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 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.

Complement activity (CH50, CH100, terminal complement component, or individual complement proteins) is measured to determine if complement is involved in the development of a disease. Complement activity is also measured to monitor how severe a disease is, or if treatment is working.

For example, patients with active systemic lupus erythematosus may have lower-than-normal levels of C3 and C4, and these component levels may be monitored as a rough index of disease activity.

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

• Cancer
• Ulcerative colitis

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

• 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

Complement is a group of proteins in the blood that facilitate immunological and inflammatory responses. The "complement cascade" is a series of reactions that take place in the blood.

There are 9 major components of complement, labeled C1 through C9. The 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 membrane, or surface, of attacking bacteria, thereby killing them.

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 and destroy bacteria.

Some bacteria don't need antibodies to set off the complement cascade. When they are exposed to fresh blood serum, complement is activated by an "alternative pathway". C3, one of the major components of the complement cascade, binds (attaches) to these bacteria directly to eliminate them.

Bound C3, or some modified form of C3, forms a stable unit with other members of the complement cascade to form the membrane attack unit.

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