What investigations are recommended to evaluate for ankylosing spondylitis?

Testing for Ankylosing Spondylitis

Start with plain radiographs of the pelvis (to visualize sacroiliac joints) as the first-line imaging study, and add cervical and lumbar spine radiographs if spinal symptoms are present. 1

Initial Imaging Approach

Radiography (First-Line)

• Obtain anteroposterior pelvis radiographs to evaluate the sacroiliac joints and hips, which are commonly involved in axial spondyloarthritis (axSpA). 1
• No need for oblique sacroiliac joint views as they provide no additional diagnostic benefit over standard anteroposterior views. 1
• If symptoms are referable to the spine, add cervical and lumbar spine radiographs to assess for syndesmophytes, erosions, shiny corners, vertebral body squaring, and ankylosis. 1
• Thoracic spine radiographs are generally not useful due to overlying structures obscuring visualization. 1

Important caveat: Radiographs have significant limitations—sensitivity ranges from only 19-72% and specificity from 47-84.5% for detecting sacroiliitis. 1 They miss more than half of patients with structural changes and cannot demonstrate active inflammation. 1

MRI (For Early Disease or Negative Radiographs)

• Consider MRI of the sacroiliac joints as the initial imaging modality in patients with short symptom duration (less than 3-7 years), as inflammatory changes on MRI precede radiographic findings by 3-7 years. 1
• If radiographs are negative but clinical suspicion remains high, MRI of the sacroiliac joints is the next appropriate study to detect early inflammatory changes (bone marrow edema/osteitis) before structural damage occurs. 1
• MRI of the spine alone is not routinely recommended as an initial study unless sacroiliac joint imaging has already been performed. 1

Essential Clinical Information to Provide

When ordering imaging studies, the referring physician must communicate specific clinical details to optimize radiologist interpretation: 1

• Patient age, sex, and HLA-B27 status (positive, negative, or unknown). 1
• Back pain characteristics: current or historical presence, duration, location, and whether inflammatory features are present (morning stiffness >30 minutes, improvement with exercise, nocturnal pain). 1
• History of physically demanding activities or childbirth (number of children and date of most recent delivery). 1
• Suspected clinical diagnosis and whether this is for primary diagnosis, disease activity assessment, or treatment response. 1

Laboratory Testing

While not imaging, HLA-B27 testing is valuable as 80-95% of AS patients carry this marker, though it is not diagnostic by itself. 2 Inflammatory markers (ESR, CRP) are non-specific but provide supporting evidence when elevated. 2

Modalities NOT Recommended

• Bone scintigraphy (SPECT/SPECT-CT): Not routinely obtained; has low to moderate sensitivity and variable specificity. 1
• CT scanning: Not recommended as initial imaging; reserved for specific situations where radiographs and MRI are inconclusive. 1
• Ultrasound: No established diagnostic utility for sacroiliitis or spinal involvement. 1
• PET/CT: Insufficient evidence to support routine use. 1

Diagnostic Algorithm Summary

1. Clinical suspicion based on inflammatory back pain features (onset <45 years, morning stiffness, improvement with exercise)
2. First imaging: Pelvis radiographs (anteroposterior view)
3. If symptoms >3-7 years and radiographs positive: Diagnosis confirmed
4. If symptoms <3-7 years or radiographs negative with high suspicion: Proceed to MRI of sacroiliac joints
5. Add spine radiographs (cervical/lumbar) if spinal symptoms present
6. Include HLA-B27 and inflammatory markers to support clinical diagnosis