What is the diagnostic approach for ankylosing spondylitis (AS)?

Diagnostic Approach for Ankylosing Spondylitis

The diagnosis of ankylosing spondylitis (AS) requires a combination of clinical assessment, laboratory testing, and imaging, with radiographs of the sacroiliac joints as the first-line imaging modality and MRI recommended when radiographs are negative but clinical suspicion remains high. 1, 2

Patient Selection for Evaluation

• Evaluate patients with chronic back pain (>3 months duration) whose symptoms began before age 45 1, 2
• Focus on identifying features of inflammatory back pain, which has a sensitivity of 75% for axial spondyloarthritis 1
• Key inflammatory back pain characteristics include:
  • Morning stiffness lasting >30 minutes
  • Improvement with exercise but not with rest
  • Pain awakening during the second half of the night
  • Alternating buttock pain 1, 2

Initial Diagnostic Workup

Clinical Assessment

• Assess for limitation of motion in the cervical, thoracic, or lumbar spine in both sagittal and frontal planes 1
• Evaluate chest expansion relative to normal values for age and sex 1
• Document peripheral manifestations such as arthritis, enthesitis, and extra-articular features (uveitis, psoriasis, inflammatory bowel disease) 1, 3

Laboratory Testing

• HLA-B27 testing is highly recommended as a screening parameter with 90% sensitivity and specificity for axial spondyloarthritis 1
• 90-95% of patients with AS are positive for HLA-B27, making it an excellent screening tool 4
• Check inflammatory markers (ESR/CRP), though these have lower sensitivity (50%) and specificity (80%) 1

Imaging

• Radiographs of the sacroiliac joints (anteroposterior view of the pelvis) should be the first imaging modality 1, 2
• Complementary radiographs of the spine (cervical and lumbar) should be performed if symptoms are referable to these areas 2
• MRI of the sacroiliac joints is recommended when:
  • Radiographs are negative or equivocal but clinical suspicion remains high
  • Symptoms have short duration (early disease)
  • Need to detect inflammatory changes before radiographic structural damage appears 1, 2

Diagnostic Criteria

• Diagnosis of axial disease should be based on the presence of two of three of the following criteria:

  1. Inflammatory back pain features
  2. Limitation of spinal motion
  3. Radiological criteria (sacroiliitis grade ≥2, syndesmophytes, or MRI changes showing bone marrow edema) 1

• The Assessment of SpondyloArthritis International Society (ASAS) classification criteria are valid for research purposes but can guide clinical diagnosis 3

Disease Activity Assessment

• Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) is recommended to measure disease activity 1
• Active disease is defined as a BASDAI score >4 1
• Treatment response is defined as a BASDAI score <3 or a reduction by 2 points 1

Common Pitfalls and Caveats

• There is typically a marked delay (average 8 years) between symptom onset and diagnosis 5
• Radiographic changes may take several years of inflammation before becoming visible, with limited sensitivity (19%-72%) for early disease 2
• Interobserver agreement for radiographic findings is only fair to moderate 2
• Radiography misses more than half of patients with structural changes when compared to CT 2
• Traditional oral DMARDs (methotrexate, leflunomide, sulfasalazine) have not been shown effective for axial disease 1

Next Steps After Diagnosis

• Refer to rheumatology for further evaluation and management, especially if initial work-up suggests AS 2
• Early diagnosis is increasingly important for effective treatment with NSAIDs and TNF inhibitors 1, 2
• Regular monitoring and appropriate adjustment of therapy should aim at reaching remission or low disease activity 6

By following this diagnostic approach, clinicians can identify patients with ankylosing spondylitis earlier, potentially improving outcomes through timely intervention with appropriate therapies.