calprotectin

Calprotectin is a calcium-binding protein complex composed of the S100A8 and S100A9 subunits. It forms a heterodimer known as calprotectin, belonging to the S100 family of calcium-binding proteins. Calprotectin is abundant in neutrophils and, to a lesser extent, in monocytes and macrophages. Upon activation of phagocytes and during migration to sites of inflammation, calprotectin is released into the extracellular space. The protein exhibits antimicrobial properties in part by sequestering zinc and manganese, contributing to innate immune defense.

Fecal calprotectin is widely used as a noninvasive biomarker of intestinal inflammation. It can be measured by immunoassays on stool samples and remains stable at room temperature for several days, facilitating outpatient testing. Elevated fecal calprotectin levels indicate neutrophil influx into the intestinal mucosa and are associated with inflammatory bowel diseases such as Crohn's disease and ulcerative colitis, as well as infections and other inflammatory conditions. In practice, fecal calprotectin testing helps distinguish inflammatory bowel disease from irritable bowel syndrome and is useful for monitoring disease activity and response to therapy, with rising levels suggesting relapse.

Serum or plasma calprotectin can also be measured and is increased in various systemic inflammatory states, including infections, autoimmune diseases, and malignancy. However, serum calprotectin is less specific for intestinal inflammation than fecal calprotectin and is influenced by comorbid conditions.

Important considerations include assay variability among different commercial tests, pre-analytical factors such as sample collection and storage, and assay-specific reference ranges. Calprotectin results are interpreted in the clinical context and alongside other laboratory and imaging data.