Board approved icon
Medically Reviewed by Expert Board.

This page was fact checked by our expert Medical Review Board for accuracy and objectivity. Read more about our editorial policy and review process.

This article was last modified on

What Is Ankylosing Spondylitis?

Ankylosing spondylitis (AS) is the chronic inflammation and swelling of the joints that connect the spine to the pelvis. These are called the sacroiliac joints.

The most common symptoms of ankylosing spondylitis are back pain and stiffness. In some cases, the affected spinal joints can fuse together, leading to limited spinal mobility and changes in posture.

Ankylosing spondylitis can affect areas of the body other than the lower back and spine. AS can cause arthritis to develop in the hips, knees, heels, ribs, and shoulders. Other complications can include inflammatory bowel disease, psoriasis, and inflammation of the pigmented inner lining of the eye, called uveitis.

Ankylosing spondylitis is a chronic health condition. This means that the symptoms of AS can be managed, but there is no cure for the disease. The goal of treatment is to slow the progression of the disease and improve quality of life. The degree of pain and disability caused by AS varies significantly for individual patients and between patients. Many patients experience flare ups during which symptoms worsen and periods when symptoms are less severe.

Ankylosing spondylitis is more commonly diagnosed in men or people assigned male at birth and typically develops between the ages of 20 and 40.

The exact cause of ankylosing spondylitis is not known. Researchers believe that AS is caused by a combination of genes and things in the environment. People with a first-degree relative, such as a parent or sibling, with ankylosing spondylitis are more likely to develop the condition. The human leukocyte antigen B27 (HLA-B27) gene has also been associated with an increased risk of developing this condition.

The Role of Ankylosing Spondylitis Tests

Ankylosing spondylitis testing is used to help diagnose AS, determine the severity of the condition, and monitor the disease’s progression and response to treatment.

No single test is able to diagnose ankylosing spondylitis. Doctors use information gathered from taking a medical history and performing a physical exam along with the results of laboratory and imaging tests to make a diagnosis.

The symptoms of ankylosing spondylitis can be similar to the symptoms of other conditions. Your doctor may perform tests to rule out other health problems that could be the source of your symptoms. This is an important part of diagnosing ankylosing spondylitis.

Because AS is a chronic disease, your doctor may recommend periodic testing to check for complications or to see if the severity of your disease has changed with time or after treatment.

Who should get testing?

Ankylosing spondylitis testing should be considered in patients under the age of 45 who have daily lower back pain lasting longer than 3 months. Testing may also be considered in patients who have other symptoms that suggest ankylosing spondylitis, which may include:

  • Inflammation of the pigmented inner lining of the eye
  • Inflammatory bowel disease
  • Arthritis in the hip, knee, heel, ankles, or shoulder
  • Psoriasis, a disease that causes itchy and sore patches of skin
  • Swelling in the fingers and toes

Early detection and treatment can help try to manage symptoms, prevent the disease from progressing, and improve quality of life. For this reason, it is important to speak with your doctor if you have symptoms that suggest ankylosing spondylitis. Your doctor is in the best position to say if testing for ankylosing spondylitis is appropriate in your situation.

For people who have already been diagnosed with ankylosing spondylitis, ongoing testing is used to monitor the evolution of symptoms and how well treatment is working.

Getting test results

Your doctor will usually give you the results of ankylosing spondylitis testing. The results of some tests may be available right away, while other test results may take a few days. Your doctor will evaluate your test results in the context of your current health and your medical history.

Your doctor may contact you by phone to discuss your test results, or they may schedule a follow-up visit to go over the results in person. If you have access to an online medical record, your test results may be available online.

Types of Ankylosing Spondylitis Tests

There is a range of tests and examinations that may be performed to diagnose and manage ankylosing spondylitis. The following sections provide information about the different types of ankylosing spondylitis testing.

Health history and physical exam

Ankylosing spondylitis testing often begins with a complete health history. Your doctor will ask about your symptoms and how long you have had them. Your doctor will also ask whether you have a history of other health conditions such as psoriasis or inflammatory bowel disease. You may be asked about whether you have a family history of ankylosing spondylitis or the HLA-B27 gene.

