Positive HLA-B27 and sacroiliitis is not always spondyloarthritis.

A 36-year-old man was treated for several years with multiple agents for ankylosing spondylitis based on positive human leukocyte antigen-B27 and sacroiliitis. He was also diagnosed with osteoporosis and hypophosphatemia. Over these years, from being an avid runner, he became dependent on a walker for ambulation. The lack of treatment response and the low phosphorus were clues that eventually led to a diagnosis of tumor-induced osteomalacia. This case discusses the importance of not solely relying on genetic markers and sacroiliitis for diagnosing ankylosing spondylitis as other conditions can cause similar presentations.