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C1 esterase inhibitor

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. This system is a group of proteins that move freely through your bloodstream. The proteins work with your immune system and play a role in the development of inflammation. There are nine major complement proteins. They are labeled C1 through C9.

Complement factors are very important in testing for autoimmune diseases, especially systemic lupus erythematosus. Low levels of C1-INH can lead to a condition called angioedema. Angioedema results in sudden swelling of the tissues of the face, upper throat and tongue and may cause difficulty breathing. Swelling in the intestine and abdominal pain may also occur. There are two types of conditions that cause angioedema from decreased levels of C1-INH. They are known as hereditary and acquired C1-INH deficiency.

This article discusses the test that is done to measure the amount of C1-INH in your blood.

How the Test is Performed

A blood sample is needed. This is most often taken through a vein. The procedure is called a venipuncture.

How to Prepare for the Test

No special preparation is needed.

How the Test will Feel

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the Test is Performed

You may need this test if you have signs of hereditary or acquired angioedema. Both forms of angioedema are caused by low levels of C1-INH.

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.


Risks associated with having blood drawn are slight but may include:

  • Excessive bleeding
  • Fainting or feeling light-headed
  • Hematoma (blood accumulating under the skin)
  • Infection (a slight risk any time the skin is broken)

Alternative Names

C1 inhibiting factor; C1-INH



Bowen T, Cicardi M, Farkas H, et al. 2010 International consensus algorithm for the diagnosis, therapy and management of hereditary angioedema. Allergy Asthma Clin Immunol. 2010;6:24. PMID: 20667127 www.ncbi.nlm.nih.gov/pubmed/20667127.

Sullivan KE, Grumach AS. The complement system. In: Adkinson NF, Bochner BS, Burks AW, et al, eds. Middleton's Allergy: Principles and Practice. 8th ed. Philadelphia, PA: Elsevier Saunders; 2013:chap 6.

Update Date 1/20/2015

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