What are the initial testing and treatment approaches for ankylosing spondylitis?

Initial Testing for Ankylosing Spondylitis

For patients with chronic low back pain (>3 months duration) starting before age 45, screen using inflammatory back pain criteria and HLA-B27 testing—these two parameters provide the highest diagnostic yield with post-test probabilities of 14% and 32% respectively. 1

Patient Selection Criteria

Apply screening only to patients meeting these specific parameters:

• Chronic back pain duration >3 months 1
• Age of symptom onset <45 years (AS starts in <4% of patients over age 40) 1

Primary Screening Tests

Clinical Assessment: Inflammatory Back Pain (IBP)

• Sensitivity: 75%, Specificity: 75% 1
• Post-test probability: 14% (1 in 7 patients referred will have axial SpA) 1
• Look for: back pain and stiffness that improves with exercise, worsens with rest, particularly morning stiffness 1
• No cost, good ease of interpretation 1
• One limitation: in one-third of patients, it cannot be clearly determined whether back pain is inflammatory or mechanical 1

Laboratory Testing: HLA-B27

• Sensitivity: 90%, Specificity: 90% 1
• Post-test probability: 32% (1 in 3 patients will have axial SpA) 1
• This is the single best screening parameter with the highest diagnostic yield 1
• Moderate cost but very good ease of interpretation 1
• 90-95% of AS patients are HLA-B27 positive 2

Alternative Clinical Test: NSAID Response

• Sensitivity: 75%, Specificity: 85% 1
• Post-test probability: 21% (1 in 5 patients will have axial SpA) 1
• Assess response to full-dose NSAID within 48 hours 1
• 75% of AS patients show good/very good response vs. only 15% with mechanical back pain 1
• Low cost, good ease of interpretation 1

Secondary Screening Parameters (Lower Sensitivity)

These should not be used as primary screening tools due to low sensitivity:

• Uveitis: Sensitivity 15%, but high specificity 98% 1
• Family history: Sensitivity 25% 1
• Peripheral arthritis: Sensitivity 40% 1
• ESR/CRP: Sensitivity only 50%, post-test probability 11.6% 1

Imaging Studies

MRI of Sacroiliac Joints

• Sensitivity: 90%, Specificity: 90% 1
• Post-test probability: 32% (equal to HLA-B27) 1
• Detects sacroiliitis years before radiographic changes appear 1
• High cost, moderate ease of interpretation 1
• Use when clinical suspicion is high and early diagnosis is critical 1, 3

Plain Radiographs

• Sensitivity: 80%, Specificity: 80% 1
• Post-test probability: 17.4% 1
• May take several years of inflammation before radiological damage is visible 1
• Less useful for early diagnosis but still part of classification criteria 4

Recommended Screening Algorithm

1. Screen all patients with chronic back pain >3 months, onset <45 years 1
2. First-line: Assess for inflammatory back pain + order HLA-B27 1
3. If either positive: refer to rheumatology 1
4. If high clinical suspicion despite negative initial tests: order MRI sacroiliac joints 1, 3

Critical Pitfall to Avoid

Do not wait for radiographic sacroiliitis to make a diagnosis—this causes the typical 5-7 year delay in diagnosis and misses the window for most effective treatment with TNF inhibitors (72% of patients with disease duration <10 years achieve ≥50% improvement vs. 50% overall). 1