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Your complement system consists of a group of important proteins in your bloodstream. These proteins help your immune system fight off harmful invaders, such as viruses and bacteria, and other foreign substances.
There are nine major complement proteins, which are labeled C1 through C9. Complement C4 plays an important role in eliminating certain infections. The complement component 4 (C4) test is a simple blood test that measures the amount of complement C4 circulating in your bloodstream. A low level of C4 is associated with autoimmune disorders and collagen vascular diseases, such as lupus and rheumatoid arthritis.
The complement C4 test only requires basic preparation and carries few risks. The sample of your blood will be sent to a laboratory for analysis. The results will then be sent to your doctor.
The complement C4 test is one of the most frequently used complement component tests. Your doctor may order a complement C4 test if you’re experiencing symptoms that indicate an autoimmune disease. These symptoms may include:
The complement C4 test is also sometimes used to monitor protein levels in people who have already been diagnosed with an autoimmune disease. Complement tests can provide doctors with valuable information about the effectiveness of current treatment for an autoimmune condition.
No special preparation is needed before undergoing a complement C4 test. However, you should tell your doctor about all the medications, vitamins, herbs, and other supplements you’re taking. Aside from prescription medications, you should also make sure to mention any over-the-counter medicines and illegal drugs you’re taking.
A complement C4 test involves taking a small sample of blood. A blood draw involves the following steps:
Your arm may be sore in the area where the needle entered your skin. You may also have some mild bruising or throbbing after the blood draw.
Most people don’t experience any serious side effects. However, rare complications from a blood test include:
Contact your doctor right away if you experience any of these symptoms.
The normal complement C4 measurement for men is typically between 12 and 72 milligrams of complement per deciliter of blood. The normal range for women is usually between 13 and 75 milligrams of complement per deciliter of blood. However, normal values may vary among different testing laboratories, so it’s important to talk to your doctor about what the results mean for you.
Higher-than-normal complement C4 values may be a sign of cancer or ulcerative colitis.
Lower-than-normal complement C4 values could indicate:
Complement component activity varies throughout the body. People with rheumatoid arthritis, for example, may have high complement levels in their blood but low complement levels in their joint fluid.
Your doctor will recommend follow-up testing or treatment based on your results.
Written by: Ann Pietrangelo and Erica Cirino
Medically reviewed on: Jan 20, 2016: Steve Kim, MD
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