Laboratory Testing for Suspected Axial Spondyloarthritis
Order HLA-B27 and C-reactive protein (CRP) as the primary laboratory tests for patients with suspected axial spondyloarthritis, but never exclude the diagnosis based on negative results alone. 1, 2, 3
Essential Laboratory Tests
HLA-B27 Testing
- HLA-B27 has 50% sensitivity and 90% specificity for axial spondyloarthritis 4
- A positive HLA-B27 supports the diagnosis and lowers the threshold for rheumatology referral (only 3 clinical criteria needed instead of 4) 3
- Critical pitfall: A negative HLA-B27 does NOT rule out axial spondyloarthritis - the disease occurs equally in HLA-B27 negative individuals 1, 2
Inflammatory Markers
- CRP has 35% sensitivity and 91% specificity for axial spondyloarthritis 4
- Elevated CRP is mandatory for initiating biologic therapy in patients with non-radiographic axial spondyloarthritis 5
- CRP is preferred over ESR for disease activity monitoring and is used in the ASDAS (Axial Spondyloarthritis Disease Activity Score) 5, 6
- Critical pitfall: Normal CRP and ESR do NOT exclude axial spondyloarthritis 1, 2
Additional Laboratory Considerations
Complete Blood Count
- Lymphocyte-to-monocyte ratio (LMR) may serve as an inflammatory marker, with lower values in axial spondyloarthritis patients compared to healthy controls 7
- The combination of ESR, CRP, and LMR showed 94.9% sensitivity and 97.4% specificity for diagnosis in one study 7
Liver Function Tests
- Baseline ALT, AST, total bilirubin, and albumin/globulin ratio may be useful, as these can be altered in axial spondyloarthritis patients 7
Clinical Context for Laboratory Interpretation
The diagnosis of axial spondyloarthritis is based on the combination of clinical features, laboratory findings, and imaging - no single test is diagnostic 4, 8. Laboratory tests support but do not establish the diagnosis, which typically requires:
- Inflammatory back pain (sensitivity 74-81%, specificity 25-44%) 4
- HLA-B27 positivity (when present) 4
- Elevated inflammatory markers (when present) 4
- Imaging evidence of sacroiliitis on radiography (sensitivity 66%, specificity 68%) or MRI (sensitivity 78%, specificity 88%) 4
The average diagnostic delay is 6-8 years from symptom onset, emphasizing the importance of not over-relying on laboratory tests to make or exclude the diagnosis 4, 8.