Complement is a blood test that measures the activity of certain proteins in the liquid portion of your blood.

The complement system is a group of proteins that move freely through your bloodstream. The proteins work with your immune system and play a role in the development of inflammation.

There are nine major complement proteins. They are labeled C1 through C9.

See also:

• Complement component 3 (C3)
• Complement component 4 (C4)
• CH50
• CH100

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

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

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

There is no special preparation.

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.

Total complement activity (CH50, CH100) looks at the overall activity of the complement system. Typically, other tests that are more specific for the suspected disease are performed first. C3 and C4 are the most commonly measured complement components.

A complement test may be used to monitor patients with an autoimmune disorder and to see if treatment for their condition is working. For example, patients with active lupus erythematosus may have lower-than-normal levels of the complement proteins C3 and C4.

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-normal 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 blood complement level: 41 to 90 hemolytic units
• C1 level: 16 to 33 mg/dL
• C3 levels:
  • Males: 88 to 252 mg/dL
  • Females: 88 to 206 mg/dL
• C4 levels:
  • Males: 12 to 72 mg/dL
  • Females: 13 to 75 mg/dL

Note: mg/dL = milligrams per deciliter.

Increased complement activity may be seen in:

• Cancer
• Certain infections
• Ulcerative colitis

Decreased complement activity may be seen in:

• Cirrhosis
• Glomerulonephritis
• Hereditary angioedema
• Hepatitis
• Kidney transplant rejection
• Lupus nephritis
• Malnutrition
• Systemic lupus erythematosus

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.

Other risks:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

The "complement cascade" is a series of reactions that take place in the blood. The cascade activates the complement proteins. The result is an attack unit that creates holes in the membrane of bacteria, killing them.