The HLA B-27 test is a blood test used to detect the presence of the human leukocyte antigen B-27 (HLA-B27) protein in a person’s blood. HLA-B27 is a protein found on the surface of white blood cells and is associated with several autoimmune diseases, including ankylosing spondylitis, reactive arthritis, and psoriatic arthritis. The test is often ordered when a person presents with symptoms suggestive of one of these conditions. However, a positive HLA-B27 test does not necessarily mean that a person has or will develop an autoimmune disease, as many people with the protein do not develop symptoms. Additionally, a negative test does not completely rule out the possibility of an autoimmune disease, as some affected individuals do not have the HLA-B27 protein. Therefore, the HLA-B27 test is just one tool among many that healthcare providers use to diagnose autoimmune diseases and to determine the best course of treatment.