What is the initial workup for a patient suspected of having ankylosing spondylitis?

Initial Workup for Suspected Ankylosing Spondylitis

Begin with plain radiographs of the sacroiliac joints (anteroposterior pelvis view) as your first-line imaging, combined with HLA-B27 testing and inflammatory markers (ESR/CRP), but only in patients with chronic back pain lasting >3 months that started before age 45. 1, 2

Patient Selection Criteria

Screen only patients meeting these specific parameters:

• Chronic back pain duration >3 months 3, 1, 2
• Symptom onset before age 45 (AS rarely starts after age 40, occurring in <4% of cases) 3, 2
• Presence of inflammatory back pain characteristics (sensitivity 75%, post-test probability 14%) 3, 2

Clinical Assessment for Inflammatory Back Pain

Identify these specific features that distinguish inflammatory from mechanical back pain:

• Morning stiffness lasting >30 minutes that improves within an hour after movement 2, 4
• Improvement with exercise but NOT with rest (critical distinguishing feature) 1, 2
• Night pain awakening during the second half of the night 1, 2
• Alternating buttock pain indicating sacroiliac joint involvement 1, 2
• Good response to full-dose NSAIDs within 48 hours (sensitivity 75%, specificity 85%, post-test probability 21%) 2

Physical Examination Measurements

Perform these specific spinal mobility assessments:

• Modified Schober test for lumbar spine mobility 2
• Chest expansion measurement 2
• Occiput-to-wall distance 2
• Finger-to-floor distance 2

Laboratory Testing

Order these tests in this priority:

• HLA-B27 testing - the single most valuable laboratory test (sensitivity 90%, specificity 90%, post-test probability 32%) 2, 4
• Inflammatory markers (ESR/CRP) to assess disease activity 4

Note that HLA-B27 is present in 74-89% of AS patients, but only 1% of HLA-B27 positive individuals develop AS, so it must be interpreted in clinical context. 2

Initial Imaging Strategy

Follow this algorithmic approach:

Step 1: Plain Radiographs

• Anteroposterior view of the pelvis to visualize sacroiliac joints 1
• Complementary spine radiographs (cervical and lumbar) if symptoms are referable to these areas 1
• Look for chronic erosions, sclerotic changes, and ankylosis as sequelae of inflammatory sacroiliitis 1

Critical limitation: Radiographs have limited sensitivity (19-72%) for early disease and may miss more than half of patients with structural changes. 1 Radiographic changes may take several years of inflammation before becoming visible. 3, 1

Step 2: MRI of Sacroiliac Joints (if radiographs negative but suspicion remains high)

• MRI detects inflammatory changes 3-7 years before radiographic structural findings appear (sensitivity 90%, specificity 90%, post-test probability 32%) 1, 2
• STIR sequences are generally sufficient to detect inflammation; contrast medium is not needed 2
• MRI is particularly important in patients with short symptom duration or when clinical suspicion remains high despite negative radiographs 1

Step 3: CT (only if MRI cannot be performed)

• CT may provide additional information on structural damage if conventional radiography is negative and MRI cannot be performed, but is not routinely recommended 2

Extra-Articular Manifestations to Screen For

Assess for these associated conditions that support the diagnosis:

• Acute anterior uveitis (occurs in 15-40% of AS patients, likelihood ratio 7.3 when present) - refer immediately to ophthalmology if symptoms present 4, 5
• Inflammatory bowel disease 5
• Peripheral joint involvement and enthesitis 2, 5

Referral Threshold

Refer to rheumatology when:

• Radiographs show sacroiliitis 1
• Clinical suspicion remains high despite negative radiographs (proceed to MRI first, then refer) 1
• HLA-B27 positive with inflammatory back pain characteristics 2, 4

Early referral is increasingly important because TNF inhibitors show 50% improvement in about half of refractory patients, with 72% of patients with disease duration <10 years showing at least 50% improvement. 3 These treatments are more effective when used in early stages of the disease. 6

Common Pitfalls to Avoid

• Do not wait for radiographic sacroiliitis before considering the diagnosis - this can delay diagnosis by several years 3, 1
• Do not screen patients with back pain onset after age 45 or duration <3 months - this reduces specificity 3, 2
• Do not rely solely on HLA-B27 - interpret in clinical context as only 1% of positive individuals develop AS 2
• Do not overlook shoulder or hip involvement as the primary presenting symptom in the absence of low back pain 7
• Interobserver agreement for radiographic findings is only fair to moderate - consider MRI if clinical suspicion is high 1