Testing for Ankylosing Spondylitis

If you experience low back pain that lasts three or more months, you may have a type of axial spondylitis (axial SpA) called ankylosing spondylitis (AS). AS is an aggressive autoimmune condition, so getting an early diagnosis and starting treatment immediately is crucial.

The tests used to diagnose ankylosing spondylitis include a physical examination, a medical history, blood tests, genetic testing, and imaging.

Symptoms of AS typically start in the late teens to mid-30s, but anyone can develop it, including children. But because symptoms start slowly, they are often ignored or mistaken for other health problems. A 2015 reports found the average diagnostic delay for axial SpA is eight years.

It is not unusual for people with AS to see multiple healthcare providers to manage the back pain and stiffness that turns out to be AS. These medical professionals can include primary care providers, orthopedists (specialists in conditions of the musculoskeletal system), physical therapists, and chiropractors.

But the type of healthcare provider who can best diagnose AS is a rheumatologist—a doctor with specialized training in musculoskeletal and autoimmune diseases like AS. If you have symptoms suggestive of AS, make an appointment to see a rheumatologist or ask your primary healthcare provider for a referral. 

This article covers tests to diagnose AS, including physical and neurological exams, blood work, genetic testing, and imaging.  

Ankylosing Spondylitis Physical Exam

A physical examination of AS typically starts with a thorough review of your medical and family history.

Your medical history looks at whether your back pain is mechanical or inflammatory. According to the Spondylitis Association of America, mechanical back pain occurs due to physical changes in the back. It is often the result of everyday activities like lifting heavy objects, incorrect posture, or poor sleep positions. 

On the other hand, inflammatory back pain related to AS is persistent, lasting for more than three months, affects people under age 35, and worsens after periods of inactivity. Inflammatory back pain will improve with movement, exercise, and nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen. 

In addition to understanding the nature of your back pain, your healthcare provider will want to know when symptoms started and what worsens and improves them. They also want to know about systemic symptoms (those that affect the entire body) and other joint pain in the neck, hips, shoulders, arms, and legs.

It is helpful to share every detail about your symptoms and overall health to help the healthcare provider make an accurate diagnosis.

The HLA-B27 gene, which can lead to AS, is inherited. Moreover, family and twin studies have shown that AS has a strong genetic component. You should inform your healthcare provider if AS or other autoimmune diseases run in your family. 

Once the healthcare provider has gotten information about your medical and family history, they will want to perform a thorough physical and neurological examination.

You will be asked to bend your back and hips in different directions during the physical exam to check for stiffness and pain. They might also push on some body areas to look for tender spots.

Since breathing difficulty is a sign of severe AS, the healthcare provider will also check your breathing. Breathing troubles in AS result from lung scarring due to inflammation, the upper body arching forward, and the chest wall stiffening. 

The neurological exam assesses sensory and motor functions, including reflexes and muscle strength. Your healthcare provider will also review your other neurological symptoms, such as nerve pain, numbness, tingling, muscle weakness or spasms, and bowel and bladder dysfunction. 

Blood Tests for Ankylosing Spondylitis

Blood work can look for signs of inflammation suggestive of AS. It is also done to rule out other conditions. Blood tests are not enough to confirm AS, but they help the healthcare provider make sense of your symptoms. 

There are no specific tests to confirm AS, but certain blood tests can check for inflammatory markers and rule out other conditions that cause similar symptoms.

The following blood tests may be ordered if AS is suspected:

• Erythrocyte sedimentation rate (ESR or sed rate): The ESR test detects inflammation in the body. The ESR rate is measured by how quickly red blood cells settle in a test tube.
• C-reactive protein (CRP): CRP is a protein made by the liver. High levels of CRP are an indication of a condition that causes inflammation. 
• Complete blood count (CBC): The CBC test checks blood cell counts. High white blood cell counts and low red blood cell counts indicate inflammation commonly linked to AS.
• Rheumatoid factor (RF): RF is often associated with other autoimmune arthritis conditions like rheumatoid arthritis and lupus. The RF test is ordered to rule out these conditions when AS is suspected. 
  

