Calprotectin belongs to a group of Ca2+ binding proteins in the S100 family. It consists of two heavy chain and one light polypeptide chain, with a total molecular mass of 36.5 kD. It is found in abundance in neutrophil granulocytes, in which it accounts for 5% of total protein and 60% of the protein in the cytosol fraction.
The amount of calprotectin reflects the number of participating neutrophils in this inflammation. This has been amply confirmed in intestinal inflammatory diseases by the significant correlation between fecal calprotectin levels and other measures of acute inflammation, be it with 111indium-labeled white cell excretion or quantitative histopathologic assessment of inflammation in colonic biopsies in controls and individuals with ulcerative colitis. In the presence of active intestinal inflammation, polymorphonuclear neutrophils migrate to the intestinal mucosa from the circulation. Any disturbance to the mucosal architecture due to the inflammatory process, results in leakage of neutrophils, and hence, calprotectin, into the lumen and its subsequent excretion in feces. The concentration of calprotectin in feces has been shown to correlate well with the “gold standard” 111indium-labeled leukocyte test and to the severity of the intestinal inflammation. Calprotectin appears to be distributed homogeneously in feces and is stable for up to 7 days at room temperature.