Introduction: When to Seek Diagnosis
Anyone experiencing persistent lower back pain and stiffness that doesn’t improve with rest should consider seeking medical evaluation. Ankylosing spondylitis (AS), a type of inflammatory arthritis that primarily affects the spine, often begins in young adulthood, typically between the ages of 17 and 45 years.[1][4] However, symptoms can start even earlier, particularly in teenagers.
You should especially consider getting evaluated if your back pain has certain characteristics that suggest inflammation rather than ordinary muscle strain. Pain that gets worse at night, wakes you from sleep, or feels most severe in the early morning hours points toward inflammatory disease.[6] Similarly, if stiffness improves with movement and exercise but worsens with prolonged rest or inactivity, this pattern suggests something more than typical back strain.[7]
People with a family history of ankylosing spondylitis should be particularly alert to these symptoms, as the condition has a strong hereditary component. If you have a close biological relative with AS, your risk is higher than the general population.[3] Additionally, individuals who already have other inflammatory conditions like Crohn’s disease, ulcerative colitis, or psoriasis should discuss any new back pain with their healthcare provider, as these conditions are associated with increased risk of developing AS.[3]
Buttock pain that alternates between the left and right sides is another characteristic symptom worth investigating. This occurs because AS typically affects the sacroiliac joints, which are located where the base of the spine meets the pelvis.[6][14] Pain in the hips, reduced mobility in the lower back, difficulty expanding the chest when breathing deeply, or persistent fatigue accompanying back pain are all reasons to seek medical attention.
Classic Diagnostic Methods
Medical History and Physical Examination
The diagnostic journey begins with a thorough discussion of your symptoms and medical history. Your healthcare provider will ask detailed questions about when your pain started, what makes it better or worse, and whether you have a family history of AS or related conditions.[8] They’ll also want to know about any other symptoms you’ve experienced, such as eye inflammation, skin rashes, or digestive problems, as these can occur alongside ankylosing spondylitis.[1]
During the physical examination, your provider will assess your posture and how well you can move different parts of your spine. They may measure how far you can bend forward, backward, and to the sides to evaluate your spinal flexibility.[7] Testing chest expansion is another important part of the exam. You’ll be asked to take a deep breath while the provider measures how much your chest expands, since AS can affect the joints between the ribs and spine, limiting this movement.[8]
Your doctor will also check for areas of tenderness, particularly around the sacroiliac joints at the base of your spine and the pelvis. Pain or limited motion in your hips, shoulders, or other joints may be noted as well, since AS can affect areas beyond the spine.[2]
Imaging Studies
X-rays have traditionally been the primary imaging tool for diagnosing ankylosing spondylitis. These images can reveal changes in the joints and bones that develop as the disease progresses.[8] When AS is visible on X-rays, it’s called radiographic axial spondyloarthritis. The X-rays particularly focus on the sacroiliac joints, looking for signs of inflammation or damage.
Healthcare providers use a grading system to describe the severity of sacroiliitis (inflammation of the sacroiliac joints) seen on X-rays. According to established diagnostic criteria, the presence of grade 2 sacroiliitis on both sides or grade 3 to 4 sacroiliitis on at least one side, combined with characteristic clinical symptoms, supports a diagnosis of AS.[6] However, there’s an important limitation: changes visible on X-rays can take years to develop, meaning early disease often won’t show up on these images.[8]
Magnetic resonance imaging (MRI) has become increasingly valuable for detecting AS earlier in its course. This imaging technique uses magnetic fields and radio waves to create detailed pictures of bones and soft tissues, including areas of inflammation.[8] MRI can identify inflammatory changes in the sacroiliac joints and spine before they become visible on X-rays, allowing for earlier diagnosis and treatment.[12]
When inflammation is detected on MRI or other imaging tests but doesn’t yet show on X-rays, the condition is called non-radiographic axial spondyloarthritis. Both forms are part of the same disease spectrum and are treated similarly.[1][5]
Laboratory Tests
Blood tests play a supporting role in diagnosing ankylosing spondylitis, though no single blood test can definitively confirm the disease. Two common tests measure markers of inflammation in the body: the erythrocyte sedimentation rate (also called sed rate or ESR) and C-reactive protein (CRP).[8] Elevated levels of these markers suggest inflammation is present, though they don’t specify where or what’s causing it, as many different conditions can raise these values.
Testing for the HLA-B27 gene is another important part of the diagnostic process. This genetic marker is strongly associated with ankylosing spondylitis. More than 90% of people with AS who are of European descent carry this gene variant.[3] In the United States, the prevalence of HLA-B27 varies among ethnic groups, appearing in approximately 7.5% of non-Hispanic whites, 4.6% of Mexican Americans, and 1.1% of non-Hispanic blacks.[4]
However, it’s crucial to understand that having the HLA-B27 gene doesn’t mean you’ll definitely develop AS. Among people who test positive for HLA-B27, only about 5% to 6% will develop ankylosing spondylitis.[4] Conversely, some people with AS don’t carry this gene at all, so a negative test doesn’t rule out the disease.[8] The HLA-B27 test is most useful when considered alongside symptoms, physical examination findings, and imaging results.
Diagnostic Criteria
Healthcare providers often use established classification criteria to help diagnose AS. The Modified New York criteria from 1984 have been widely used for both clinical and research purposes.[6] These criteria require at least one clinical symptom combined with radiographic evidence of sacroiliitis. Clinical symptoms include back pain lasting at least three months that improves with exercise but not with rest, limitation of motion in the lower back in multiple directions, and reduced chest expansion compared to normal values for the person’s age and sex.
Because many people with early AS don’t yet have changes visible on X-rays, newer criteria have been developed that also consider MRI findings and other clinical features. These more recent approaches help identify people earlier in the disease process when treatment may be most effective at preventing long-term damage.[12]
Diagnostics for Clinical Trial Qualification
When patients are being considered for enrollment in clinical trials studying new treatments for ankylosing spondylitis, they typically need to meet specific diagnostic requirements. These standards ensure that study participants truly have the disease and that researchers can accurately measure whether treatments are working.
Clinical trials usually require participants to meet established classification criteria for AS, such as the Modified New York criteria. This means they must have both characteristic symptoms and objective evidence of sacroiliitis on imaging studies.[6] Documentation of sacroiliitis through X-rays or MRI is typically mandatory for trial enrollment.
Trials often assess disease activity using standardized measurement tools. One commonly used measure is a calculation that combines the patient’s own ratings of back pain, morning stiffness duration, and overall health, along with blood test results for inflammatory markers like C-reactive protein.[13] Higher scores indicate more active inflammation and more severe symptoms. Many trials specifically recruit patients with high disease activity to test whether new treatments can reduce inflammation and improve symptoms.
Blood tests documenting elevated inflammatory markers are frequently required for clinical trial participation. The C-reactive protein test is particularly common, as measurable inflammation provides a way to objectively track whether a treatment is working.[14] Some trials may also require confirmation of HLA-B27 status or documentation that participants have not responded adequately to standard treatments before enrolling.
Assessment of spinal mobility and function is another standard component of clinical trial diagnostics. Researchers measure things like how far forward participants can bend, the range of motion in their spine, and their ability to perform daily activities. These measurements establish a baseline that can be compared to measurements taken during and after treatment to evaluate effectiveness.
Imaging requirements for clinical trials are often more rigorous than for routine clinical diagnosis. Participants may need recent MRI scans showing active inflammation in the spine or sacroiliac joints, not just old damage. X-rays documenting the extent of structural changes in the spine may also be required. These imaging studies are typically repeated during the trial to monitor whether treatment prevents progression of joint damage.
