C4 level

Definition:

C4 level is a test that measures the concentration of a component of the complement system in the blood.

Alternative Names:

How the test is performed:

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

The sample is then sent to the laboratory where the quantity of C4 is measured.

How to prepare for the test:

There are no special preparations.

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.

If your child is to have this test performed it may be helpful to explain how the test will feel, and even practice or demonstrate on a doll. The more familiar your child is with what will happen to them, and the purpose for the procedure, the less anxiety he or she will feel.

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.

Normal Values:

Males: 12 to 72 mg/dl (120 to 720 mg/L)
Females: 13 to 75 mg/dl (130 to 750 mg/L)

Note: mg/dl = milligrams per deciliter

What abnormal results mean:

Increased complement activity may be seen in:

Decreased complement activity may be seen in:

Hereditary angioedema
Bacterial infections (especially Neisseria)
Cirrhosis
Glomerulonephritis
Hepatitis
Lupus nephritis
Malnutrition
Renal 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:

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: 10/6/2001
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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