Diagnosing ankylosing spondylitis can be somewhat difficult because:

  • Low back pain is a common problem and usually caused by sore muscles or other conditions, such as osteoarthritis
  • X-rays are often normal, particularly in the early stages of the disease

See Osteoarthritis Symptoms and Signs

While it can be a challenge to recognize ankylosing spondylitis in its early stages, early diagnosis is ideal. When patients are diagnosed early, they can begin treatment before the disease has progressed very far, helping them retain joint flexibility and movement.

See Progression of Ankylosing Spondylitis in the Spine

During diagnosis, doctors will review a patient’s medical history, perform a physical exam, and—when necessary—order diagnostic tests, such as x-rays and blood tests.


Medical History

A patient will be asked to describe:

  • When and how the joint pain or other symptoms first occurred
  • If pain improves or worsens with exercise
  • If joint pain has changed in severity or location
  • If any treatment has provided pain relief
  • If he or she has ever experience symptoms other than joint pain, such as eye inflammation, skin rash, gastrointestinal problems, and fatigue
  • His or her family history of back pain and joint diseases

See 5 Unusual Ankylosing Spondylitis Symptoms

Patients who have upcoming appointments are encouraged to think about these issues ahead of time and perhaps even write down a dates and details. An accurate medical history can help a doctor make a diagnosis.

Physical Exam

The physician will conduct a full examination, including:

  • Looking at the patient’s posture to see if the lumbar spine (lower back) is losing its normal curve and beginning to flatten out, a condition called lumbar lordosis
  • Examining the whole spine to look for evidence of inflammation
  • Evaluating other joints, particularly the hips and ankles, that may be affected (particularly in women and children)
  • Checking for eye and skin abnormalities
  • Assessing each joint’s range of motion and trying to identify tender points

See Development and Progression of Ankylosing Spondylitis

During a physical exam a doctor will look at the patient’s spine and may ask the patient to do simple movements, such as bend over.

Seeing a specialist
If the physician suspects a patient has ankylosing spondylitis, he or she will likely refer the patient to a rheumatologist . A rheumatologist is a doctor trained to recognize the symptoms of ankylosing spondylitis and similar conditions. The rheumatologist will often conduct a medical history, physical exam, and other testing before ruling out or diagnosing a patient with ankylosing spondylitis.

See Arthritis Treatment Specialists

Medical Imaging

X-ray evidence of sacroiliitis—inflammation of the sacroiliac joint at the base of the spine—is one of the most telling signs of ankylosing spondylitis. However, a patient might feel sacroiliitis or other back pain years before changes in the spine’s anatomy can be seen on x-rays.

See Possible Causes of Sacroiliitis and SI Joint Pain

Magnetic resonance imaging (MRI scans), CAT scans (CT scans), and ultrasound can provide more accurate and detailed images, helping doctors identify earlier changes in the joints. These medical imaging techniques are more expensive and time consuming than x-rays, so they are only done if needed.

Blood Tests

There is no single lab test that can be used to diagnose ankylosing spondylitis. Instead, a variety of blood tests can be used to help rule out or provide evidence for an ankylosing spondylitis diagnosis. In addition to a routine complete blood count (CBC), a doctor may order testing for:

C-Reactive protein (CRP). When there is inflammation in the body, the liver produces a protein called CRP. A high CRP level indicates an inflammatory condition, including infection. For this reason, a high CRP level does not automatically mean a patient has ankylosing spondylitis. In addition to using this test for diagnosis, doctors often monitor CRP levels to gauge a patients’ response to treatment.

Erythrocyte sedimentation rate (ESR). Similar to CRP, this test also screens for inflammation. Because inflammation is a factor in many conditions, this test is not specific for ankylosing spondylitis and by itself does not diagnose a patient, but it can support the diagnosis. It is also used to see how active the condition is.

HLA-B27 gene. The majority of people with ankylosing spondylitis carry a gene called HLA-B27. A positive test for this gene can help confirm a suspected case of ankylosing spondylitis, but this test is not required for diagnosis when a patient has several obvious symptoms, or when sacroiliitis can be seen on an x-ray.


Doctors may also order other blood tests to rule out other possible diagnoses, including spinal or bone tumors, infection, pelvic inflammatory disease, metabolic bone disease, diffuse idiopathic skeletal hyperostosis (DISH), and prolapsed intervertebral disc.

For example, a doctor may order a test for rheumatoid factor, an antibody (protein). This protein is often found in high levels in the blood of people with rheumatoid arthritis and lupus but not in people with ankylosing spondylitis. (For this reason, ankylosing spondylitis and is called a seronegative spondyloarthropathy—the blood serum is negative for the rheumatoid factor.)

See Blood Tests to Help Diagnose Rheumatoid Arthritis (RA)

Dr. Judith Frank is a rheumatologist and internal medicine physician. She has been practicing for nearly 30 years, specializing in osteoarthritis, rheumatoid arthritis, gout, and lupus. She completed her Doctor of Medicine degree, residency, and fellowship training from Rush University.