If you are receiving treatment for AS, your doctor will work with you to understand how treatment is going. You may be asked about your symptoms and if they have improved. You may also be asked to complete standard questionnaires that allow your doctor to track your symptoms and response to treatment over time.

Your doctor will also perform physical exams as part of your testing for ankylosing spondylitis. A physical exam is part of the initial evaluation for the disease. A physical exam for AS will assess the range of motion in your spine and examine other joints for signs of arthritis. Your doctor will check your eyes, hands, feet, and skin for signs that suggest ankylosing spondylitis.

Physical exams are also an important part of monitoring patients undergoing treatment. Over time, changes in your physical exam can help the doctor assess the progression of AS and its response to treatment.

Imaging tests

Imaging tests play an important role in the diagnosis of ankylosing spondylitis. Your doctor will order an x-ray of your spine and pelvis if your physical exam and health history suggest a diagnosis of ankylosing spondylitis. X-rays can show abnormalities of the sacroiliac joints and help your doctor make a diagnosis of AS.

An MRI may be suggested if no changes of the sacroiliac joints are detected on x-ray but your doctor suspects that you have ankylosing spondylitis. An MRI can show subtle changes and inflammation in the joints that are not visible on an x-ray.

Less often, ultrasound may be used to obtain an image of inflammation around where tendons or ligaments connect to bones.

Laboratory tests

Several laboratory tests are typically ordered as part of testing for ankylosing spondylitis. The results of these tests alone are not enough to make a diagnosis, but they may help to confirm that symptoms are not being caused by another condition.

Examples of laboratory tests for ankylosing spondylitis include:

  • Human leukocyte antigen B27 (HLA-B27): Many patients with ankylosing spondylitis carry a gene variant called HLA-B27. This gene variant causes the body to produce HLA-B27 antigen, a protein found on the surface of white blood cells. Testing for this antigen in the blood can help identify people with the HLA-B27 gene variant. While many patients with AS carry the HLA-B27 gene variant, testing positive for it does not mean you will develop ankylosing spondylitis.
  • C-reactive protein (CRP): CRP is made in the liver and is one of several proteins that increase in response to inflammation. Testing a sample of blood for CRP levels can provide information about inflammation in the body. CRP is elevated in about 40% of patients with ankylosing spondylitis.
  • Erythrocyte sedimentation rate (ESR): ESR is a test performed on a sample of blood that measures how quickly red blood cells fall to the bottom of a test tube. Red blood cells that settle more quickly than normal indicate inflammation in the body.

Your doctor may also order other laboratory tests such as a complete blood count (CBC) or synovial fluid analysis depending on your situation.

Getting Tested for Ankylosing Spondylitis

Ankylosing spondylitis testing is typically ordered by a doctor after a patient has reported symptoms that suggest AS. Testing is also ordered for patients being treated for ankylosing spondylitis. Testing usually takes place in a medical office, laboratory, or hospital.

In addition to working with your primary care provider, you may be referred to a specialist called a rheumatologist, a doctor who specializes in joint diseases, for testing and ongoing medical care related to your condition. Depending on the severity and symptoms of your disease, you may work with other specialists such as physical therapists, dermatologists, surgeons, or ophthalmologists.

At-home testing

At-home testing for ankylosing spondylitis is not commercially available. Given the complexity of AS testing, diagnosing and monitoring this condition should always be performed by a trained health professional.

