Fecal calprotectin correlates with active colonic inflammatory bowel disease but not with small intestinal Crohn's disease activity

Background: The utility of fecal calprotectin (FC) in small intestinal Crohn's disease (CD) is unclear. We examined how reliably FC reflects clinical and mucosal disease activity in small intestinal CD, colonic CD, and ulcerative colitis (UC). Methods: A total of 72 Inflammatory Bowel Disease (IBD) patients (23 colonic CD, 14 isolated small intestinal CD, and 35 UC) were included. Clinical activity was assessed using the Harvey–Bradshaw Index (HBI) (CD) and Mayo score (UC). Inflammatory activity was assessed through ileocolonoscopy, cross-sectional imaging, C-reactive protein (CRP), and FC. Clinical activity was defined as HBI > 4 or Mayo clinical score ≥ 3. Endoscopy activity was defined as Mayo endoscopic subscore ≥ 1, SES-CD score ≥ 3, and Rutgeerts > i1. Results: In UC, FC was correlated with the Mayo clinical score (P < 0.0001) and was highly correlated with the total Mayo score (P < 0.0001). A cut-off value of FC 100 μg/g provided sensitivity of 88% and specificity 100% for endoscopic activity. FC was lower for patients with endoscopic and clinical remission compared to active endoscopic disease (median 100 vs 1180 μg/g, P < 0.0001). In colonic CD, there was a significant correlation between FC and endoscopic activity (P < 0.001). For an FC cut-off value of 100 μg/g, sensitivity was 100%, and specificity was 67%. In contrast, for isolated small intestinal CD, there was no significant correlation between FC and objective disease activity measured by either endoscopy or imaging (AUC 0.52, P = 0.58). Conclusion: FC is reliable for the detection of colonic mucosal inflammation in both UC and CD but is less sensitive and reliable in the detection of small intestinal CD.