C-reactive protein (CRP) is a marker of inflammation.

Ankylosing spondylitis (AS) is one of several health conditions that may cause elevated CRP. Other autoimmune conditions, infections, or lifestyle factors may also increase CRP levels.

The standard diagnostic process for AS does not include a CRP test. But if a person has AS, their doctor might order a CRP test to check for signs of disease activity or another health condition that may be causing symptoms. They will also ask about symptoms and likely order imaging tests.