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Blood test
Blood test

Complement component 3 (C3)
Definition:
A test that measures the concentration of a component of complement in the blood.

Alternative Names:
C3

How the test is performed:
Adult or child:
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

Infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.
How to prepare for the test:
Adults:
There are no special preparations.

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:
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:
Complement activity (CH50, CH100, terminal complement component, or individual complement proteins) is measured to determine if complement is involved in the development of a number of diseases. Complement activity is also measured to monitor severity of a disease or determine efficiency of treatment. For example, patients with active lupus erythematosus may have depressed levels of C3 and C4, and these component levels may be followed as a rough index of disease activity.

Patients with gram negative septicemia and shock are often depleted of C3 and components of the alternative pathway. C3 is often also depressed in fungal infections and some parasitic infections such as malaria.

Red blood cells from patients with paroxysmal nocturnal hemoglobinuria (PNH) bind more C3 than normal cells. This acquired defect is associated with chronichemolysis inside blood vessels caused by activation of the alternative pathway.
Normal Values:
The normal range is 75 to 135 mg/dl.

Note: mg/dl = milligrams per deciliter
What abnormal results mean:
Increased complement activity may be seen in:Decreased complement activity may be seen in:
What the risks are:
  • excessive bleeding
  • fainting or feeling light-headed
  • hematoma (blood accumulating under the skin)
  • infection (a slight risk any time the skin is broken)
  • multiple punctures to locate veins
Special considerations:

Serum complement comprises a group of proteins that facilitate immunological and
inflammatory responses. The so-called complement cascade involves a series of enzymatic reactions that take place in the blood. There are 9 major components labeled C1 through C9.
The cascade can be initiated by various means, especially antigen-antibody complexes. The end-product of the cascade is the so-called membrane attack unit (also called terminal complement component), which creates holes in the membranes of attacking bacteria, thereby causing lysis and death of the cells. CH50 or CH100 are tests for the activity of the complement system. There are also a number of side products of the complement cascade that attract white blood cells and increase the efficiency of certain types of white blood cells to engulf bacteria.

When many bacteria are exposed to fresh serum, complement is activated by an "alternative pathway" that does not require the presence of specific antibodies to the bacteria. C3, one of the major components of the complement cascade binds to these bacteria directly. Bound C3, or some modified form of C3, is able to associate with other members of the complement cascade to form a stable unit capable of activating other complement proteins to form the membrane attack unit.

Typically, other tests that are more specific for the suspected disease are performed first.
Review Date: 11/17/2001
Reviewed By: Birgit Kovacs, M.D., Division of Rheumatology, University of Pennsylvania Medical Center, Philadelphia, PA. Review provided by VeriMed Healthcare Network.
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