Complement component 3 (C3)

Alternative names

This is a blood test that measures one component of the complement cascade. Complement is a group of blood proteins that cause immune responses and
inflammation. The complement cascade is a series of reactions that take place in the blood. There are 9 major complement components, labeled C1 through C9. This test measures C3.

How the test is performed

Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to 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 band 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.

For an 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
There are no special preparations.

For infants and children:
The preparation you can provide for this test depends on your child’s age, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics:

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 how severe a disease is or to determine if treatment is working. For example, patients with active lupus erythematosus may have low levels of C3 and C4, and these component levels may be watched as an indicator of disease activity.

Patients with gram negative septicemia and shock often have very low or no C3, and C3 is often also low in fungal infections and some parasitic infections such as malaria.

Red blood cells from patients with paroxysmal nocturnal hemoglobinuria (PNH) carry more C3 than do normal cells.

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

What abnormal results mean
Increased complement activity may be seen in:

  • cancer  
  • ulcerative colitis

Decreased complement activity may be seen in:

  • hereditary angioedema  
  • bacterial infections (especially Neisseria)  
  • cirrhosis  
  • glomerulonephritis  
  • hepatitis  
  • lupus nephritis  
  • malnutrition  
  • kidney transplant rejection  
  • systemic lupus erythematosus

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

The complement cascade can be started in several ways, especially by antigen-antibody complexes. The end-product of the cascade is the “membrane attack unit” (also called terminal complement component), which creates holes in the membranes of attacking bacteria, thereby killing them.

CH50 and 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 and destroy bacteria.

Some bacteria don’t need specific antibodies to be present for the complement system to be activated. C3, one of the major components of the complement cascade, attaches to and kills these bacteria directly.

Typically, other tests that are more specific for the suspected disease are performed first.

Johns Hopkins patient information

Last revised: December 3, 2012
by Martin A. Harms, M.D.

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