What blood tests are used to diagnose sacroiliitis, including tests for inflammation such as Erythrocyte Sedimentation Rate (ESR) and C-Reactive Protein (CRP)?

Blood Tests for Diagnosing Sacroiliitis

HLA-B27 is the most valuable blood test for diagnosing sacroiliitis, with 90% sensitivity and specificity, while inflammatory markers like ESR and CRP have limited diagnostic utility with only 50% sensitivity. 1

Key Laboratory Tests for Sacroiliitis Evaluation

Primary Blood Tests

• HLA-B27: The most reliable laboratory test for sacroiliitis screening with 90% sensitivity, 90% specificity, and a post-test probability of 32% 1
• Erythrocyte Sedimentation Rate (ESR): Limited utility with only 50% sensitivity and 80% specificity 1
• C-Reactive Protein (CRP): Similar limitations to ESR with approximately 50% sensitivity 1

Diagnostic Value of Inflammatory Markers

• ESR and CRP are easy to measure but provide only a post-test probability of about 12% if elevated 1
• The sensitivity of ESR/CRP in patients with ankylosing spondylitis and early sacroiliitis is no higher than 50%, making them unsuitable as primary screening parameters 1, 2
• ESR may correlate moderately with physical mobility limitations (r = 0.427) in established ankylosing spondylitis 2
• CRP also shows moderate correlation with physical mobility limitations (r = 0.410) in established disease 2

Additional Laboratory Tests

• Full blood count: Should be included to assess for anemia, leukocytosis, and thrombocytosis, which may be present in inflammatory conditions 1
• Liver enzymes: Important to evaluate as they may be elevated in inflammatory conditions or affected by treatments 1
• Serum albumin and pre-albumin: Helpful to assess nutritional status and degree of inflammation 1

Diagnostic Algorithm for Sacroiliitis

1. Initial laboratory evaluation:

   • HLA-B27 testing (highest diagnostic value) 1
   • Inflammatory markers (ESR, CRP) 1
   • Complete blood count 1
   • Liver function tests and serum albumin 1

2. Interpretation of results:

   • Positive HLA-B27: Significantly increases likelihood of sacroiliitis (post-test probability 32%) 1
   • Elevated ESR/CRP: Supports but does not confirm diagnosis (post-test probability only 12%) 1
   • Normal ESR/CRP: Does not rule out sacroiliitis (sensitivity only 50%) 1

3. Additional considerations:

   • Rule out infectious causes that may mimic inflammatory sacroiliitis 1
   • Consider that ESR and CRP may correlate with disease activity in established cases but have limited diagnostic value in early disease 3, 2

Clinical Pearls and Pitfalls

• Key pitfall: Relying solely on ESR/CRP for diagnosis can miss approximately 50% of sacroiliitis cases 1
• Important consideration: HLA-B27 testing needs to be done only once, as genetic markers do not change over time 1
• Clinical pearl: The combination of ESR × disease duration or CRP × disease duration may provide better correlation with physical mobility limitations than either marker alone in established disease 2
• Diagnostic challenge: Sacroiliitis may present with normal inflammatory markers, especially in early disease 4, 5
• Imaging correlation: Laboratory tests should be interpreted alongside imaging findings, as MRI has 90% sensitivity and specificity for sacroiliitis compared to the limited sensitivity of blood tests 1, 5