Workup for Ankylosing Spondylitis
Begin with plain radiographs of the sacroiliac joints (anteroposterior pelvis view) as your first-line imaging, and order HLA-B27 testing concurrently in patients with chronic back pain (≥3 months) that started before age 45 with inflammatory characteristics. 1, 2, 3
Initial Clinical Assessment
Target patients with these specific inflammatory back pain features, which have 75% sensitivity for axial spondyloarthritis 2, 3:
- Insidious onset before age 40-45 years with symptoms lasting ≥3 months 2
- Morning stiffness lasting >30 minutes that is prolonged 2, 3
- Improvement with exercise but NOT with rest (key distinguishing feature) 2, 3
- Night pain awakening in the second half of the night 2, 3
- Alternating buttock pain indicating sacroiliac joint involvement 2, 3
Additional Clinical Features to Document
- Peripheral arthritis: Large joint involvement (especially knees) in 30-50% of patients, typically oligoarticular and asymmetric 2
- Extra-articular manifestations: Uveitis, psoriasis, or inflammatory bowel disease 2
- Family history: Personal or family history of related spondyloarthropathies 4
Initial Laboratory and Imaging Workup
Laboratory Testing
- HLA-B27: Present in 74-89% of AS patients, though NOT diagnostic alone 2, 3
- The American Academy of Family Physicians explicitly states: do NOT rule out spondyloarthritis based solely on a negative HLA-B27 test 2
- C-reactive protein (CRP): May be elevated but is NOT always elevated, so normal inflammatory markers do not exclude disease 2, 5
Initial Imaging
Radiography is the appropriate first-line imaging modality 1, 3:
- Anteroposterior pelvis radiograph to evaluate sacroiliac joints (rated 9/9 appropriateness by ACR) 1, 3
- Add cervical and lumbar spine radiographs if symptoms are referable to these areas 1, 3
- Radiographs demonstrate chronic erosions, sclerotic changes, and ankylosis but have limited sensitivity (19-72%) for early disease 1, 3
Critical Pitfall with Radiography
Radiographs miss more than half of patients with structural changes compared to advanced imaging, and interobserver agreement is only fair to moderate 1, 3. One study showed 41.3% of radiography reports gave incorrect diagnoses 1. Do NOT stop at negative radiographs if clinical suspicion remains high 1, 3.
Next Step: MRI When Indicated
Order MRI of the sacroiliac joints (without contrast is sufficient) if 1, 3:
- Radiographs are negative or equivocal BUT clinical suspicion remains high 1, 3
- Short duration of symptoms (early disease suspected) 1, 3
- MRI can detect inflammatory changes 3-7 years before radiographic findings appear 1, 3, 5
MRI of both sacroiliac joints AND spine may be considered for comprehensive evaluation, though sacroiliac joint MRI alone is often sufficient initially 1, 6
Imaging Modalities to AVOID
The ACR guidelines are explicit about what NOT to order 1:
- CT: Not routinely obtained for initial evaluation 1
- Bone scan with SPECT or SPECT/CT: No relevant literature supporting use 1
- Fluoride PET/CT: Not routinely obtained 1
- Ultrasound: Not a routine diagnostic tool, limited to superficial posterior margins 1
Diagnostic Algorithm Summary
- Screen clinically: Chronic back pain ≥3 months, onset <45 years, with inflammatory characteristics 2, 3
- Order concurrently: Plain radiographs (AP pelvis + spine if symptomatic) AND HLA-B27 2, 3
- If radiographs show sacroiliitis: Diagnosis established, refer to rheumatology 3
- If radiographs negative but high clinical suspicion: Proceed to MRI sacroiliac joints 1, 3
- Refer to rheumatology if positive HLA-B27 with high clinical suspicion OR if MRI shows inflammatory changes 2, 3
Why Early Diagnosis Matters
Diagnosis is frequently delayed by 4.9-8 years from symptom onset 2, yet early treatment with NSAIDs and TNF inhibitors is increasingly effective and may slow radiographic progression 3, 5, 7. This makes aggressive workup in appropriate patients clinically imperative despite the diagnostic challenges.