What is the diagnostic approach for ankylosing spondylitis?

Diagnostic Approach for Ankylosing Spondylitis

Diagnose ankylosing spondylitis by first screening patients with chronic back pain (>3 months duration, onset before age 45) for inflammatory back pain characteristics, then confirm with HLA-B27 testing and imaging—starting with conventional radiography of the sacroiliac joints, followed by MRI if radiographs are negative but clinical suspicion remains high. 1

Initial Clinical Screening

Screen only patients meeting these criteria:

• Chronic back pain lasting >3 months 1
• Onset before age 45 (AS rarely starts after age 40, occurring in <4% of cases) 1

Assess for inflammatory back pain pattern (sensitivity 75%, specificity 75%, post-test probability 14%): 1

• Insidious onset before age 40-45 years 1
• Prolonged morning stiffness (distinguishes from mechanical back pain) 1
• Improvement with exercise but NOT with rest 1
• Night pain, particularly awakening in the second half of the night 1
• Alternating buttock pain indicating sacroiliac joint involvement 1

Test response to NSAIDs (sensitivity 75%, specificity 85%, post-test probability 21%):

• Good response to full-dose NSAIDs within 48 hours strongly supports the diagnosis 1

Laboratory Testing

Order HLA-B27 testing as the single most valuable laboratory test (sensitivity 90%, specificity 90%, post-test probability 32%): 1

• Present in 74-89% of AS patients 1
• Only 1% of HLA-B27 positive individuals develop AS, so interpret in clinical context 1

Measure inflammatory markers (ESR/CRP) to assess disease activity, though these are less specific for diagnosis 1

Imaging Strategy

Step 1: Conventional radiography of sacroiliac joints 2, 1

• This is the initial recommended imaging modality 2
• Look for chronic inflammatory changes (erosions, sclerosis, joint space narrowing, ankylosis) 3
• Radiographic changes typically appear late in disease (diagnostic delay averages 7-10 years) 4

Step 2: MRI of sacroiliac joints if radiography is negative but axial SpA is still suspected (sensitivity 90%, specificity 90%, post-test probability 32%): 2, 1

• MRI detects sacroiliac joint inflammation years before radiographic changes appear 1
• Assess for both active inflammatory lesions (primarily bone marrow edema) AND structural lesions (bone erosion, new bone formation, sclerosis, fat infiltration) 2
• STIR sequences are generally sufficient to detect inflammation; contrast medium is not needed 2

MRI of the spine is NOT generally recommended for initial diagnosis of axial SpA 2

CT may provide additional information on structural damage if conventional radiography is negative and MRI cannot be performed, but is not routinely recommended 2

Assessment of Extra-Articular Manifestations

Screen for acute anterior uveitis (occurs in 15-40% of AS patients, likelihood ratio 7.3 when present): 5

• May precede spinal symptoms or occur during apparent remission 5
• Immediate ophthalmology referral is mandatory if symptoms present to prevent vision loss 5

Evaluate for peripheral joint involvement, enthesitis, and other extra-articular manifestations: 6

• Peripheral arthritis, particularly of lower extremities 6
• Enthesitis (inflammation at tendon/ligament insertions) 6
• Inflammatory bowel disease, cardiac involvement, pulmonary involvement 6

Physical Examination Findings

Perform spinal mobility assessments: 1

• Modified Schober test (positive test indicates reduced lumbar spine mobility) 5
• Chest expansion measurement 2
• Occiput-to-wall distance 2
• Finger-to-floor distance 2

Common Diagnostic Pitfalls

Do not rely solely on radiography in early disease—MRI is critical for detecting pre-radiographic changes 4, 7

Do not order HLA-B27 testing indiscriminately—only test patients meeting clinical screening criteria, as the general population prevalence is low 1

Do not dismiss the diagnosis if inflammatory markers (ESR/CRP) are normal—they lack close correlation with disease activity in many AS patients 8

Do not overlook extra-articular manifestations, particularly uveitis, which can be vision-threatening and requires urgent treatment 5, 6

Documentation for Ongoing Monitoring

Use the ASAS core set for clinical record keeping: 1

• Bath Ankylosing Spondylitis Functional Index (BASFI) for physical function 1
• Modified Schober test for spinal mobility 1
• Patient global assessment and pain scores 1
• Assessment of peripheral joints and entheses when involved 1