Test Details: Citrulline is an amino acid made from ornithine and carbamoyl phosphate in one of the central reactions in the urea cycle. Plasma citrulline has been used as a marker for reduced small intestinal mass, intestinal injury and rejection of small bowel transplants. Plasma citrulline rises in proportion to enterocytes mass, reflecting the unique capacity of enterocytes for net conversion of glutamine into citrulline. Both classic and type II citrullinemia are associated with known gene alterations. Plasma citrulline elevation is also used as a marker of postoperative intestinal failure and a marker of reduced intestinal mass in patients of villous atrophy.