Fecal calprotectin (FCAL) is a protein marker of gut inflammation. It is used as a diagnostic tool and marker of disease activity for Crohn disease and ulcerative colitis.
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Biochemistry
Calprotectin is a protein complex from the S-100 family, which is formed of three polypeptide chains, two heavy and one light 2. It was first purified in 1980 and called L1 protein by the group that discovered it. However, like many proteins, it has had multiple names, and calprotectin is now preferred 2.
Distribution
It is found in especially heavy concentrations in neutrophils and monocytes, and more generally within skin cells, with an organ distribution including the lungs, gastrointestinal tract, oral and cervical mucosae and associated body fluids/secretions.
During the inflammatory process, it is secreted by neutrophils and macrophages in the gut and is assayable in the feces.
Assay
There are many commercially available testing kits for calprotectin, the majority using ELISA (enzyme-linked immunosorbent assay) techniques.
Clinical
Fecal calprotectin is primarily employed in two scenarios:
distinguish inflammatory bowel disease from irritable bowel syndrome
assess disease activity in inflammatory bowel disease, including acute exacerbations and response to treatment
The use of fecal calprotectin as a screening tool for colorectal carcinoma has been explored, however, its specificity is too low to be useful in the general population 1.
No consensus exists on the correlation between calprotectin levels and activity of bowel disease on cross-sectional imaging, as studies have come to different conclusions. One study, albeit a small cohort, showed a weak correlation at best 4.