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Complement component 4 is a blood test that measures the activity of a certain protein that is part of the complement system. 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. This test measures C4.
A blood sample is needed. This is usually taken through a vein. The procedure is called a venipuncture.
No special preparation is needed.
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.
If your child is to have this test performed it may be helpful to explain how the test will feel, and even practice or demonstrate on a doll. The more familiar your child is with what will happen to them, and the purpose for the procedure, the less anxiety he or she will feel.
C3 and C4 are the most commonly measured complement components. Complement activity may be measured to determine how severe a disease is or if treatment is working.
A complement test may be used to monitor patients with an autoimmune disorder. 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 may be normal or higher-than-normal in the blood, but much lower-than-normal in the joint fluid.
Note: mg/dL = milligrams per deciliter
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:
Introduction to the Complement System. In: Adkinson NF Jr, ed. Middleton’s Allergy: Principles and Practice. 7th ed. Philadelphia, Pa: Mosby Elsevier; 2008:chap 6.