C-reactive protein: a marker for disease activity in ankylosing spondylitis and Reiter's syndrome.

Forty-four patients, 22 with ankylosing spondylitis (AS) and 22 with Reiter's syndrome (RS) were studied to determine the reliability of C-reactive protein (CRP) levels and erythrocyte sedimentation rate (ESR) as indicators of disease activity. CRP levels were significantly elevated in patients with active disease for both AS and RS while ESR values for active and inactive disease were not statistically different. Misdiagnosis was more likely when ESR rather than CRP was used as the variable for activity. CRP as measured by nephelometry is a more sensitive and specific indicator of disease activity in AS and RS than ESR.