019 | Last updated: February 2023

Testing for and receiving a diagnosis of ankylosing spondylitis (AS) is unfortunately not a simple or straightforward process, and many people go through multiple doctor visits and tests over several years before receiving an accurate diagnosis. An average of 8-11 years has been reported between the onset of symptoms and a diagnosis. This continues to be a challenge for people with AS, as early diagnosis and effective treatment are important to reduce the inflammation in the body and delay or stop the progression of the disease.1

Challenges of diagnosing ankylosing spondylitis

One of the challenges in properly diagnosing AS is that its primary symptom is low back pain, a common condition among the general public which can be caused by several other conditions or circumstances. Early symptoms such as joint pain may not be accompanied by visible changes on x-rays, and some doctors may fail to recognize early stages of AS. Symptoms of AS typically begin in late adolescence or early adulthood (before age 40), and early intervention with effective treatment is important to maintain functional ability during a time of life that is typically active and productive.1

Diagnostic criteria

The diagnosis of AS is generally made by a combination of criteria of inflammatory back pain and enthesitis or arthritis with x-rays or other imaging techniques.1 AS is the primary form of spondyloarthritis (SpA), a group of arthritis conditions that cause joint inflammation and can also cause inflammation of the entheses, the points where ligaments and tendons attach to the bones. Diagnostic criteria have been developed by the Assessment of SpondyloArthritis International Society (ASAS) and classifies SpA as axial SpA (affecting the spine) or peripheral SpA (affecting joints in the arms or legs). Axial SpA can also be classified as radiographic or non-radiographic. Radiographic SpA means that inflammation and/or joint damage is visible on imaging, such as x-ray or magnetic resonance imaging (MRI). The term radiographic SpA is also used in place of AS, and these terms refer to the same condition. Non-radiographic means that the joints appear normal on imaging, but they have other symptoms of SpA.1,3,4

Diagnostic tests for ankylosing spondylitis

Diagnosis of AS is commonly made by a rheumatologist, a specialist who focuses on disorders of the joints, bones, and connective tissue. The diagnosis is determined through a combination of tests, including2,6:

During a medical history, the physician or nurse will ask about a patient’s personal health experiences, as well as family health conditions. Having a family member who has AS increases a person’s risk of developing it. Any symptoms the patient is experiencing are important to note at this time, such as areas of pain or stiffness, how long the pain has been present, anything that relieves the pain or stiffness (like activity), or fatigue.6,7

Any points of inflammation are evaluated during the physical exam. Doctors may also want to evaluate a patient’s flexibility in their spine.6

While there is not a blood test that can determine if a person has AS, blood tests may be used to rule out other conditions or to assess inflammatory markers, such as erythrocyte sedimentation rate (ESR) and/or C-reactive protein (CRP).6

The classic sign of AS is sacroiliitis (inflammation of the joint between the pelvis and the base of the spine) seen on x-rays. However, it may take many years of having AS before this progression is visible on x-rays. MRI scans can be more sensitive in measuring inflammation but may be expensive and not always covered by insurance.6

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