How Is Ankylosing Spondylitis Diagnosed?

Ankylosing spondylitis (AS) is a chronic inflammatory disease that mostly affects the spine and back. AS is a form of arthritis that causes pain and stiffness in the joints and ligaments of your back, especially in the mornings and after periods of inactivity.

Symptoms of AS can be painful and interrupt your daily life if left untreated. That’s why an early diagnosis is so important. If you have a family history of AS or notice symptoms, it’s good practice to see your healthcare provider to get tested. 

During your diagnostic process, your healthcare provider will ask you about your medical history and perform a physical exam. They may also order a variety of tests (e.g., blood or imaging tests) to learn more about your symptoms. It is also common for your primary care provider to work with a rheumatologist—or, a doctor who specializes in the bones, joints, and muscles—to help give you an official diagnosis.

doctor showing man imaging scans of spine

Ross Helen / Getty Images

Medical History 

During your appointment, your healthcare provider will ask you about your personal medical and family history, current symptoms, and lifestyle habits. The information you tell them will help them better understand your condition. They may ask you questions such as:

  • What symptoms do you have?
  • When did symptoms start?
  • How severe are your symptoms?
  • Is there anything that makes your symptoms better or worse?
  • Do you have a parent or sibling with AS?
  • Do you have any other inflammatory conditions, like arthritis, psoriasis, or inflammatory bowel disease?
  • Do you smoke cigarettes or drink alcohol?
  • Have you received a diagnosis for any other medical condition? 

Physical Exam

After learning about your medical history, your provider will perform a physical exam to look for signs of AS, including inflammation, limited range of motion in your joints, or tenderness and pain in parts of your body.

During your physical exam, your provider may also:

  • Measure your vital signs (e.g., temperature, heart rate, blood pressure)
  • Feel your spine and other joints (e.g., wrist, ankles, or pelvis) to check for inflammation signs such as redness, warmth, or swelling 
  • Assess the range of motion in your joints to identify any loss of flexibility  
  • Ask you to take deep breaths to measure how far your chest expands when inhaling and exhaling 
  • Check your posture to look for rounded shoulders, hunched back, or a curved spine 
  • Take a look at your eyes for signs of uveitis—a related condition to AS that causes inflammation in your eyes
  • Examine your skin and scalp for signs of psoriasis—an inflammatory skin condition 

Imaging Tests

Your provider may order imaging tests to look for signs of bone erosion (loss), issues in the spine, or inflammation in the joints. The most common imaging tests for AS are X-rays and MRI scans. 


X-rays can show changes in the bones of the spine and other joints in the body. These changes may include:

  • Erosions: Areas of bone damage or loss of bone 
  • Osteophytes: Bone growths that can form on the edges of the spine and other bones as the body attempts to repair the damage caused by inflammation 
  • Ankylosis: The fusion (or joining together) of the bones and joints in the spine, which causes stiffness and immobility 
  • Reduced joint space: A loss of space between the spine's bones, which can indicate inflammation in the joints 

Healthcare providers can also use these X-rays periodically throughout your lifetime if you receive a diagnosis for AS. X-rays can help you and your provider learn how your bone structure and joints are changing as your condition progresses over time. 

Magnetic Resonance Imaging (MRI) 

Magnetic resonance imaging (MRI) is a type of scan that can create detailed images of the body's soft tissues. MRI scans can help your provider look for inflammation and early signs of AS that aren’t always visible on X-rays. Similarly to X-rays, MRIs can also monitor the disease's progression over time and understand how your treatment is working, if you receive a diagnosis for AS. 

To diagnose AS, doctors will look for the following on an MRI:

  • Inflammation: Swelling in the spine and other joints in the body 
  • Bone edema: Fluid retention in the bones 
  • Synovitis: Inflammation of the synovial membrane—the lining of the joints that are located between two or more bones
  • Enthesitis: Inflammation of the enthesis—the area where tendons or ligaments attach to the bone 

Blood tests

No single blood test alone can diagnose AS. But, your provider may still order blood tests to look for biomarkers (molecules in your blood) that can indicate that you have the condition.    


The HLA-B27 test is a blood test that specifically checks for the HLA-B27 gene. HLA-B27 is a protein located on the surface of white blood cells. The protein plays an important role in the immune system and can cause your white blood cells to attack healthy tissues in the body by mistake. As a result, this can promote inflammation in your body and make you more vulnerable to inflammatory conditions, like AS.

