Citrulline

Citrulline is an amino acid made mostly by the cells that line your small intestine. It is also a key intermediate in the urea cycle, the process your body uses to convert ammonia, a waste product of protein metabolism, into urea for safe removal in urine.

This test measures the amount of citrulline in your blood plasma. Because citrulline reflects both small intestinal function and urea cycle activity, it can give your care team insight into metabolic health and the integrity of your intestinal lining.

Clinicians use plasma citrulline to help evaluate suspected urea cycle disorders, a group of inherited conditions that affect how the body removes ammonia. It is also useful to assess small intestinal health, since low levels can suggest reduced intestinal mucosal mass that may occur with short bowel syndrome or other causes of intestinal failure.

Your clinician may order this test if you or your child has unexplained vomiting, lethargy, or confusion, particularly in infancy, or if there is concern for metabolic disease. It can also be used to monitor intestinal recovery after surgery or transplant, and to track enteral nutrition response in conditions that damage the small intestine.

Your result is interpreted alongside symptoms, medical history, and other laboratory findings such as ammonia and a full plasma amino acid profile. Higher citrulline levels can be seen in some urea cycle enzyme deficiencies, while lower levels can point to reduced small intestinal mucosal mass or impaired intestinal function. Kidney and liver function, as well as recent supplements, can influence the value.

If your result is outside the expected range, your clinician may repeat the test, review medications and supplements, and consider additional testing. This may include ammonia testing, other amino acids, urine orotic acid, imaging, or genetic testing. Treatment plans are individualized and may involve diet changes, medications to manage ammonia, or nutrition support if intestinal function is reduced.