Diagnostic Tests for Ankylosing Spondylitis
The diagnosis of ankylosing spondylitis requires a combination of imaging studies, laboratory tests, and clinical assessment, with radiography of the sacroiliac joints being the first-line imaging modality followed by MRI when radiographs are negative but clinical suspicion remains high. 1, 2
Imaging Studies
Radiography
Magnetic Resonance Imaging (MRI)
- Indicated when:
- Radiographs are negative but clinical suspicion remains high
- Modified New York criteria are not met on radiography
- Patient has an immature skeleton 1
- Should include:
- T1 sequences
- Fat-suppressed fluid-sensitive sequences (T2-weighted fat-suppressed or STIR images) 2
- Bone marrow edema on MRI is the hallmark of active sacroiliitis 2
- STIR sequences are generally sufficient to detect inflammation; contrast medium is not needed 1
- Follow-up MRI can be considered when diagnosis remains uncertain 1
Other Imaging
- Conventional radiography of the spine may be used for long-term monitoring of structural changes
- Should not be repeated more frequently than every two years 1
- Scintigraphy is not recommended for diagnosis 1
Laboratory Tests
HLA-B27 Testing
- Recommended as a screening tool with:
- A positive HLA-B27 test increases the likelihood of peripheral spondyloarthritis 1
- HLA-B27 positivity is an ideal screening test for patients with chronic low back pain for axial SpA 1
Inflammatory Markers
- C-reactive protein (CRP) measurement
- Erythrocyte sedimentation rate (ESR)
- Important caveat: Spondyloarthritis should not be ruled out based only on negative laboratory results from HLA-B27, CRP, or ESR testing 1
- Both ESR and CRP have limited sensitivity (around 50%) in patients with AS 1, 3, 4
- Neither ESR nor CRP is superior for assessing disease activity 3, 4
Clinical Assessment Tools
- ASDAS-CRP (Ankylosing Spondylitis Disease Activity Score): Measures disease activity based on patient-reported outcomes and CRP levels 2
- BASDAI (Bath Ankylosing Spondylitis Disease Activity Index): Measures fatigue, pain, stiffness, and discomfort 2
- BASFI (Bath Ankylosing Spondylitis Functional Index): Measures functional ability 2
Referral Criteria to Rheumatologist
Patients with chronic low back pain should be referred to a rheumatologist if they have:
- Back pain occurring before age 45 that has lasted more than three months
- At least four of the following criteria:
- Back pain occurring before 35 years of age
- Waking at night to alleviate symptoms
- Buttock pain
- Improved pain when moving or within two days of taking an NSAID
- First-degree family member with spondyloarthritis
- Current or previous arthritis, enthesitis, or psoriasis 1
- Or if they have three of these criteria plus a positive HLA-B27 test 1
Common Pitfalls in Diagnosis
- Over-reliance on radiographs alone, which may miss early disease 2
- Ignoring inflammatory back pain patterns 2
- Neglecting extra-articular manifestations (uveitis, psoriasis, inflammatory bowel disease) 2
- Misinterpreting normal inflammatory markers (ESR/CRP can be normal in up to 50% of cases) 1, 3, 4
- Delayed diagnosis (average 7-10 years from symptom onset) 2
- Ruling out spondyloarthritis based on a single negative test result 1
The diagnostic approach should be comprehensive, considering both clinical features and objective test results, with early referral to a rheumatologist when suspicion is high to prevent diagnostic delays that can impact quality of life and disease progression.