Fecal Calprotectin Correlates with Disease Extent but Remains a Reliable Marker of Mucosal Healing in Ulcerative Colitis

Fecal calprotectin (FC) is a marker of mucosal inflammatory activity in ulcerative colitis (UC). FC levels may also be influenced by the extent of disease. We aimed to examine the relationships between FC, mucosal activity, and disease extent, and to assess how disease extent affects the diagnostic accuracy of FC. We conducted a single-center observational study of patients with UC. Mucosal inflammatory activity was rated by the Mayo Endoscopic Score (MES) and Nancy Histological Index (NHI). The endoscopic and histological extents of disease were categorized by the Montreal classification and colorectal distribution of histologically active inflammation (NHI ≥ 2), respectively. FC was measured by EliA Calprotectin Enzyme fluoroimmunoassay (Phadia). A total of 518 visits by 254 patients were analyzed. In endoscopically active UC (MES ≥ 2), FC levels were significantly lower in proctitis (440 [IQR: 175-1350] mg/kg) as compared to left-sided colitis (840 [IQR: 298-2011] mg/kg, p = 0.048) or pancolitis (1690 [IQR: 723-2582] mg/kg, p = 0.00005). In MES ≤ 1, FC levels were significantly higher in pancolitis (85 [IQR: 43-350] mg/kg) as compared to proctitis (24 [IQR: 15-116] mg/kg, p = 0.00032) or left-sided colitis (40 [IQR: 15-160] mg/kg, p = 0.012). However, FC remained a reliable marker of mucosal healing across all disease extents, with AUC ranging from 0.878 to 0.915, and no significant differences between the extent categories (DeLong´s test, p ≥ 0.2919). FC showed a significant association with disease extent yet remained a reliable surrogate marker for mucosal healing across all disease extents.