What blood work is recommended for a patient suspected of having ankylosing spondylitis?

Blood Work for Ankylosing Spondylitis

HLA-B27 testing is the most valuable blood test for suspected ankylosing spondylitis, with 90% sensitivity and 90% specificity, and should be ordered for all patients with chronic inflammatory back pain with onset before age 45. 1

Primary Laboratory Tests

• HLA-B27: The cornerstone laboratory test for AS screening

  • Sensitivity: 90%, Specificity: 90% 2, 1
  • Post-test probability: 32% (only 3 HLA-B27 positive patients need to be seen by a rheumatologist to diagnose one case) 2, 1
  • Only needs to be performed once in a patient's lifetime 1

• Inflammatory markers:

  • ESR (erythrocyte sedimentation rate): Sensitivity 50%, Specificity 80% 2
  • CRP (C-reactive protein): Sensitivity 50%, Specificity 80% 2
  • Note: Normal inflammatory markers do not rule out AS, as approximately 50% of AS patients may have normal ESR/CRP 1

Interpretation and Clinical Context

When evaluating a patient with suspected ankylosing spondylitis:

1. HLA-B27 positive result: Significantly increases likelihood of AS (likelihood ratio of 9) 2

   • Most valuable in patients with inflammatory back pain starting before age 45 that has lasted >3 months
   • Only 10% of AS patients are HLA-B27 negative 1

2. Inflammatory markers (ESR/CRP):

   • Elevated levels support but do not confirm diagnosis
   • Regular monitoring is conditionally recommended to assess disease activity and treatment response 2, 1
   • Less useful as screening tools (post-test probability only 11.6%) 2

Important Considerations

• No single blood test can diagnose AS - diagnosis requires a combination of clinical, laboratory, and imaging findings 1

• Clinical criteria for referral to rheumatologist when AS is suspected:

  • Back pain starting before age 45 lasting >3 months, plus
  • At least 4 of: back pain before age 35, night pain, buttock pain, improvement with movement/NSAIDs, family history of spondyloarthritis, current/previous arthritis/enthesitis/psoriasis
  • OR 3 of the above criteria plus positive HLA-B27 1

• Imaging remains essential: Radiographs of the sacroiliac joints are the initial imaging modality of choice 2, 3

  • If x-rays are negative but clinical suspicion remains high, HLA-B27 testing is indicated 3

Monitoring Disease Activity

• Regular-interval monitoring of CRP/ESR is conditionally recommended to assess response to treatment 2, 1
• Use of validated AS disease activity measures is conditionally recommended at regular intervals 2

Remember that while blood tests are valuable tools in the diagnostic process, they must be interpreted in the context of clinical presentation and imaging findings. HLA-B27 testing provides the highest diagnostic utility among laboratory tests for AS screening.