What tests should be done if ankylosing spondylitis is suspected?

Diagnostic Testing for Suspected Ankylosing Spondylitis

When ankylosing spondylitis is suspected in patients with chronic back pain (>3 months duration) starting before age 45, begin with plain radiographs of the sacroiliac joints and HLA-B27 testing, followed by MRI of the sacroiliac joints if radiographs are negative but clinical suspicion remains high. 1, 2

Initial Patient Selection Criteria

Before ordering any tests, confirm the patient meets screening criteria:

• Chronic back pain lasting >3 months with onset before age 45 years (AS rarely starts after age 40 in <4% of cases) 1, 3
• Look for inflammatory back pain features: insidious onset, morning stiffness >30 minutes, improvement with exercise but not rest, pain awakening during the second half of the night, and alternating buttock pain 1, 3, 2

Laboratory Testing

HLA-B27 is the single most valuable laboratory test and should be ordered early in the diagnostic workup 1, 3:

• Sensitivity: 90%, specificity: 90%, post-test probability: 32% 1, 3
• Only 3 patients with chronic back pain who are HLA-B27 positive need to be evaluated by a rheumatologist to diagnose one case 1
• Present in 74-89% of AS patients, though only 1% of HLA-B27 positive individuals develop AS 3
• The test is binary (positive/negative), easy to interpret, requires testing only once, and costs are comparable to standard radiographs 1

Acute phase reactants (ESR/CRP) are NOT recommended for screening because sensitivity is only 50% in AS patients, resulting in a post-test probability of merely 12% 1. However, CRP may be useful for monitoring disease activity once diagnosis is established 1.

Imaging Strategy

First-Line Imaging: Plain Radiographs

Obtain anteroposterior radiographs of the pelvis (to visualize sacroiliac joints) as the initial imaging study 1, 2:

• Look for bilateral grade 2 sacroiliitis or unilateral grade 3+ sacroiliitis (modified New York criteria) 1
• Add radiographs of symptomatic spine regions (cervical, thoracic, or lumbar) if symptoms are referable to those areas 1, 2
• Radiographs demonstrate chronic erosions, sclerotic changes, and ankylosis but have limited sensitivity (19-72%) for early disease 2
• Radiographic changes may take several years of inflammation before becoming visible 2

Critical pitfall: Radiographs can miss more than half of patients with structural changes compared to CT, and interobserver agreement is only fair to moderate 2. Do not stop the workup if radiographs are negative but clinical suspicion remains high.

Second-Line Imaging: MRI

If radiographs are negative but clinical suspicion remains high (especially with short symptom duration), proceed to MRI of the sacroiliac joints 1, 3, 2:

• MRI detects inflammatory changes 3-7 years before radiographic structural findings appear 3, 2
• Sensitivity and specificity both 90%, post-test probability 32% 1, 3
• STIR sequences are generally sufficient to detect inflammation; contrast medium is not needed 3
• MRI assesses both active inflammatory lesions (bone marrow edema) and structural lesions 3

Important clinical information to provide on imaging requisitions (per ASAS 2024 recommendations) 1:

• Patient age, sex, and HLA-B27 status
• Duration and localization of back pain
• Presence or absence of inflammatory features
• History of physically demanding activities or childbirth (can cause bone marrow edema mimicking AS)

Alternative Imaging: CT

CT is NOT routinely recommended for initial diagnosis but may provide additional information on structural damage if radiography is negative and MRI cannot be performed 3. CT is primarily reserved for evaluating suspected fractures in patients with established spinal ankylosis 1.

Do NOT use bone scintigraphy, fluoride PET/CT, or ultrasound for AS diagnosis—there is no supporting evidence for these modalities in this context 1.

Clinical Assessment Parameters

While not "tests" per se, document these clinical findings to support the diagnosis:

• Response to NSAIDs: Good response to full-dose NSAIDs within 48 hours has sensitivity 75%, specificity 85%, post-test probability 21% 3
• Spinal mobility measurements: Modified Schober test, chest expansion, occiput-to-wall distance, finger-to-floor distance 3
• Extra-articular manifestations: Anterior uveitis, inflammatory bowel disease, psoriasis, peripheral arthritis 3

Diagnostic Algorithm Summary

1. Screen patients with chronic back pain >3 months, onset <age 45, with inflammatory features 1, 2
2. Order simultaneously: Plain radiographs of sacroiliac joints (AP pelvis view) + HLA-B27 testing 1, 2
3. If radiographs show sacroiliitis: Diagnosis established, refer to rheumatology 2
4. If radiographs negative but HLA-B27 positive or strong clinical suspicion: Order MRI of sacroiliac joints 1, 3, 2
5. If MRI shows active inflammation or structural changes: Refer to rheumatology for definitive diagnosis and treatment 2

Do not delay referral to rheumatology if multiple screening parameters are positive, even if imaging is pending—early diagnosis is critical for preventing irreversible structural damage 1, 3.