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    Complement component 3 (C3)

    C3

    Complement C3 is a blood test that measures the activity of a certain protein that is part 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. This test measures C3.

    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

    There is no special preparation 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

    C3 and C4 are the most commonly measured complement components.

    A complement test may be used to monitor patients with an autoimmune disorder and to see if treatment for their condition is working. For example, patients with active lupus erythematosus may have lower-than-normal levels of the complement proteins C3 and C4.

    Complement activity varies throughout the body. For example, in patients with rheumatoid arthritis, complement activity in the blood may be normal or higher-than-normal, but much lower-than-normal in the joint fluid.

    Additional conditions under which the test may be performed:

    • Fungal infections
    • Gram negative septicemia
    • Parasitic infections such as malaria
    • Paroxysmal nocturnal hemoglobinuria (PNH)
    • Shock

    Normal Results

    The normal range is 75-135 mg/dl (milligrams per deciliter).

    Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

    The examples above show the common measurements for results for these tests. Some laboratories use different measurements or may test different specimens.

    What Abnormal Results Mean

    Increased complement activity may be seen in:

    • Cancer
    • Ulcerative colitis

    Decreased complement activity may be seen in:

    • Bacterial infections (especially Neisseria)
    • Cirrhosis
    • Glomerulonephritis
    • Hepatitis
    • Hereditary angioedema
    • Kidney transplant rejection
    • Lupus nephritis
    • Malnutrition
    • Systemic lupus erythematosus

    Risks

    Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

    Other 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)

    Considerations

    The "complement cascade" is a series of reactions that take place in the blood. The cascade activates the complement proteins. The result is an attack unit that creates holes in the membrane of bacteria, killing them. C3 attaches to and kills bacteria directly.

    References

    Introduction to the Complement System. In: Adkinson NF Jr, ed. Middleton’s Allergy: Principles and Practice. 7th ed.Philadelphia, Pa:Mosby Elsevier; 2008:chap 6.

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      A Closer Look

        Tests for Complement component 3 (C3)

        Review Date: 2/11/2013

        Reviewed By: Ariel D. Teitel, MD, MBA, Clinical Associate Professor of Medicine, NYU Langone Medical Center. Review provided by VeriMed Healthcare Network. Also reviewed by A.D.A.M. Health Solutions, Ebix, Inc., Editorial Team: David Zieve, MD, MHA, Bethanne Black, Stephanie Slon, and Nissi Wang.

        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. 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.
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