COVID-19 (Coronavirus) Information
Your health and safety are our top priorities. Learn more about our COVID-19 evaluation and testing and our commitment to providing great care while maintaining the safest environment possible.
Complement C3 is a blood test that measures the activity of a certain protein.
This protein is part of the complement system. The complement system is a group of nearly 60 proteins that are in blood plasma or on the surface of some cells. The proteins work with your immune system and play a role to protect the body from infections, and to remove dead cells and foreign material. Rarely, people may inherit deficiency of some complement proteins. These people are prone to certain infections or autoimmune disorders.
There are nine major complement proteins. They are labeled C1 through C9. This article describes the test that measures C3.
Blood is drawn from a vein. Most often, a vein from the inside of the elbow or the back of the hand is used.
The procedure is as follows:
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 needed.
When the needle is inserted to draw blood, some people feel moderate pain. Others may feel only a prick or stinging sensation. Afterward, there may be some throbbing.
C3 and C4 are the most commonly measured complement components.
A complement test may be used to monitor people with an autoimmune disorder. It is done to see if treatment for their condition is working. When the complement system is turned on during inflammation, levels of complement proteins may go down. For example, people 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 people with rheumatoid arthritis, complement activity in the blood may be normal or higher-than-normal, but much lower-than-normal in the joint fluid.
The test may also be done for the following conditions:
The normal range is 88 to 201 milligrams per deciliter (mg/dL) (0.88 to 2.01 g/L).
Note: Normal value ranges may vary slightly among different laboratories. Talk to your provider 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:
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. C3 attaches to bacteria and kills them directly.
Chernecky CC, Berger BJ. C3 complement (beta-1c-globulin) - serum. In: Chernecky CC, Berger BJ, eds. Laboratory Tests and Diagnostic Procedures. 6th ed. St Louis, MO: Elsevier Saunders; 2013:267-268.
Holers VM. Complement and its receptors: new insights into human disease. Annu Rev Immunol. 2014;32:433-459. PMID: 24499275 www.ncbi.nlm.nih.gov/pubmed/24499275.
Massey HD, McPherson RA, Huber SA, Jenny NS. Mediators of inflammation: complement. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 23rd ed. St Louis, MO: Elsevier; 2017:chap 47.
Merle NS, Church SE, Fremeaux-Bacchi V, Roumenina LT. Complement system part I - molecular mechanisms of activation and regulation. Front Immunol. 2015;6:262. PMID: 26082779 www.ncbi.nlm.nih.gov/pubmed/26082779.
Merle NS, Noe R, Halbwachs-Mecarelli L, Fremeaux-Bacchi V, Roumenina LT. Complement system part II: role in immunity. Front Immunol. 2015;6:257. PMID: 26074922 www.ncbi.nlm.nih.gov/pubmed/26074922.
Morgan BP, Harris CL. Complement, a target for therapy in inflammatory and degenerative diseases. Nat Rev Drug Discov. 2015;14(12):857-877. PMID: 26493766 www.ncbi.nlm.nih.gov/pubmed/26493766.BACK TO TOP
Review Date: 1/10/2019
Reviewed By: Gordon A. Starkebaum, MD, MACR, ABIM Board Certified in Rheumatology, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.
The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. © 1997- 2021 A.D.A.M., a business unit of Ebix, Inc. Any duplication or distribution of the information contained herein is strictly prohibited.