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.

The blood is tested in a laboratory.

• infant test or procedure preparation (birth to 1 year)
• toddler test or procedure preparation (1 to 3 years)
• preschooler test or procedure preparation (3 to 6 years)
• schoolage test or procedure preparation (6 to 12 years)
• adolescent test or procedure preparation (12 to 18 years)

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 lower-than-normal levels of C3 and C4, and the levels of these complement components may be monitored to gauge disease activity.

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-nomral in the joint fluid.

Patients with gram negative septicemia and shock often have very low C3 and components of what's known as the alternative pathway. C3 is often also low in fungal infections and some parasitic infections such as malaria.

• Total hemolytic complement level: 41 to 90 hemolytic units
• C1 esterase inhibitor level: 16 to 33 mg/dl (160 to 330 mg/L)
• C3 levels:
  • males: 88 to 252 mg/dl (880 to 2520 mg/L)
  • females: 88 to 206 mg/dl (880 to 2060 mg/L)
• C4 levels:
  • 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
• 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 involved in immunological and inflammatory responses. There are 9 major components of complement labeled C1 through C9.

The "complement cascade" is a series of reactions that take place in the blood. The cascade can be started in several ways, especially by antigen-antibody complexes. The end product of the cascade is called the "membrane attack unit" (also called the terminal complement component). This attack unit creates holes in the membranes of attacking bacteria, killing them.

CH50 or CH100 are tests for the activity of the complement system. Typically, other tests that are more specific for the suspected disease are performed first.

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.