What are the diagnostic criteria and methods for ankylosing spondylitis?

Diagnosing Ankylosing Spondylitis

The diagnosis of ankylosing spondylitis (AS) requires a combination of clinical features, laboratory testing, and imaging findings, with radiography of the sacroiliac joints being the recommended initial imaging modality followed by MRI when radiographs are negative or equivocal. 1

Clinical Criteria for Suspicion of AS

• Inflammatory back pain characteristics:

  • Morning stiffness lasting >30 minutes
  • Pain at night/early morning
  • Improvement with exercise but not rest
  • Age of onset <45 years
  • Symptoms lasting >3 months
  • Alternating buttock pain 1, 2

• Additional clinical features:

  • Limitation of motion of the lumbar spine in both sagittal and frontal planes
  • Limitation of chest expansion relative to normal values for age and sex
  • Peripheral arthritis (typically large joints)
  • Enthesitis (inflammation at insertion of tendons/ligaments into bone) 1, 3

Diagnostic Testing Algorithm

Step 1: Initial Laboratory Testing

• HLA-B27 testing - highest utility with likelihood ratio of 9 (90-95% of AS patients are HLA-B27 positive) 1, 2, 4
• C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) - Note: normal values don't rule out AS (sensitivity only 50%) 1, 2

Step 2: Imaging

1. Radiography of sacroiliac joints - first-line imaging modality 1

   • Look for: erosions, sclerosis, joint space narrowing, ankylosis

2. If radiographs are negative or equivocal:

   • MRI of sacroiliac joints - can detect early inflammatory changes before radiographic damage 1
   • Include T1 sequences and fat-suppressed fluid-sensitive sequences (T2-weighted fat-suppressed or STIR images) 1
   • Bone marrow edema on MRI is the hallmark of active sacroiliitis 1

3. CT of sacroiliac joints - may be helpful when radiographs are equivocal to identify subtle erosions and soft-tissue ossification 1

4. Spine imaging - to assess for syndesmophytes and other spinal changes in established disease 1

Formal Diagnostic Criteria

Modified New York Criteria for AS (1984)

Requires radiological criterion plus at least one clinical criterion:

• Radiological criterion:

  • Sacroiliitis grade ≥2 bilaterally or grade ≥3 unilaterally

• Clinical criteria:

  • Low back pain for ≥3 months, improved by exercise but not relieved by rest
  • Limitation of lumbar spine motion in sagittal and frontal planes
  • Limitation of chest expansion relative to normal values for age and sex 1, 5

ASAS Classification Criteria for Axial Spondyloarthritis

For patients with back pain ≥3 months and age of onset <45 years:

Either:

• Sacroiliitis on imaging (MRI or radiographs) plus ≥1 SpA feature
• HLA-B27 positive plus ≥2 other SpA features 1

SpA features include:

• Inflammatory back pain
• Arthritis
• Enthesitis
• Uveitis
• Dactylitis
• Psoriasis
• Inflammatory bowel disease
• Good response to NSAIDs
• Family history of SpA
• HLA-B27 positive
• Elevated CRP 1

Disease Activity Assessment

Once diagnosed, disease activity should be assessed using:

• ASDAS-CRP (Ankylosing Spondylitis Disease Activity Score) - recommended for axial SpA 1
• BASDAI (Bath Ankylosing Spondylitis Disease Activity Index) - measures fatigue, pain, stiffness, and discomfort 1
• BASFI (Bath Ankylosing Spondylitis Functional Index) - measures functional ability 1, 6

Common Pitfalls to Avoid

1. Delayed diagnosis - average delay of 7-10 years from symptom onset to diagnosis 1, 4
2. Over-reliance on radiographs - radiographic changes may take years to develop 1
3. Ignoring inflammatory back pain patterns - key distinguishing feature from mechanical back pain 1, 2
4. Neglecting extra-articular manifestations - uveitis, inflammatory bowel disease, psoriasis can provide diagnostic clues 7
5. Misinterpreting normal inflammatory markers - ESR/CRP can be normal in up to 50% of cases 1, 2

When to Refer to a Rheumatologist

Refer patients with chronic low back pain to a rheumatologist when they have:

• Inflammatory back pain characteristics
• Positive HLA-B27 test
• Family history of AS or related conditions
• Extra-articular manifestations (uveitis, psoriasis, inflammatory bowel disease)
• Elevated inflammatory markers with no other explanation 2

Early referral is crucial as newer therapies can effectively manage the condition and potentially induce remission when started early 4, 8.