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Complement C3 (Blood)

Does this test have other names?

Complement component 3, C3

What is this test?

This test measures the amount of C3 proteins in your blood. These proteins are part of your complement system, an important part of your immune system that helps kill disease-causing bacteria and viruses.

Complement component C3 is the most important and abundant protein in the complement system. It is placed on microbes to destroy them. By measuring complement C3 levels, especially in how they compare with other parts of the complement system, your healthcare provider can diagnose and monitor treatment of certain diseases. One of the diseases that commonly involves abnormal C3 is systemic lupus erythematosus, or lupus, an autoimmune disorder.

Why do I need this test?

You may need this test if your healthcare provider suspects you have an autoimmune disorder, especially lupus. Symptoms of lupus may include:

  • Rash in the shape of a butterfly across your cheeks

  • Mouth or nose ulcers

  • Hair loss

  • Joint pain and swelling

  • Extreme tiredness

  • Swelling around your eyes, and in your hands and feet

  • Pain in your chest when breathing deeply

You may also have the test if you get repeated bacterial infections. And if you have already been diagnosed with an autoimmune disease, you may have this test to monitor its progress.

What other tests might I have along with this test?

Your healthcare provider may also order a total complement activity test, or CH50. This test measures all nine components of the complement system, from C1 to C9.

Healthcare providers often order a complement C4 test along with a complement C3 test. In certain diseases, both components are low, but in others only one component is low. In lupus, both C3 and C4 levels are usually low.

If your healthcare provider suspects lupus, you may have a number of other blood tests to see how your immune system is functioning. These may include:

  • Tests to measure antibodies in your blood

  • Sedimentation rate, or ESR, and C-reactive protein tests, which measure inflammation

  • Panels of tests to determine whether your kidneys, liver, and muscles are involved 

What do my test results mean?

Test results may vary depending on your age, gender, health history, the method used for the test, and other things. Your test results may not mean you have a problem. Ask your healthcare provider what your test results mean for you. 

The normal range for a complement C3 blood test is 80 to 160 milligrams per deciliter (mg/dL), or 0.8 to 1.6 grams per liter (g/L).

Your complement levels will often shoot up dramatically just after an infection or injury. When your complement system is activated in response to ongoing disease such as lupus, levels usually go down.

You can inherit a deficiency in your complement C3, but it's much more common to acquire a deficiency. If only your C3 complement level is low and all other complement components are normal, it's usually because of an inherited component deficiency. This makes it more likely that you will develop certain autoimmune disorders.

More often, you will have lowered levels of several complement components at once. This is the result of an acquired disease. If your C3 and C4 levels are reduced, this may be a sign that you have lupus. Usually your total complement level is also slightly lower in this situation. Low C3 and C4 levels may also be a sign of alcoholic liver disease, but this is less common.

Other conditions sometimes linked to low C3 levels include:

  • C3 deficiency, a condition characterized by recurrent bacterial infections

  • Different forms of kidney disease

If you are being treated for a disease like lupus and your complement C3 and C4 levels go up, it is usually a sign that your treatment is working.

How is this test done?

The test is done with a blood sample. A needle is used to draw blood from a vein in your arm or hand. 

Does this test pose any risks?

Having a blood test with a needle carries some risks. These include bleeding, infection, bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may feel a slight sting or pain. Afterward, the site may be sore. 

What might affect my test results?

If the blood sample is mishandled, your C3 levels may be falsely low. If the C3 test is done as part of a total complement activity test, the test should be repeated if low levels are found.

How do I get ready for this test?

You don't need to prepare for this test. Be sure your healthcare provider knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.

Online Medical Reviewer: Haldeman-Englert, Chad, MD
Online Medical Reviewer: Walton-Ziegler, Olivia, MS, PA-C
Date Last Reviewed: 9/1/2017
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