Ankylosing Spondylitis Diagnosis and Monitoring

Introduction to Immune Markers

• The American College of Radiology suggests that HLA-B27, C-reactive protein (CRP), and erythrocyte sedimentation rate (ESR) are the most clinically relevant immune markers in ankylosing spondylitis, with HLA-B27 being positive in 74-89% of patients 1

HLA-B27 as a Genetic Marker

• HLA-B27 is the strongest immune marker associated with ankylosing spondylitis, present in 74% to 89% of patients with axial spondyloarthritis, according to the American College of Radiology 1
• The American Academy of Family Physicians recommends that HLA-B27 positivity should be combined with clinical criteria for referral decisions, as it increases the likelihood of peripheral spondyloarthritis 2

Inflammatory Markers

• The American Academy of Family Physicians advises that spondyloarthritis should not be ruled out based solely on normal CRP or ESR values, as these can be normal even in active disease 2
• CRP and ESR should be monitored regularly alongside validated AS disease activity measures to assess treatment response and disease progression, as suggested by the American College of Rheumatology 3, 4
• These markers provide additional information when combined with MRI findings for monitoring disease activity in axial spondyloarthritis, according to the European League Against Rheumatism 5, 6

Clinical Pitfalls

• The American Academy of Family Physicians warns that the diagnosis of spondyloarthritis cannot be excluded based on negative laboratory results alone, including normal C-reactive protein levels and normal erythrocyte sedimentation rate 2
• All three markers must be interpreted within the clinical context of inflammatory back pain characteristics, imaging findings, and extra-articular manifestations, as recommended by the American Academy of Family Physicians 2

HLA-B27 Testing for Ankylosing Spondylitis Diagnosis

Test Methodology and Clinical Context

• The molecular method detects HLA-B27 genotype directly and only needs to be performed once in a patient's lifetime, according to the European League Against Rheumatism 7
• HLA-B27 should be used as a screening parameter, not a definitive diagnostic test, as recommended by the European League Against Rheumatism and the American College of Rheumatology 7, 8

Interpretation and Cost Considerations

• Approximately 30-40% of patients with chronic back pain and positive HLA-B27 will ultimately receive an AS diagnosis, as reported by the American College of Rheumatology 7, 8
• A negative HLA-B27 test should NOT rule out spondyloarthritis, as approximately 10% of AS cases are HLA-B27 negative, according to the American College of Rheumatology 8, 9
• HLA-B27 testing costs approximately €40, equivalent to a chest or pelvis X-ray in Germany, and is cost-effective when used appropriately as a screening parameter in the right patient population, as stated by the European League Against Rheumatism 7, 8
• In IBD-associated AS, HLA-B27 prevalence is lower (25-75%), making it less reliable as a diagnostic test in this population, according to the European Crohn’s and Colitis Organisation 10

Management of HLA-B27 Positive Patients

Diagnosis and Referral

• A positive HLA-B27 test in a patient with chronic back pain (>3 months) starting before age 45 warrants referral to a rheumatologist for evaluation of axial spondyloarthropathy, as only 3 HLA-B27 positive patients with chronic back pain need to be evaluated to diagnose one case of spondyloarthritis 11
• The presence of HLA-B27 positivity increases disease likelihood substantially, with approximately 30-40% of patients with chronic back pain and positive HLA-B27 ultimately receiving an AS diagnosis, and a post-test probability of 32% in patients with chronic back pain 11
• Refer immediately if the patient has chronic back pain >3 months, first symptoms before age 45, and inflammatory back pain characteristics, or HLA-B27 positivity, or sacroiliitis on imaging (X-ray or MRI) 11

Imaging and Diagnostic Parameters

• X-ray sacroiliitis (grade 2 bilaterally or grade 3+ unilaterally) has sensitivity and specificity of only ~80% 11
• MRI of sacroiliac joints has good sensitivity and specificity but is expensive and not recommended for primary screening 11
• If sacroiliitis is present on either X-ray or MRI in a young patient with chronic back pain, this supports referral to rheumatology 11

Laboratory Tests

• ESR/CRP have only 50% sensitivity in AS patients and provide only 12% post-test probability when raised, and should be monitored alongside validated AS disease activity measures but cannot exclude disease 11

Risk Assessment and Diagnostic Recommendations for Axial Spondyloarthritis in HLA‑B27‑Positive Patients with Chronic Back Pain

Pre‑test and Post‑test Probabilities

• In individuals with chronic lower‑back pain and HLA‑B27 positivity, the probability of having axial spondyloarthritis (axSpA) is approximately 30‑40 % (about 1 in 3 patients). 12
• HLA‑B27 positivity alone raises the post‑test probability of axSpA to 32 % among patients with chronic back pain. 12
• On average, 3 HLA‑B27‑positive patients with chronic back pain need to be evaluated by a rheumatologist to diagnose 1 case of axSpA. 12

Test Characteristics of HLA‑B27

• Sensitivity of HLA‑B27 for axSpA is 90 % (i.e., 10 % of axSpA patients are HLA‑B27 negative). 12
• The positive likelihood ratio of HLA‑B27 positivity is 9, indicating excellent discriminative ability as a screening marker. 12

Impact of Inflammatory Back Pain Features

• Presence of morning stiffness > 30 minutes, night or early‑morning pain that awakens the patient, symptom improvement with exercise but not with rest, and insidious onset before age 45 each further increase the likelihood of axSpA in HLA‑B27‑positive individuals. 12
• When inflammatory back‑pain features are combined with HLA‑B27 positivity, the sensitivity of the overall assessment falls to 68 %, meaning more cases may be missed if relying solely on these criteria. 13

Interpretation of Laboratory and Imaging Findings

• Normal inflammatory markers (ESR/CRP) do not exclude axSpA; only about 50 % of patients have elevated values. 12
• Negative plain‑film X‑rays do not rule out disease; radiographic sacroiliitis shows roughly 80 % sensitivity and specificity. 12

Referral and Imaging Recommendations

• Patients who are HLA‑B27 positive with chronic back pain should be referred promptly to a rheumatologist for further evaluation. 12
• Once under rheumatology care, magnetic resonance imaging (MRI) of the sacroiliac joints is the appropriate imaging modality to confirm sacroiliitis and support the axSpA diagnosis. [12][13]