Background and aims: C-reactive protein (CRP) levels are often used in the follow-up of patients with inflammatory bowel disease (IBD). The aims of this study were to establish the relationship of CRP levels to disease extent in patients with ulcerative colitis and to phenotype in patients with Crohn’s disease, and to investigate the predictive value of CRP levels for disease outcome.

Methods: CRP was measured at diagnosis and after 1 and 5 years in patients diagnosed with IBD in south-eastern Norway. After 5 years, 454 patients with ulcerative colitis and 200 with Crohn’s disease were alive and provided sufficient data for analysis.

Results: Patients with Crohn’s disease had a stronger CRP response than did those with ulcerative colitis. In patients with ulcerative colitis, CRP levels at diagnosis increased with increasing extent of disease. No differences in CRP levels at diagnosis were found between subgroups of patients with Crohn’s disease as defined according to the Vienna classification. In patients with ulcerative colitis with extensive colitis, CRP levels above 23 mg/l at diagnosis predicted an increased risk of surgery (odds ratio (OR) 4.8, 95% confidence interval (CI) 1.5 to 15.1, p = 0.02). In patients with ulcerative colitis, CRP levels above 10 mg/l after 1 year predicted an increased risk of surgery during the subsequent 4 years (OR 3.0, 95% CI 1.1 to 7.8, p = 0.02). A significant association between CRP levels at diagnosis and risk of surgery was found in patients with Crohn’s disease and terminal ileitis (L1), and the risk increased when CRP levels were above 53 mg/l in this subgroup (OR 6.0, 95% CI 1.1 to 31.9, p = 0.03).

Conclusions: CRP levels at diagnosis were related to the extent of disease in patients with ulcerative colitis. Phenotype had no influence on CRP levels in patients with Crohn’s disease. CRP is a predictor of surgery in subgroups of patients with either ulcerative colitis or Crohn’s disease.