What investigations should be ordered in a case of suspected ankylosing spondylitis?

Investigations for Suspected Ankylosing Spondylitis

Radiographs of the sacroiliac joints should be ordered as the initial imaging modality for all patients with suspected ankylosing spondylitis, followed by HLA-B27 testing and inflammatory markers. 1, 2

Initial Diagnostic Workup

Laboratory Tests

1. HLA-B27 testing

   • Sensitivity of 90% and specificity of 90% for ankylosing spondylitis 1
   • Positive result significantly increases likelihood of disease with a likelihood ratio of 9 1
   • Note: A negative HLA-B27 result does not rule out ankylosing spondylitis 2

2. Inflammatory markers

   • Erythrocyte sedimentation rate (ESR)
   • C-reactive protein (CRP)
   • Important caveat: Both have sensitivity of only about 50% 1, 3, 4
   • Normal levels do not exclude the diagnosis as approximately 50% of patients may have normal values 1, 3

Imaging Studies

1. Radiography of sacroiliac joints (first-line)

   • Initial imaging modality of choice 2, 1
   • Assess for sacroiliitis meeting modified New York criteria 2

2. If radiographs are negative or equivocal:

   • MRI of sacroiliac joints (without IV contrast is sufficient, though contrast may help in initial evaluation) 2
   • MRI can detect inflammatory changes 3-7 years before they appear on radiographs 2
   • Request should specify "evaluation for axial spondyloarthritis" to ensure proper sequences are used 2

3. If sacroiliac joint imaging is negative but clinical suspicion remains high:

   • Radiography of the spine (cervical, thoracic, and/or lumbar based on symptoms) 2
   • MRI of the spine (without IV contrast) 2
   • CT of sacroiliac joints may be considered to identify subtle erosions, especially in patients unable to undergo MRI 2

Special Considerations

Patients with Spine Ankylosis and Suspected Fracture

• These patients are at high risk for unstable fractures even with minor trauma 2
• Radiography has poor sensitivity for fractures in ankylosing spine 2
• CT with multiplanar reformatted images is necessary for fracture exclusion 2
• MRI without contrast is recommended if neurologic symptoms are present 2

When to Refer to a Rheumatologist

Refer patients with back pain starting before age 45, lasting >3 months, plus at least 4 of:

• Back pain occurring before age 35
• Waking at night due to pain
• Buttock pain
• Improvement with movement or NSAIDs
• First-degree relative with spondyloarthritis
• Current or previous arthritis, enthesitis, or psoriasis

OR 3 of the above criteria plus positive HLA-B27 2, 1

Monitoring Disease Activity

• Regular monitoring of ESR and CRP is conditionally recommended 1
• Neither ESR nor CRP is superior for assessing disease activity 3, 4, 5
• Use of validated ankylosing spondylitis disease activity measures at regular intervals 1

Common Pitfalls to Avoid

1. Ruling out ankylosing spondylitis based solely on negative HLA-B27 or normal inflammatory markers 2, 1
2. Failing to order MRI when radiographs are negative but clinical suspicion is high 2
3. Using standard MRI protocols without fluid-sensitive sequences (STIR or T2-weighted fat-saturated) needed to detect inflammatory features 2
4. Missing fractures in patients with ankylosis by relying on radiographs alone 2
5. Overreliance on bone scintigraphy (not recommended as initial imaging) 2

Remember that no single test can diagnose ankylosing spondylitis, and diagnosis requires a combination of clinical, laboratory, and imaging findings 1.