View Sources

A.D.A.M. Medical Encyclopedia. ESR. Updated May 6, 2019. Accessed January 6, 2022. https://medlineplus.gov/ency/article/003638.htm

A.D.A.M. Medical Encyclopedia. Ankylosing spondylitis. Updated May 31, 2020. Accessed January 4, 2022. https://medlineplus.gov/ency/article/000420.htm

A.D.A.M. Medical Encyclopedia. C-reactive protein. Updated January 31, 2021. Accessed January 6, 2022. https://medlineplus.gov/ency/article/003356.htm

A.D.A.M. Medical Encyclopedia. HLA-B27 antigen. Updated May 2, 2021. Accessed January 6, 2022. https://medlineplus.gov/ency/article/003551.htm

Gower T. Enthesitis and PsA. Arthritis Foundation. Date unknown. Accessed January 9, 2022. https://www.arthritis.org/health-wellness/about-arthritis/related-conditions/physical-effects/enthesitis-and-psa

MedlinePlus: National Library of Medicine. Ankylosing spondylitis. Updated September 1, 2014. Accessed January 4, 2022. https://medlineplus.gov/genetics/condition/ankylosing-spondylitis/

MedlinePlus: National Library of Medicine. HLA-B gene. Updated July 1, 2015. Accessed January 9, 2022. https://medlineplus.gov/genetics/gene/hla-b/

MedlinePlus: National Library of Medicine. Erythrocyte sedimentation rate (ESR). Updated July 31, 2020. Accessed January 6, 2022. https://medlineplus.gov/lab-tests/erythrocyte-sedimentation-rate-esr/

National Institute of Arthritis and Musculoskeletal and Skin Disease. Ankylosing spondylitis. Updated February 2020. Accessed January 4, 2022. https://www.niams.nih.gov/health-topics/ankylosing-spondylitis/advanced

UpToDate. Patient education: Ankylosing spondylitis (the basics). Date unknown. Accessed January 5, 2022. https://www.uptodate.com/contents/ankylosing-spondylitis-the-basics

Wenker KJ, Quint JM. Ankylosing spondylitis. In: StatPearls. Updated August 4, 2021. Accessed January 5, 2022. https://www.ncbi.nlm.nih.gov/books/NBK470173/

Yu DT, Van Tubergen A. Patient education: Axial spondyloarthritis, including ankylosing spondylitis (beyond the basics). In: Sieper J, ed. UpToDate. Updated January 7, 2020. Accessed January 4, 2022. https://www.uptodate.com/contents/axial-spondyloarthritis-including-ankylosing-spondylitis-beyond-the-basics

Yu DT, Van Tubergen A. Clinical manifestations of axial spondyloarthritis (ankylosing spondylitis and nonradiographic axial spondyloarthritis) in adults. In: Sieper J, ed. UpToDate. Updated January 9, 2020. Accessed January 4, 2022. https://www.uptodate.com/contents/clinical-manifestations-of-axial-spondyloarthritis-ankylosing-spondylitis-and-nonradiographic-axial-spondyloarthritis-in-adults

Yu DT, Van Tubergen A. Diagnosis and differential diagnosis of axial spondyloarthritis (ankylosing spondylitis and nonradiographic axial spondyloarthritis) in adults. In: Sieper J, ed. UpToDate. Updated March 11, 2020. Accessed January 4, 2022. https://www.uptodate.com/contents/diagnosis-and-differential-diagnosis-of-axial-spondyloarthritis-ankylosing-spondylitis-and-nonradiographic-axial-spondyloarthritis-in-adults

Yu DT, Van Tubergen A. Treatment of axial spondyloarthritis (ankylosing spondylitis and nonradiographic axial spondyloarthritis) in adults. In: Sieper J, ed. UpToDate. Updated November 20, 2020. Accessed January 5, 2022. https://www.uptodate.com/contents/treatment-of-axial-spondyloarthritis-ankylosing-spondylitis-and-nonradiographic-axial-spondyloarthritis-in-adults

Yu DT, Van Tubergen A. Pathogenesis of spondyloarthritis. In: Sieper J, ed. UpToDate. Updated February 16, 2021. Accessed January 9, 2022. https://www.uptodate.com/contents/pathogenesis-of-spondyloarthritis

Ask a Laboratory Scientist

Ask a Laboratory Scientist

This form enables patients to ask specific questions about lab tests. Your questions will be answered by a laboratory scientist as part of a voluntary service provided by one of our partners, American Society for Clinical Laboratory Science. Please allow 2-3 business days for an email response from one of the volunteers on the Consumer Information Response Team.

Send Us Your Question