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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, which plays an important role in your immune system. Its job is to help kill disease-causing bacteria and viruses. It also responds to such invaders with inflammation that protects your body from disease.  

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 doctor 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 have this test if your doctor suspects you have an autoimmune disorder, especially lupus. Symptoms of lupus may include:

  • Rash in the shape of a butterfly across your cheeks

  • Mouth ulcers

  • Hair loss

  • Joint pain and swelling

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 health care 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.

Doctors 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 doctor 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?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

The normal range for a complement C3 blood test is 70 to 160 milligrams per deciliter (mg/dL), or 0.7 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 requires a blood sample, which is drawn through a needle from a vein in your arm.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly 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 doctor 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.