If you test positive for the HLA-B27 blood test, you have the HLA-B27 gene. But, keep in mind that:

  • Having the gene does not guarantee that you will develop AS 
  • People who don’t have the gene can still develop AS 

Erythrocyte Sedimentation Rate (ESR) 

The erythrocyte sedimentation rate (ESR) blood test measures the amount of inflammation in your body. Healthcare providers often use this test for a number of inflammatory conditions, including AS, psoriasis, and arthritis.

In general, the higher the ESR, the more inflammation you have in your body. The ESR score in people with AS is typically higher than in people without AS. However, the test result is not specific to just AS. 

A high ESR score may also indicate the presence of other conditions such as viral infections, anemia, or other autoimmune diseases. Your ESR results may also vary depending on your age, alcohol consumption, exercise or activity levels, pregnancy status, or medications. That said, your healthcare providers will likely ask you about any medications or supplements you are taking before ordering an ESR blood test.

An estimated 40% to 50% of people with AS have a higher ESR result. It’s important to note that receiving a normal ESR result doesn’t exclude the possibility of AS. This just means that AS is a condition that affects each person’s body differently. 

C-Reactive Protein (CRP) 

C-reactive protein (CRP) is a blood test that measures the level of CRP in the blood—a protein that your liver produces in response to inflammation in the body. People with AS usually have higher levels of CRP in their blood due to inflammation in their spine and other joints.

Just like the ESR test, other conditions and infections can also affect your CRP levels. For this reason, your provider will holistically look at your blood tests results and imaging scans to better understand your condition. 

Diagnostic Criteria

Healthcare providers use a specific set of criteria—known as the Assessment of Spondyloarthritis International Society (ASAS) Classification Criteria—to give you a proper diagnosis for AS. 

These criteria were developed by an international group of experts in spondyloarthritis in 2009.  Your healthcare provider will use your symptoms and results from your imaging and blood tests to give you a diagnostic score for AS. 

In order to receive an accurate AS diagnosis, you must meet the following criteria:

  1. Chronic back pain and stiffness for at least three months and a limited range of motion or flexibility in the spine in at least two areas of the spine (e.g., cervical, thoracic, or lumbar spine) 
  2. Imaging scan that shows the presence of sacroiliitis—or, inflammation of the sacroiliac joints, which are the joints that connect the spine and pelvis
  3. And, one of the following factors: 
  • Having the HLA-B27 gene
  • A family history of AS
  • Evidence of peripheral arthritis (inflammation in the joints outside of the spine)
  • Evidence of uveitis (inflammation of the eye)
  • Evidence of psoriasis (inflammation on the skin)
  • Evidence of inflammatory bowel disease (inflammation in the digestive system)

Screening for Related Conditions 

AS can sometimes be difficult to diagnose because symptoms of AS can mimic the symptoms of other conditions. As a result, it is common for healthcare providers to perform additional tests to rule out other conditions and provide you with an accurate diagnosis for AS.

Some conditions that share some symptoms with AS include:

  • Degenerative disc disease 
  • Herniated disc
  • Spinal fracture
  • Spinal stenosis 
  • Osteoarthritis
  • Rheumatoid arthritis 
  • Reactive arthritis 
  • Psoriatic arthritis 
  • Diffuse idiopathic skeletal hyperostosis (DISH)
  • Mechanical back pain (from overuse or injury) 

Roughly 80% of people with AS also have other conditions that co-occur with AS. These conditions are known as comorbid conditions, meaning that you have AS and another condition at the same time. If you receive an AS diagnosis, you may also be at an increased risk for developing:

  • Hypertension
  • Depression
  • Diabetes 
  • Osteoporosis
  • Inflammatory bowel disease 
  • Psoriasis 

Your healthcare provider may test for other conditions periodically if you receive a diagnosis for AS. Some conditions can worsen your AS symptoms, so it’s important to get a diagnosis for co-occurring conditions sooner rather than later.  

A Quick Review

Ankylosing spondylitis (AS) is a type of inflammatory arthritis that affects the spine. The condition can be difficult to diagnose because symptoms of AS can mimic symptoms of other conditions. 

If you notice symptoms of AS or have a family history of the condition, your healthcare provider will ask you about your medical history, perform a physical exam, and order a variety of imaging and blood tests to learn more about your symptoms.

AS symptoms can be painful and debilitating, so it’s important to get tested for AS early. Getting an early diagnosis can help you start treatment sooner, slow disease progression, and improve your quality of life. 

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