What blood tests are used to diagnose ankylosing spondylitis?

Blood Tests for Diagnosing Ankylosing Spondylitis

HLA-B27 testing is the most valuable blood test for screening patients with suspected ankylosing spondylitis (AS), with a high sensitivity of 90% and specificity of 90%, making it an ideal screening tool when combined with clinical symptoms. 1

Key Blood Tests for AS Diagnosis

Primary Blood Tests

• HLA-B27:

  • Highest sensitivity (90%) and specificity (90%) for AS
  • Positive likelihood ratio of 9 and post-test probability of 32%
  • Only 10% of patients with AS are HLA-B27 negative 1
  • Clear "positive" or "negative" result with minimal variability over time

• Inflammatory Markers:

  • ESR (Erythrocyte Sedimentation Rate) and CRP (C-reactive protein)
  • Limited sensitivity (≤50%) in AS patients 1
  • Post-test probability of only about 12% if elevated
  • Regular monitoring recommended to assess disease activity 1

Diagnostic Algorithm

1. Initial Screening:

   • HLA-B27 testing is recommended for patients with chronic low back pain (>3 months) with onset before age 45 1
   • Normal inflammatory markers do not rule out AS - approximately 50% of AS patients may have normal ESR/CRP 1

2. Interpretation of Results:

   • Positive HLA-B27: Increases likelihood of AS (only 3 HLA-B27 positive patients with chronic low back pain need to be seen by a rheumatologist to diagnose one case) 1
   • Negative HLA-B27: Does not rule out AS but makes it less likely
   • Elevated inflammatory markers: Supportive but not diagnostic

3. Monitoring Disease Activity:

   • Regular-interval monitoring of CRP/ESR is conditionally recommended 1
   • These markers help assess response to treatment and disease activity

Important Considerations

• Diagnostic Limitations: No single blood test can diagnose AS; diagnosis requires a combination of clinical, laboratory, and imaging findings 1
• False Negatives: Normal inflammatory markers (ESR/CRP) do not rule out AS 1
• Family History Impact: HLA-B27 positive relatives of AS patients have a 16 times greater risk of developing AS compared to HLA-B27 positive individuals in the general population 2
• Cost Effectiveness: HLA-B27 testing is cost-effective as it only needs to be performed once and has high diagnostic utility 1

When to Refer to a Rheumatologist

Referral to a rheumatologist is recommended for patients with:

• Back pain starting before age 45 that has lasted more than 3 months, plus
• At least 4 of the following: back pain before age 35, night pain, buttock pain, improvement with movement/NSAIDs, family history of spondyloarthritis, or current/previous arthritis/enthesitis/psoriasis
• Or 3 of the above criteria plus positive HLA-B27 1

Remember that while blood tests are valuable tools in the diagnostic process, they should be interpreted in conjunction with clinical symptoms and imaging findings for a definitive diagnosis of ankylosing spondylitis.