The first clue in diagnosing ankylosing spondylitis is the presence of symptoms, especially back pain. In addition, X-rays are taken to look for signs of the disorder, such as fused joints. A blood test for the HLA-B27 gene, which is found in about 90 percent of the people with ankylosing spondylitis, also may be performed. However, only 10 percent to 15 percent of people who inherit the gene develop ankylosing spondylitis.