Initial Workup for Suspected Ankylosing Spondylitis
Begin with anteroposterior radiographs of the pelvis to visualize the sacroiliac joints as your first-line imaging study, complemented by spine radiographs if symptoms localize to cervical or lumbar regions. 1, 2
Clinical Assessment
Target the right patient population:
- Evaluate patients with chronic back pain lasting >3 months that began before age 45 2
- Specifically screen for inflammatory back pain characteristics: morning stiffness >30 minutes, improvement with exercise (not rest), pain awakening during the second half of the night, and alternating buttock pain 2
- These inflammatory features have 75% sensitivity for axial spondyloarthritis 2
Document the following clinical parameters:
- Patient history using standardized questionnaires 1
- Physical function assessment (Bath Ankylosing Spondylitis Functional Index) 1
- Pain assessment via visual analog scale for spine pain in the past week 1
- Spinal mobility measurements: chest expansion, modified Schober test, occiput-to-wall distance 1
- Patient global assessment 1
- Morning stiffness duration 1
- Peripheral joint and entheses examination (number of swollen joints, painful entheses) 1
Laboratory Testing
Order acute phase reactants:
- Erythrocyte sedimentation rate (ESR) as part of the ASAS core set 1
- Consider HLA-B27 testing if radiographs are negative but clinical suspicion remains high 3
- Note that 90-95% of AS patients are HLA-B27 positive, though only 1% of HLA-B27 positive individuals develop AS 4
Initial Imaging Strategy
Radiography is your starting point:
- Obtain anteroposterior pelvis radiographs to evaluate both sacroiliac joints and hips 1, 2
- Add cervical and lumbar spine radiographs if symptoms are referable to these areas 2
- Oblique sacroiliac views provide no additional benefit over standard anteroposterior views 1
Understand radiography limitations:
- Sensitivity ranges only 19-72% with specificity 47-84.5% for sacroiliitis 1
- Radiographs miss more than half of patients with structural changes compared to CT 1
- Interobserver agreement is only fair to moderate 1
- Radiographs show chronic changes (erosions, sclerosis, ankylosis) but cannot demonstrate active inflammation 1, 2
When to Advance to MRI
Proceed to MRI of the sacroiliac joints in these specific scenarios:
- Short duration of symptoms where early disease is suspected 1, 2
- Negative radiographs but high clinical suspicion persists 2
- MRI detects inflammatory changes 3-7 years before radiographic structural findings appear 1, 2
MRI is not routinely obtained as initial imaging but serves as the critical next step when radiographs are unrevealing 1, 2
Imaging Modalities to Avoid Initially
Do not order these studies for initial workup:
- CT of sacroiliac joints or spine (not routinely obtained initially) 1
- Bone scintigraphy with SPECT or SPECT/CT (no supporting literature) 1
- Fluoride PET/CT (not routinely obtained) 1
- MRI of spine alone without sacroiliac joint imaging (no supporting literature for initial evaluation) 1
Assessment of Extra-Articular Manifestations
Screen for associated conditions:
- Anterior uveitis (most common extra-articular manifestation) 5
- Inflammatory bowel disease 5
- Cardiac involvement 5
- Psoriasis 1
Diagnostic Criteria Application
Use ASAS classification criteria to formalize diagnosis once imaging and clinical data are collected 3
Common Pitfalls to Avoid
- Do not dismiss AS in patients with normal initial radiographs - inflammatory changes precede structural radiographic findings by years 1, 2
- Do not order MRI spine without including sacroiliac joints - sacroiliitis is the primary site of early disease 1
- Do not rely solely on HLA-B27 testing - it has limited specificity as only 1% of positive individuals develop AS 4
- Do not delay referral to rheumatology if initial workup suggests AS, as early diagnosis enables effective treatment with NSAIDs and TNF inhibitors 2, 4
Monitoring Frequency
Individualize monitoring based on symptoms, severity, and treatment: