What are the diagnostic criteria and management options for a young to middle-aged adult presenting with chronic back pain and stiffness, suggestive of Ankylosing Spondylitis (AS)?

Diagnosis of Ankylosing Spondylitis

Begin with plain radiographs of the sacroiliac joints as first-line imaging in any patient under 45 years old with chronic back pain (>3 months) who has inflammatory back pain features, and proceed to MRI if radiographs are negative or equivocal. 1

Clinical Recognition of Inflammatory Back Pain

The diagnosis hinges on recognizing inflammatory back pain, which differs fundamentally from mechanical back pain. Inflammatory back pain is present in 70-80% of patients with axial spondyloarthritis and includes at least 2 of these 4 features: 1, 2, 3

• Morning stiffness lasting >30 minutes 2, 3, 4
• Improvement with exercise but not with rest 1, 2, 3
• Awakening due to back pain during the second half of the night 1, 2, 4
• Alternating buttock pain 1, 4

When at least 2 of these 4 parameters are present, sensitivity is 70% and specificity is 81% (likelihood ratio 3.7), yielding a post-test probability of 14%. 1, 4 If 3 or more features are present, the likelihood ratio increases to 12.4. 4

A critical pitfall: Do not confuse this with mechanical back pain, which worsens with movement and improves with rest—the exact opposite pattern. 2, 3

Imaging Algorithm

Step 1: Plain Radiographs

Order anteroposterior radiographs of the pelvis to visualize both sacroiliac joints. 1 This remains the recommended first imaging modality despite limitations. 1

• Sacroiliitis grade ≥2 bilaterally or grade ≥3 unilaterally has sensitivity of 66-80% and specificity of 68-80%. 1, 2
• Radiographic changes often lag 7 or more years behind symptom onset. 1
• The modified New York criteria still require radiographic sacroiliitis for definitive AS diagnosis. 1

Step 2: MRI if Radiographs Negative/Equivocal

If clinical suspicion remains high but radiographs show no sacroiliitis, proceed immediately to MRI of the sacroiliac joints with STIR or T2-weighted fat-saturated sequences. 1

• MRI detects bone marrow edema indicating active inflammation with sensitivity of 78-90% and specificity of 88-90%. 2, 1
• Gadolinium contrast is optional and does not significantly increase diagnostic accuracy. 1
• MRI identifies "pre-radiographic" spondyloarthropathy years before structural changes appear. 1, 5

Do not use CT as a screening tool due to radiation exposure in young patients, though it may help when MRI is contraindicated and radiographs are equivocal. 1

Laboratory Testing

HLA-B27

Order HLA-B27 testing in all patients with suspected AS. 1, 3, 6

• Positive in 74-89% of patients with axial spondyloarthritis. 1, 6
• When positive in a patient with chronic back pain, post-test probability increases to 32% (likelihood ratio 9). 1, 6
• HLA-B27 has the highest overall rating as a screening parameter. 1

Inflammatory Markers

Check ESR and CRP, but recognize their limitations. 1, 3, 6

• Normal in approximately 50% of patients with active disease. 6
• Cannot rule out AS if negative. 3, 6
• When elevated, they support the diagnosis but add limited discriminatory value (likelihood ratio 2.5). 1

Diagnostic Criteria Application

Use the ASAS (Assessment of SpondyloArthritis International Society) classification criteria, which have superseded the modified New York criteria for early diagnosis. 1, 7

The ASAS criteria allow diagnosis based on:

• Clinical features (inflammatory back pain, HLA-B27 positivity, peripheral manifestations) combined with
• Either radiographic sacroiliitis OR MRI evidence of active inflammation 1

This represents a major advance because it permits diagnosis before irreversible structural damage occurs. 1

Screening for Extra-Articular Manifestations

Actively screen for associated conditions at initial evaluation: 6

• Acute anterior uveitis (present in up to 40% of patients)—ask about eye pain, redness, photophobia, and refer urgently to ophthalmology if present 6
• Inflammatory bowel disease—inquire about diarrhea, abdominal pain, blood in stool 7
• Psoriasis—examine skin and nails 1
• Peripheral arthritis or enthesitis (present in 30-50%)—examine peripheral joints and entheses 1

When to Refer to Rheumatology

Refer immediately to rheumatology when chronic low back pain (>3 months) starting before age 45 is present along with any of the following: 1, 3, 6

• Inflammatory back pain features (≥2 of the 4 criteria above) 1, 3
• HLA-B27 positivity 1, 6
• Radiographic or MRI evidence of sacroiliitis 1, 6
• Good response to full-dose NSAIDs within 48 hours (75% of AS patients respond) 1, 3
• Family history of spondyloarthritis in first-degree relative 1

The average diagnostic delay is 4.9-8 years from symptom onset, emphasizing the critical importance of early recognition. 1, 6, 8

Disease Activity Assessment

Use the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) to quantify disease activity. 2, 3, 9

• BASDAI score >4 indicates active disease requiring treatment escalation. 2, 3
• Also assess functional capacity with Bath Ankylosing Spondylitis Functional Index (BASFI) and spinal mobility with Bath Ankylosing Spondylitis Metrology Index (BASMI). 2, 9

Common Diagnostic Pitfalls

Avoid these errors: 2, 3

• Waiting for radiographic changes before considering the diagnosis—MRI allows much earlier detection 1, 2
• Dismissing the diagnosis because inflammatory markers are normal—they are frequently normal despite active disease 3, 6
• Failing to distinguish inflammatory from mechanical back pain patterns—the improvement with exercise is the key distinguishing feature 2, 3
• Not screening for extra-articular manifestations, particularly uveitis, which requires urgent ophthalmologic evaluation 6