Which HLA (Human Leukocyte Antigen) B27 lab test is used for the diagnosis of ankylosing spondylitis?

HLA-B27 Testing for Ankylosing Spondylitis Diagnosis

The standard HLA-B27 test for ankylosing spondylitis diagnosis is performed using either flow cytometry (antigen detection) or PCR-based molecular methods, with PCR melting assays demonstrating superior sensitivity (99.6%) and specificity (100%) compared to flow cytometry antigen assays. 1

Test Methodology Options

PCR-Based Testing (Preferred)

• Allele-specific PCR melting assays provide the highest diagnostic accuracy with sensitivity of 99.6% and specificity of 100% when compared to DNA sequencing as the gold standard 1
• This molecular method detects HLA-B27 genotype directly and only needs to be performed once in a patient's lifetime 2
• PCR methods can differentiate between specific HLA-B27 subtypes including B2702, B2705, and B*2708 3

Flow Cytometry (Alternative)

• Two-color direct immunofluorescence using HLA-B27/CD3 antibodies on erythrocyte-lysed whole blood is the standard flow cytometry approach 4
• Flow cytometry antigen assays have slightly lower performance with sensitivity of 98.2% and specificity of 97.6% for the best single assay 1
• The accuracy depends on the specific monoclonal antibody used and the positive cutoff values assigned 1
• Using a reflex combination of both antigen assays improves sensitivity to 98.8% while maintaining specificity of 97.6% 1

Clinical Context for Ordering

When to Order HLA-B27

HLA-B27 should be used as a screening parameter, not a definitive diagnostic test 2, 5

Order HLA-B27 testing when the patient has:

• Chronic low back pain lasting >3 months 2, 5
• Symptom onset before age 45 years 2
• Features suggesting inflammatory back pain 5, 6

Interpretation Guidelines

• Approximately 30-40% of patients with chronic back pain and positive HLA-B27 will ultimately receive an AS diagnosis 2, 5
• HLA-B27 is found in 90% of patients with ankylosing spondylitis 1
• In IBD-associated AS, HLA-B27 prevalence is lower (25-75%), making it less reliable as a diagnostic test in this population 2
• A negative HLA-B27 test should NOT rule out spondyloarthritis, as approximately 10% of AS cases are HLA-B27 negative 5, 6

Critical Pitfalls to Avoid

• Never use HLA-B27 early in the diagnostic approach before increasing pre-test probability to 30-50% with clinical parameters 2, 5
• Do not rule out spondyloarthritis based solely on negative HLA-B27, normal CRP, or normal ESR 5
• Do not delay referral to rheumatology or appropriate treatment based on HLA-B27 negative status, as disease severity can be equivalent in HLA-B27 negative patients 6

Cost Considerations

• HLA-B27 testing costs approximately €40 (equivalent to a chest or pelvis X-ray in Germany) 2, 5
• The test only needs to be performed once in a patient's lifetime 2, 5
• When used appropriately as a screening parameter in the right patient population, HLA-B27 testing is cost-effective 2