C3 Complement Blood Test

A C3 complement blood test gives your healthcare provider information about your immune system. It shows how parts of your immune system are responding to harmful substances. This test can help your healthcare provider diagnose autoimmune disorders (like lupus), as well as other conditions. It also helps your healthcare provider monitor treatment for autoimmune disease.


What is a C3 complement blood test?

A C3 complement blood test measures the levels of a specific type of protein in your blood (C3 proteins). As part of your immune system, C3 proteins destroy microbes (germs) that can make you sick. Sometimes, C3 proteins launch an attack against healthy cells by mistake.

The results of this blood test give your healthcare provider information about how your immune system is working. By measuring the amount of C3 proteins, your healthcare provider can diagnose infection, disease, immune deficiency and other health conditions, like autoimmune disorders. This test also helps your healthcare provider monitor treatment for autoimmune disease.


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What is the complement system?

C3 proteins are part of the complement system, which is part of your immune system. The complement system includes more than 30 different proteins. They work together in a sequence to protect you from infection and illness.

When your immune system identifies a harmful substance or recognizes signs of an infection, it activates a protein in the complement system to launch an attack. That protein activates another protein, which activates another and another. Healthcare providers call this process a complement cascade. The proteins, like C3, are consumed in this process.

Proteins in the complement system have many jobs, including:

  • Helping your immune system create antibodies (proteins that respond to specific germs) and making antibodies more effective.
  • Helping your body get rid of dead cells and other waste.
  • Neutralizing viruses (making them less harmful).
  • Recognizing and killing viruses, bacteria and other germs.
  • Working with other parts of the immune system to help them destroy bacteria.

Sometimes, complement proteins like C3 mistakenly attack tissues that aren’t dangerous. Autoimmune disorders cause the body to believe its own tissues are foreign and harmful. The complement system responds by making antibodies to fight its own tissues.

When is a C3 complement blood test performed?

Your healthcare provider may order this test if you have:

  • Bacterial infections that don’t go away or keep coming back.
  • Edema (swelling) or inflammation.
  • Symptoms of an autoimmune disorder, like lupus.

Your healthcare provider might also recommend a C3 complement blood test to monitor:


Test Details

How does a C3 complement test work?

Your healthcare provider removes a sample of your blood through a vein in your arm. They send the sample to a lab to check the levels of C3 proteins in your blood.

The lab may also look at the levels of other proteins (such as C4 proteins) and see how they compare to the amount of C3 proteins. Your healthcare provider will contact you when the results are ready and explain what they mean.

What should I expect during a C3 complement test?

You don’t need to do anything to prepare for a C3 complement blood test. After cleaning your arm, your healthcare provider inserts a needle. You may feel a sting or pinch from the needle, but it should not hurt. Your healthcare provider collects some of your blood in a tube.


What should I expect after a C3 complement test?

Your healthcare provider will tape some gauze to your arm where they inserted the needle. Your arm may be sore for a little while, and you may get a small bruise.

What are the benefits of a C3 complement test?

Using just a small amount of blood, a C3 complement test gives your healthcare provider important information about your health. It helps them check for disease and disorders. It also helps your healthcare provider monitor treatments.

What are the risks of a C3 complement test?

Blood tests are very safe. Your healthcare provider only removes a small amount of blood, and your body replenishes the supply. Some people may feel like they are going to faint, or they may have some temporary dizziness after their blood is taken.

Results and Follow-Up

When should I know the C3 complement blood test results?

Results from this test are usually ready in a few days. But it can also take up to a week for them to be ready. Your healthcare provider will contact you to discuss the results.

What do low levels of C3 mean?

The normal range of C3 proteins varies depending on age, gender and overall health. Usually, the normal range is between 80 and 178 milligrams per deciliter (mg/dl). Ask your healthcare provider what the normal range is for you and your laboratory.

Lower-than-normal levels of C3 proteins can be a sign of:

What do high levels of C3 mean?

Higher-than-normal levels of C3 proteins can indicate that you’ve just had an infection and your immune system is responding to it. If you’re getting treatment for lupus or another autoimmune disease, higher levels of C3 proteins usually mean that treatment is working.

Increased C3 levels may also be a sign of:

When should I call my doctor?

Call your healthcare provider if you have any questions about the results of your C3 complement blood test. If you have bleeding or pain where the needle entered your arm, talk to your healthcare provider.

A note from Cleveland Clinic

A C3 complement blood test gives your healthcare provider important information about how your immune system is working. The blood test is usually quick and painless. The results of this test can show signs of an autoimmune disorder, infection or other condition. But even if your levels are higher or lower than the normal range, it doesn’t necessarily mean you have one of these conditions. Your healthcare provider will discuss the results with you and may recommend follow-up tests.

Medically Reviewed

Last reviewed by a Cleveland Clinic medical professional on 11/24/2021.

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