C1 esterase inhibitor (C1-INH) is a protein found in the fluid part of your blood. It controls a protein called C1, which is part of the complement system.

The complement system is a group of nearly 60 proteins in blood plasma or on the surface of some cells. The complement proteins work with your immune system to protect the body from infections. They also help remove dead cells and foreign material. There are nine major complement proteins. They are labelled C1 through C9. Rarely, people may inherit deficiency of some complement proteins. These people are prone to certain infections or autoimmune disorders.

Why the Test is Performed
You may need this test if you have signs of hereditary or acquired angioedema, which is a type of tissue swelling. Both forms of angioedema are caused by low levels of C1-INH.

Complement factors may also be important in testing for autoimmune diseases, such as systemic lupus erythematosus.

Normal Results
Normal value ranges may vary slightly among different laboratories. Your health care provider will also measure the functional activity level of your C1 esterase inhibitor. Talk to your provider about the meaning of your specific test results.

What Abnormal Results Mean
Low levels of C1-INH may cause certain types of angioedema. Angioedema results in sudden swelling of the tissues of the face, upper throat and tongue. It may also cause difficulty breathing. Swelling in the intestine and abdominal pain may also occur. There are two types of angioedema that result from decreased levels of C1-INH. Hereditary angioedema affects children and young adults under age 20. Acquired angioedema is seen in adults older than age 40. Adults with acquired angioedema are much more likely to also have other conditions, such as cancer or autoimmune disease.