What are the diagnostic criteria for Ankylosing Spondylitis?

Diagnostic Criteria for Ankylosing Spondylitis

The diagnosis of ankylosing spondylitis requires a combination of clinical features, laboratory testing, and imaging findings, with sacroiliitis on imaging plus at least one spondyloarthritis feature being the most definitive diagnostic approach. 1

Clinical Criteria

The clinical assessment focuses on identifying inflammatory back pain characteristics:

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

Additional clinical features that support the diagnosis:

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

Laboratory Testing

• HLA-B27: Most valuable laboratory test with 90-95% sensitivity in AS patients

  • Likelihood ratio of 9
  • Post-test probability of 32% when positive
  • Only 3 HLA-B27 positive patients with chronic low back pain need to be seen by a rheumatologist to diagnose one case 2, 1

• Inflammatory markers:

  • ESR/CRP have limited sensitivity (only 50%)
  • Normal values do not rule out AS 2, 1
  • Should not be relied upon exclusively for diagnosis

Imaging Criteria

Radiography

• First-line imaging modality 2, 1
• Sacroiliitis grading:
  • Grade 2 bilaterally OR
  • Grade 3 or higher unilaterally 2, 1
• Limitations: Low sensitivity for early disease, changes may lag behind symptoms by 7+ years 2, 3

MRI

• Recommended when radiographs are negative but clinical suspicion remains high 2, 1
• Should include:
  • T1-weighted sequences
  • Fat-suppressed fluid-sensitive sequences (T2-weighted fat-suppressed or STIR) 1
• Bone marrow edema on MRI is the hallmark of active sacroiliitis 1
• Allows identification of "pre-radiographic" spondyloarthropathy 2, 4, 5

CT

• May be helpful when radiographs are equivocal
• Better for detecting subtle erosions and reparative changes
• Higher radiation exposure limits routine use 2, 1

Formal Diagnostic Criteria

Modified New York Criteria (1984)

Requires radiological criterion plus at least one clinical criterion:

• Radiological criterion: Sacroiliitis grade ≥2 bilaterally or grade ≥3 unilaterally
• Clinical criteria (at least one):
  • Low back pain for ≥3 months, improved by exercise but not relieved by rest
  • Limitation of lumbar spine motion
  • Limitation of chest expansion 1, 5

ASAS Classification Criteria for Axial Spondyloarthritis

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

Pathway 1: Sacroiliitis on imaging plus ≥1 SpA feature

• Imaging: MRI showing active inflammation or radiographs showing sacroiliitis

Pathway 2: HLA-B27 positive plus ≥2 other SpA features

• SpA features include:
  • Inflammatory back pain
  • Arthritis
  • Enthesitis
  • Uveitis
  • Dactylitis
  • Psoriasis
  • Inflammatory bowel disease
  • Good response to NSAIDs
  • Family history of SpA
  • HLA-B27 positivity
  • Elevated CRP 1, 5

Diagnostic Algorithm

1. Initial screening: Assess for inflammatory back pain characteristics in patients with chronic back pain (>3 months) with onset before age 45
2. Laboratory testing: Order HLA-B27 and inflammatory markers (ESR/CRP)
3. Initial imaging: Radiographs of sacroiliac joints
4. If radiographs negative but clinical suspicion remains high: Order MRI of sacroiliac joints
5. Apply diagnostic criteria: Use Modified New York or ASAS criteria based on available findings
6. Refer to rheumatologist: For confirmation of diagnosis and management 2, 1

Common Pitfalls in Diagnosis

• Delayed diagnosis (average 7-10 years from symptom onset) 1, 4
• Over-reliance on radiographs which miss early disease 2, 3
• Ignoring inflammatory back pain patterns 1
• Over-reliance on inflammatory markers (normal levels don't rule out AS) 1
• Referring to inappropriate specialists instead of rheumatologists 1
• Failure to recognize extra-articular manifestations (uveitis, psoriasis, IBD) 1

Early diagnosis is crucial as newer treatments like TNF-α inhibitors are more effective when started early in the disease course 4, 3.