Definition of Complement:

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

How the test is performed:

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.

How to prepare for the test:

There is no special preparation.

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:

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.