Ankylosing Spondylitis (AS) is one of the major forms of chronic inflammatory arthritis and is the prototypical example of the spondyloarthropathies, a group of chronic autoimmune joint diseases.
Ankylosing Spondylitis has a global distribution, though rarer in Africans. It is characterized by arthritis affecting the spine and pelvis. It specifically involves the sacroiliac joint and initially causes pain and reversible stiffness (stiffness in the mornings that goes away later in the day with exercise) but leads to progressive joint fusion with irreversible stiffness and deformity in a number of cases.
There is a strong association with the HLA class I molecule HLA-B27, found in upwards of 90% of patients. Association of AS to B27 is amongst the strongest genetic associations with a common disease, although the mechanism of action remains uncertain. The strong association of HLA-B27 with AS makes B27 testing a useful component of the diagnostic work up which includes a physical examination, use of X-rays, MRI, and a check for family history of AS. The value of B27 testing is that it allows a presumptive diagnosis and early treatment in patients showing some of the symptoms and before the damage to joints would already have happened.