The full name of b27 is HLA-B27, or human leukocyte antigen B27.Bilateral sacroiliac arthritis is positive for HLA-B27, suggesting possible ankylosing spondylitis. HLA-B27 is a surface antigen marker commonly used in the diagnosis of spondyloarthropathies with good sensitivity and specificity.HLA-B27 is closely associated with ankylosing spondylitis with a correlation level of 90% or more, but a positive test alone is not conclusive of a diagnosis of ankylosing spondylitis. If a patient presents with symptoms of spinal pain and is positive for HLA-B27, the diagnosis is essentially confirmed. Ankylosing spondylitis is a chronic progressive inflammatory disease of the spine. As the disease progresses, the spine becomes ankylosed from the bottom up, mainly affecting the spine, the median bones, and the large joints of the limbs. If ankylosing spondylitis is present, the sacroiliac joints will be involved causing bilateral sacroiliitis. If HLA-B27 is positive with bilateral sacroiliac arthritis, ankylosing spondylitis may be present. If HLA-B27 is positive, it is recommended to seek prompt medical treatment, and the clinician will make appropriate diagnosis and treatment according to the test results.