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Blood test
Blood test

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.

Alternative Names:

C1 inhibiting factor; C1-INH

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)


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 .

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.

Review Date: 1/20/2015
Reviewed By: Gordon A. Starkebaum, MD, Professor of Medicine, Division of Rheumatology, University of Washington School of Medicine, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.

The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. © 1997- A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.

Contact Us: 1-800-245-3314 or (501) 329-3831 2302 College Avenue, Conway, Arkansas 72034
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