Definition of C1 esterase inhibitor:

C1 esterase inhibitor (C1-INH) is a protein found in the fluid part of your blood that controls C1, the first component of the complement system. The complement 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. C1-INH is an important marker for hereditary angioedema, and it also plays a role in other diseases.

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

See also:

• Complement
• Complement component 3 (C3)
• Complement component 4 (C4)
• CH50
• CH100

How the test is performed:

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

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:

Your doctor may order this test if you have signs of hereditary angioedema. Hereditary angioedema is caused by low levels of C1-INH.