The cause of ankylosing spondylitis (AS) is not fully understood. However, certain factors have been identified which may increase your risk of developing the condition.

Genetic markers (HLA-B27)

Research has shown that most people who have ankylosing spondylitis carry a particular genetic marker, known as human leukocyte antigen B27 (HLA-B27). This means that people who have this antigen are more likely to develop ankylosing spondylitis than those who do not.

However, as most people with the HLA-B27 gene never develop ankylosing spondylitis, genetics cannot be the only cause of the condition.

Also, most people who have the HLA-B27 gene do not develop ankylosing spondylitis, so testing for the marker is not a very reliable method of diagnosis.

Family history

Ankylosing spondylitis can run in families and the HLA-B27 antigen can be inherited (passed on) from one member of a family to another.

If you have a close relative, such as a parent, or sibling, who has ankylosing spondylitis, you are three times more likely to develop it compared with someone who does not have a relative with the condition.

However, as ankylosing spondylitis is a relatively rare condition, your overall likelihood of developing it is very small.