Kalprotektin

Kalprotektin, commonly known as calprotectin, is a calcium-binding protein complex of the S100 family. It consists of two subunits, S100A8 (calgranulin A) and S100A9 (calgranulin B), and forms a heterodimer that is abundant in neutrophils and to a lesser extent in monocytes and other phagocytes. The protein is stored in the cytosol and released when these cells are activated or damaged.

Calprotectin binds calcium and zinc and exhibits antimicrobial activity by limiting essential metals needed by microbes, a process known as nutritional immunity. It can also function as a damage-associated molecular pattern, engaging receptors such as TLR4 and RAGE to promote inflammatory signaling. Through these actions, calprotectin participates in innate immune responses and contributes to the propagation of local inflammation.

Clinically, calprotectin is best known as a biomarker of inflammation. Fecal calprotectin is used as a noninvasive indicator of gastrointestinal inflammation and is helpful in distinguishing inflammatory bowel disease from functional disorders like irritable bowel syndrome, as well as in monitoring disease activity and response to therapy. Elevated calprotectin levels can also occur in infections, colorectal cancer, and other inflammatory or ischemic conditions, though levels vary with age and the source of the sample (stool versus serum).

Measurement is typically performed by immunoassays, such as ELISA or nephelometry, on stool or plasma/serum samples. Normal ranges depend on the assay and patient factors; in healthy adults, fecal calprotectin is generally below about 50 µg/g of stool. Clinicians interpret results with consideration of clinical context and pre-analytical factors, including NSAID use and sample handling.