The HLA-B27 test is a blood test that detects the presence of a specific protein called HLA-B27 on the surface of white blood cells. This protein is part of the human leukocyte antigen (HLA) system and is considered a genetic marker associated with certain inflammatory autoimmune diseases. The test is commonly used to help diagnose conditions such as ankylosing spondylitis (chronic inflammation of the spine), reactive arthritis, psoriatic arthritis, or uveitis (eye inflammation). The presence of HLA-B27 in certain individuals—particularly those of specific ethnic backgrounds, such as Caucasians—is associated with an increased risk of developing these conditions. However, a positive result alone does not confirm the disease and must be interpreted in conjunction with clinical symptoms and other tests. The sample is collected through a simple blood draw, and the result is reported as either positive or negative. About 6–8% of healthy individuals may also be HLA-B27 positive, so physicians consider this result alongside imaging studies (such as MRI) and clinical evaluations.