Main human histocompatibility complex HLA B27
Why this test?
For differential diagnosis of joint syndrome (seronegative spondyloarthritis, rheumatoid and septic arthritis, gout and others);
for screening, diagnosis and prognosis of ankylosing spondylitis;
to assess the risk of atlanto-axial subluxation in rheumatoid arthritis.
In what cases is it prescribed?
With joint syndrome: asymmetric oligoarthritis, especially in combination with pain in the lumbar region of an inflammatory nature (morning stiffness for more than 1 hour, improvement with physical exertion, worsening at night) and signs of enthesitis; with a heavy hereditary history of ankylosing spondylitis; in rheumatoid arthritis.
HLA-B27 antigen is a specific protein found on the surface of immune cells. It belongs to the proteins of the main human histocompatibility complex, which ensures various immune reactions. Carrying the HLA-B27 antigen is associated with an increased risk of developing diseases from the group of seronegative spondyloarthritis.
Thus, this antigen can be detected in 90-95% of patients with ankylosing spondylitis (Bekhterev's disease), 75% of patients with reactive arthritis (Reiter's syndrome), 50-60% of patients with psoriatic arthropathy, 80-90% of patients with juvenile ankylosing spondylitis and 60-90% of patients with enteropathic arthritis.
The presence of the HLA-B27 antigen in patients with other joint diseases (gout, rheumatoid arthritis, septic arthritis) does not exceed 7-8%. Given this feature, detection of the HLA-B27 antigen is of great diagnostic value in the clinic of rheumatological diseases. Determination of the HLA-B27 antigen is of the greatest importance in the diagnosis of early ankylosing spondylitis. In most cases, 5-10 years pass between the appearance of the first signs of the disease and the establishment of the final diagnosis. This is due to the fact that the main diagnostic criterion of the disease is radiological signs of sacroiliitis, which develops only after several years of the inflammatory process in the sacroiliac joints. Patients with complaints of back pain without radiological signs of sacroiliitis actually do not fall into the field of vision of a rheumatologist.
Detection of HLA-B27 in such a situation may be a sufficient reason for referring the patient to a narrowly focused specialist. Determination of the HLA-B27 antigen is indicated when examining a patient with complaints of inflammatory back pain in the absence of radiological signs of sacroiliitis or when examining a patient with asymmetric oligoarthritis. The presence of HLA-B27 is associated with an increased risk of extra-articular manifestations of ankylosing spondylitis. Associations of the HLA-B27 antigen and acute anterior uveitis, aortic valve insufficiency, acute leukemia, IgA nephropathy and psoriasis are of greatest importance. HLAB27-positive patients are more prone to the risk of tuberculosis and malaria.
On the other hand, the presence of HLA-B27 also plays a certain protective role: some viral infections (influenza, herpes virus type 2 infection, infectious mononucleosis, hepatitis C and HIV) occur in a milder form in HLA-B27 carriers. Determination of the HLA-B27 antigen is carried out to predict the complications of rheumatoid arthritis. The presence of HLA-B27 is associated with a threefold increase in the risk of atlanto-axial subluxation.
Various laboratory methods can be used to determine the HLA-B27 antigen: a lymphocytotoxic test, molecular diagnostic methods (PCR), enzyme-linked immunosorbent assay (ELISA), and flow cytometry. The flow cytometry method is a fast and reliable way to detect the HLA-B27 antigen. At the same time, it has some limitations that should be taken into account when interpreting the result. Thus, monoclonal antibodies to the HLA-B27 antigen used in the test are not completely specific, but can also react with other antigens of the HLA-B family (primarily - HLA-B7, and to a lesser extent - HLA-B40, 73 , 22, 42, 44). Given this feature, in order to avoid diagnostic errors, modern protocols for determining the HLA-B27 antigen use double antibodies that allow differentiation of the HLA-B27 antigen from other antigens of the HLA-B family. This approach increases the specificity and sensitivity of the test to 97.6 and 98.8%, respectively.
Despite the presence of a stable association of the HLA-B27 antigen and the risk of developing spondyloarthritis, a positive test result does not always reflect the real risk of the disease in a particular patient. This is due to the fact that the HLA-B27 antigen is represented by 49 different variants characterized by varying degrees of association with this group of diseases.
Thus, the HLA-B2708 variant has the greatest association with the disease, and the HLA-B2706 and HLA-B2709 variants are apparently not associated with the risk of the disease at all. About 7-8% of healthy people of the European population are carriers of the HLA-B27 antigen. When interpreting a positive result, additional data on the patient's heredity can help. It should be noted that there are other, both hereditary and acquired risk of developing seronegative spondyloarthritis.
The absence of HLA-B27 does not contradict the diagnosis of ankylosing spondylitis. In this case, ankylosing spondylitis is classified as HLA B27-negative.