Genetic Testing for Ankylosing Spondylitis

A genetic blood test can be requested to see if you carry the HLA-B27 gene. According to a 2018 report, this gene is only found in 8% of the general population. HLA-B27 is present in 90% of people with AS. 

Testing positive for HLA-B27 does not mean you will go on to develop AS. Only around 5% to 6% of people with the gene will develop AS, according to a 2019 report in the journal Bone Research.

If you carry the HLA-B27 gene, your healthcare provider is more likely to suspect AS. This gene is also linked to more severe AS symptoms, such as eye inflammation and a condition called uveitis (inflammation of the middle layer of tissue in the eye wall).

Ankylosing Spondylitis Imaging Tests

Different imaging studies can help with making a diagnosis of AS, including conventional radiology (X-rays), magnetic resonance imaging (MRI), computed tomography (CT), and ultrasound. X-rays are standard for diagnosing AS, while a CT, MRI, or ultrasound scan might be requested to evaluate bone and soft tissues of the spine in greater detail. 

If your healthcare provider requests imaging, they might be looking for the following signs of AS:

• Sacroiliitis (inflammation of one or both SI joints)
• Loss of cartilage in the facet joints (located between the vertebrae), which can lead to fusing 
• Loss of normal spinal curvature leading to kyphosis (humpback) or lordosis (swayback)
• Spinal and pelvic fractures
• Bone erosion (bone loss) in the low back
• Bony growths in the vertebrae
• Calcification (bone hardening)
  

Your healthcare provider might use X-rays and MRI to track AS disease progression or to look for fractures or joint damage.

Confirming an Ankylosing Spondylitis Diagnosis

Your healthcare provider will rely on your physical and neurological exams, symptom history, imaging results, and blood work to diagnose AS.

According to a 2020 Rheumatology report, a healthcare provider can make a diagnosis of AS based on specific criteria, which include:

• Back symptoms: This includes back pain that has lasted three or months and started before age 40 that resolves with exercise and is worse in the morning and after periods of inactivity, back pain that alternates into buttock pain, and limited movement of the lower back
• Finding relief with NSAIDs, which indicates back pain is the result of inflammation
• Peripheral manifestations, such as arthritis in the arms, legs, and shoulders, dactylitis (swelling of the fingers and toes), and enthesitis (inflammation of the entheses—the sites where tendons and ligaments attach to bone)
• Extra-musculoskeletal manifestations, such as uveitis, psoriasis (a chronic skin condition), and inflammatory bowel disease
• Positive family history of AS
• HLA-B27 positivity 
• High CRP and ESR levels
• Evidence of sacroiliitis on imaging 
• Osteitis (bone inflammation) and/or bone marrow edema (fluid buildup in bone marrow), which might be seen on MRI

If you have experienced inflammatory back pain for at least three or more months, along with other characteristics that are part of AS diagnostic criteria, your healthcare provider will likely diagnose you with AS. 

Summary

Ankylosing spondylitis is an autoimmune disease that occurs when the immune system malfunctions and attacks healthy tissues—mainly the small bones of the spine. AS is an aggressive condition and early diagnosis is crucial to reducing spine complications, such as spinal fusion, fractures, and abnormal curvatures.

If you experience signs and symptoms of AS, like pain and stiffness in the low back and hips, reach out to a healthcare provider so they can request testing to determine the cause. Tests used to diagnose AS include physical and neurological exams, blood tests, genetic testing, and imaging. 

A diagnosis of AS is typically made based on symptoms, blood work, imaging results, and HLA-B27 positivity. 

A Word From Verywell 

Ankylosing spondylitis is a lifelong condition. There is no cure for the disease, but it is treatable and manageable. 

You will have many options for treating your condition, which can reduce symptoms and prevent disease progression. Taking your medications as prescribed is crucial to slowing down the disease’s effects on you, keeping you moving and independent, and maintaining a good quality of life. 

In addition to following your treatment plan, you should see a rheumatologist regularly. And make sure you speak up if your treatment plan isn’t managing your symptoms adequately.