HLA-B27

HLA-B27 is a class I major histocompatibility antigen strongly associated with axial spondyloarthritis (ankylosing spondylitis and non-radiographic axial spondyloarthritis), reactive arthritis, acute anterior uveitis, psoriatic spondylitis and spondyloarthritis linked to inflammatory bowel disease.

It is detected by flow cytometry (monoclonal antibody binding on lymphocytes) or by molecular methods (PCR with sequence specific primers or probes for the B*27 allele group). Testing is qualitative and is done once in a lifetime, since the genotype does not change.

HLA-B27 is present in 6 to 8% of white Europeans, 2 to 4% of South Asians, under 1% of Black Africans and Japanese, and up to 50% in some Native American groups. In axial spondyloarthritis, 80 to 90% of white patients are positive, but only 50 to 70% in Black African, Middle Eastern and Asian patients.

Only 1 to 5% of all B27 carriers develop spondyloarthritis, rising to 15 to 20% of B27-positive first degree relatives of patients. The antigen therefore has high sensitivity in some populations but low specificity, and is used as a supporting test.

Indications. Chronic inflammatory back pain starting before 45 years (morning stiffness, improvement with exercise, night pain, alternating buttock pain). Suspected axial spondyloarthritis with normal radiographs of the sacroiliac joints. Recurrent acute anterior uveitis. Unexplained oligoarthritis, enthesitis or dactylitis. Juvenile enthesitis related arthritis.

Reactive arthritis after enteric or genitourinary infection. Family history of spondyloarthritis with symptoms. Not for screening of healthy people or for back pain without inflammatory features.

Specimen. EDTA whole blood 2 to 3 mL for flow cytometry (analysed within 24 to 48 hours, kept at room temperature) or for DNA extraction (stable for days, longer if frozen). No fasting.

Reference values. Reported as negative or positive. Flow cytometry reports the ratio of fluorescence to control, and borderline results are confirmed by a molecular method.

Interpretation.

• Positive HLA-B27 with inflammatory back pain, sacroiliitis on MRI or radiographs, and elevated inflammatory markers: axial spondyloarthritis is very likely. HLA-B27 counts as one of the features in classification of axial disease when sacroiliitis is absent on imaging (together with at least two other features).
• Positive HLA-B27 without symptoms: not a disease, and no treatment or imaging follows. Carrying the gene has a lifetime risk of 1 to 5%.
• Negative HLA-B27 with typical inflammatory back pain and positive MRI: axial spondyloarthritis is still possible (10 to 20% of white and up to 50% of Asian and African patients are negative).
• Positive HLA-B27 with acute anterior uveitis: 50% of cases are B27 positive, and 30 to 50% of those later develop or have spondyloarthritis. Recurrence risk is higher.
• Positive HLA-B27 in a young man with acute peripheral arthritis after urethritis or diarrhoea: reactive arthritis, with more chronic or severe disease and uveitis.
• Subtypes: B*27:05 (most common in whites) and B*27:04 (Chinese) are strongly disease associated, while B*27:06 and B*27:09 (Southeast Asian, Sardinian) are not.
• HLA-B27 positivity predicts earlier onset, hip involvement, uveitis and radiographic progression, but not response to drugs.

Critical values and findings needing immediate action.

• Positive HLA-B27 with a red painful eye, photophobia and blurred vision: acute anterior uveitis, same day ophthalmology review (steroid drops and mydriatics).
• Positive HLA-B27 with a hot swollen joint, fever and recent infection: septic arthritis must be excluded by aspiration before reactive arthritis is assumed.
• Positive HLA-B27 with sudden neurological deficit or severe back pain after minor trauma in long standing ankylosing spondylitis: spinal fracture (rigid spine), urgent imaging.
• Positive HLA-B27 with aortic regurgitation or conduction block: aortitis and cardiac involvement, echocardiography and ECG.

Interferences and pitfalls.

• HLA-B27 is not a diagnostic test. A positive result in a patient with mechanical or non-specific back pain does not make the diagnosis, and a negative result does not exclude disease.
• Testing should follow clinical suspicion, imaging (MRI of sacroiliac joints showing bone marrow oedema) and inflammatory markers, because the positive predictive value is low in the general population (under 5%) but rises to about 50% in patients with chronic inflammatory back pain.
• Flow cytometry can give false positives from cross-reacting antigens (B7, B22) and false negatives from poorly stored samples, so a molecular confirmation is used for borderline results.
• Transfusion and haematopoietic stem cell transplantation can alter results (donor genotype after transplant).
• C-reactive protein is normal in about half of axial spondyloarthritis patients and is not an exclusion.
• Do not repeat the test, since genotype is fixed.
• Result reports should state the method and, if known, the allele.

Special groups.

• Children: enthesitis related arthritis is B27 associated in 60 to 80%, with onset in boys over 6 years. Symptoms may be peripheral first.
• Pregnancy: back pain is common, and B27 testing has no pregnancy specific role. Disease often remains stable in pregnancy.
• Older adults: new back pain at older age is rarely axial spondyloarthritis, and other causes are considered first.
• Regions: prevalence of B27 varies widely, from near zero in sub-Saharan Africa to over 10% in northern Scandinavian and Arctic populations, so the pre-test probability depends on origin.

Follow up tests. MRI of sacroiliac joints and spine, plain radiograph of the pelvis, C-reactive protein and ESR, full blood count, liver and renal function, urinalysis, rheumatoid factor and anti-CCP (usually negative), ophthalmology slit lamp examination, stool culture and urethral swab for chlamydia in reactive arthritis, faecal calprotectin when bowel symptoms are present,

echocardiography, bone density scan, and screening for tuberculosis and hepatitis B before biologic therapy.