- A wide variety of markers are available to assess the function and pathology of the gastrointestinal (GI) tract. This review describes some of these markers with special emphasis given to markers used in dogs and cats. Small intestinal disease can be confirmed and localized by the measurement of serum concentrations of folate and cobalamin. Fecal 1-proteinase inhibitor concentration can increase in individuals with excessive GI protein loss. A wide variety of inflammatory markers are available for a variety of species that can be used to assess the inflammatory activity of various types of inflammatory cells in the GI tract, although most of these markers assess neutrophilic inflammation, such as neutrophil elastase, calprotectin, or S100A12. N-methylhistamine can serve as a marker of mast cell infiltration. Markers for lymphocytic or eosinophilic inflammation are currently under investigation. Exocrine pancreatic function can be assessed by measurement of serum concentrations of pancreatic lipase immunoreactivity (PLI) and trypsin-like immunoreactivity (TLI). Serum PLI concentration is increased in individuals with pancreatitis and has been shown to be highly specific for exocrine pancreatic function and sensitive for pancreatitis. Serum TLI concentration is severely decreased in individuals with exocrine pancreatic insufficiency.