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.
A blood sample is needed. This is usually taken through a vein. The procedure is called a venipuncture.
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.
Note: mg/dL = milligrams per deciliter.
Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.
The examples above show the common measurements for results for these tests. Some laboratories use different measurements or may test different specimens.
Increased complement activity may be seen in:
Decreased complement activity may be seen in:
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 associated with having blood drawn are slight but may include:
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.
Complement assay; Complement proteins
Updated by: Ariel D. Teitel, MD, MBA, Clinical Associate Professor of Medicine, NYU Langone Medical Center. Review provided by VeriMed Healthcare Network. Also reviewed by A.D.A.M. Health Solutions, Ebix, Inc., Editorial Team: David Zieve, MD, MHA, Bethanne Black, Stephanie Slon, and Nissi Wang.
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