Which blood tests are recommended for evaluating suspected ankylosing spondylitis?

Blood Tests for Ankylosing Spondylitis

For suspected ankylosing spondylitis, order HLA-B27 testing along with inflammatory markers (ESR and CRP), but critically understand that negative results do not rule out the disease. 1

Primary Recommended Blood Tests

HLA-B27 Testing

• HLA-B27 is the single most valuable blood test for screening, with 90% sensitivity and 90% specificity in patients with chronic back pain (>3 months duration, onset <45 years) 2
• Provides a clear positive/negative result with no variability over time
• Only needs to be performed once
• Positive HLA-B27 increases post-test probability to 32% - meaning only 3 patients with positive results need rheumatology referral to diagnose one case of AS 2
• HLA-B27 positivity also predicts worse prognosis 2

Inflammatory Markers (ESR/CRP)

• ESR and CRP should be measured but have limited screening value - sensitivity only 50% in AS patients 2
• Post-test probability with elevated ESR/CRP is only 12% 2
• More useful for monitoring disease activity than diagnosis
• Critical caveat: Normal ESR and CRP do NOT exclude ankylosing spondylitis 1

Key Clinical Algorithm

Order blood tests when patient presents with:

• Chronic back pain >3 months duration
• Symptom onset before age 45 years
• Features of inflammatory back pain (morning stiffness >30 min, night pain, improvement with exercise)

Then proceed as follows:

1. If HLA-B27 positive → Refer to rheumatology regardless of ESR/CRP results
2. If HLA-B27 negative but strong clinical suspicion → Still refer to rheumatology; 10% of AS patients are HLA-B27 negative 2
3. If inflammatory markers elevated → Consider referral even with negative HLA-B27 if clinical features present

Important Pitfalls to Avoid

Do not rule out spondyloarthritis based solely on:

• Negative HLA-B27 test 1
• Normal C-reactive protein 1
• Normal erythrocyte sedimentation rate 1

The 2017 NICE guidelines explicitly state that AS occurs equally in men and women, can be found in HLA-B27 negative persons, and is possible even without sacroiliitis on radiography 1. This represents a critical shift from older diagnostic paradigms.

Additional Laboratory Considerations

While newer research has explored various hematological indices (neutrophil-to-lymphocyte ratio, systemic immune-inflammation index, serum amyloid A) 3, 4, 5, 6, these remain investigational and are not recommended for routine clinical screening or diagnosis. The evidence base for these markers is limited to recent observational studies without guideline endorsement.

Infective antibody testing should NOT be routinely performed to confirm reactive arthritis in patients with prior gastrointestinal infection 1.

Practical Testing Strategy

Order the following panel for suspected AS:

• HLA-B27 (primary screening test)
• ESR (for baseline and monitoring)
• CRP (for baseline and monitoring)

This focused approach balances diagnostic yield with cost-effectiveness, as HLA-B27 testing costs approximately the same as standard radiography but provides superior screening performance 2.