What is the role of C-reactive protein and fecal calprotectin in evaluating Crohn's disease activity?

Historically, the evaluation of patients with Crohn's disease (CD) has centered on use of subjective symptom-based assessment. However, patients with CD experience a broad spectrum of non-specific symptoms that may not directly correlate with objective measures of inflammation. Endoscopy has been the gold standard for evaluating the burden and severity of mucosal disease. However, use of ileocolonoscopy for disease monitoring in long-term follow-up is limited by considerations of cost, resource utilization, and invasiveness. As treatment goals in CD have shifted towards 'treat-to-target' paradigms that emphasize tight control of inflammation, it has become increasingly evident that sensitive, accurate, and reliable measures of disease activity are required. The use of non-invasive serum and fecal biomarkers such as C-reactive protein (CRP) and fecal calprotectin (FC) has been evaluated in patients with CD for categorizing disease activity, predicting treatment response, identifying patients at risk for disease relapse, and as a potential therapeutic target. In this review, we summarize the interpretation of CRP and FC in patients with CD within specific clinical contexts and according to assay performance characteristics.