The Human Leukocyte Antigen B27 (HLA-B27) test is a specialized blood examination that physicians use to help diagnose a group of inflammatory conditions, primarily those affecting the spine and joints. The test detects the HLA-B27 antigen, a specific protein found on the surface of white blood cells and a marker within the body’s complex immune system. Understanding this antigen aids in accurate diagnosis when a patient presents with chronic, unexplained symptoms like joint pain or eye inflammation. While the presence of HLA-B27 does not confirm a disease, it significantly increases the likelihood of developing specific autoimmune disorders, making it a valuable diagnostic tool.
Understanding the HLA-B27 Antigen
The Human Leukocyte Antigen (HLA) system is a collection of proteins located on the surface of most cells in the body, which are responsible for regulating the immune system. These proteins, also known as Major Histocompatibility Complex (MHC) molecules, function to help the immune system distinguish between the body’s own cells and foreign invaders like bacteria or viruses. HLA-B27 is a particular variant of the Class I MHC molecules, which are found on nearly all nucleated cells.
The primary role of these Class I molecules is to present small pieces of proteins, called peptides, to T-cells. This presentation process allows T-cells to check for and destroy cells that are infected or abnormal. The HLA-B27 protein is present in about 6% of the general United States population and is considered normal for many people. However, its specific structure is thought to sometimes trigger an inappropriate immune response. Theories suggest that HLA-B27 may improperly fold or present certain foreign peptides in a way that leads to a misguided attack on the body’s own tissues, resulting in chronic inflammation.
How the HLA-B27 Test is Performed and Interpreted
The HLA-B27 test is a straightforward procedure involving drawing a small sample of blood, similar to a routine blood test. The sample is then sent to a laboratory where specialized techniques are used to determine if the HLA-B27 antigen is present on the surface of the white blood cells. Common laboratory methods utilized for this analysis include flow cytometry, which detects the protein directly, or Polymerase Chain Reaction (PCR), which identifies the specific gene that codes for the HLA-B27 protein.
The test result is reported as either “positive” or “negative.” A positive result confirms the presence of the HLA-B27 antigen, indicating an increased genetic susceptibility to certain autoimmune conditions. Crucially, a positive result does not mean a person has or will definitely develop a disease, as many individuals with the antigen never experience any associated health problems. Conversely, a negative result means the antigen was not detected, which suggests a lower risk for HLA-B27-associated diseases. However, a negative result does not completely rule out the possibility of these conditions, as some patients develop them regardless of their HLA-B27 status. Therefore, the test is not a standalone diagnostic tool but must be interpreted alongside a patient’s clinical symptoms, medical history, and other diagnostic findings.
Conditions Associated with a Positive HLA-B27 Result
Physicians primarily order the HLA-B27 test to help diagnose spondyloarthropathies, a group of inflammatory diseases strongly linked to this antigen.
Ankylosing Spondylitis (AS)
AS is a chronic inflammatory disease that primarily affects the joints of the spine and pelvis, leading to chronic low back pain and stiffness. The association is particularly strong, as the HLA-B27 antigen is found in approximately 80% to 90% of patients with AS. The disease typically begins in young adulthood, with symptoms often worse in the morning and improving with activity. In advanced cases, inflammation can cause the vertebrae of the spine to fuse together, resulting in a loss of mobility.
Reactive Arthritis (ReA)
ReA, previously known as Reiter’s syndrome, is another condition strongly linked to HLA-B27. This condition develops as an autoimmune reaction triggered by a bacterial infection, most commonly in the gastrointestinal or genitourinary tracts (e.g., Chlamydia, Salmonella, or Yersinia). Reactive Arthritis is characterized by a triad of symptoms: inflammation of the joints, the urethra (urethritis), and the eyes (conjunctivitis or uveitis). About 60% to 85% of patients diagnosed with ReA are positive for the HLA-B27 marker.
Psoriatic Arthritis (PsA)
PsA is included in the family of spondyloarthropathies, affecting up to 30% of individuals who have the skin condition psoriasis. The association with HLA-B27 is less pronounced than with AS. However, its presence is relevant, particularly in cases where the disease involves the spine. Patients with psoriatic arthritis who are HLA-B27 positive are more likely to experience spinal involvement.
Acute Anterior Uveitis (AAU)
AAU is an eye disorder closely associated with the HLA-B27 antigen. It is an inflammation of the iris and ciliary body, typically presenting as a sudden onset of pain, redness, and light sensitivity in one eye. The HLA-B27 antigen is found in about 40% to 80% of patients who experience AAU, making it the most common cause of non-infectious uveitis. An episode of AAU may be the first manifestation of an underlying spondyloarthropathy, such as Ankylosing Spondylitis. The test helps confirm a suspected diagnosis when symptoms align with these inflammatory diseases, especially in early stages before physical joint damage is visible on imaging.