What Is the C3 Complement Test?

What to Expect When Undergoing This Test

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A C3 complement test is a blood test that can tell your healthcare provider how your immune system is functioning. The test measures the amount of a protein called C3 that enhances the action of antibodies and other disease-fighting immune cells.

C3 is part of the complementary immune system. The complementary immune system is part of the larger innate immune system that serves as the body's frontline defense against disease, attacking anything that the body considers abnormal.

The C3 complement test can provide valuable information about the status of your immune system based on whether C3 values are high or low. It can reveal if you have a specific immune deficiency that places you at risk of infection or an autoimmune disease that causes the body to attack its own cells or tissues.

C3 proteins can also rise or fall in response to certain infections and diseases, including liver and kidney disease. The test can also assess whether a treatment for an autoimmune disorder is working.

This article explains the purpose of a C3 complement test and why your healthcare provider may order one. It also describes what to expect if you are scheduled to undergo the test, including what the test results mean.

Serum separator blood draw tube yellow top

Oleksandr Boliukh / iStock / Getty Images

Purpose of the Test

A C3 complement test may be ordered if you have signs or symptoms of certain diseases or infections. Among the reasons your healthcare provider may order the test:

The C3 complement test cannot diagnose any medical condition but can be used in tandem with other tests to narrow or confirm the possible causes.

For example, if C3 protein levels are low, it could mean that you have an autoimmune disease, a genetic disorder like C3 deficiency, or chronic kidney or liver disease.

On the flip side, C3 protein levels may be high if you have an inflammatory condition like ulcerative colitis, sarcoidosis, or certain types of cancer. They may also rise if you are responding to autoimmune therapies.

What to Expect

The C3 complement test involves a simple blood draw. No preparation is needed. The test may be performed at a hospital, clinic, lab, or your healthcare provider's office.

There is nothing that would prevent someone from having a C3 complement test. Furthermore, there are very few risks associated with the procedure.

This test generally costs between $35 to $60. Most health insurance plans will cover all or part of the cost when used for a medical purpose. Keep in mind that if other blood tests are ordered, the out-of-pocket costs may be higher.

Before the Test

You do not need to bring anything specific to the test other than your ID and insurance card. Wear either a short-sleeved top or a shirt with sleeves you can roll up.

Waiting times can vary depending on the facility. The blood draw itself only takes a couple of minutes. You can also call the lab or clinic in advance to ask for an estimated wait time.

There are no food, drink, or medication restrictions involved with a C3 complement test. If you are having other tests done at the same time, ask your healthcare provider if they require fasting or medication adjustments.

Handy Tip

If you have tiny veins, it often helps to drink plenty of water in advance of the test to "plump up" the veins in your arms.

During the Test

After checking in with the receptionist and signing a consent form, you will be led to a room to have your blood drawn. The blood sample will be obtained by a nurse or a technician known as a phlebotomist specially trained in blood draws.

Once seated, will be asked to select the arm you want your blood drawn from. Thereafter, the steps are pretty straightforward:

  1. A tight elastic band known as a tourniquet is placed around your forearm.
  2. The nurse or phlebotomist will touch around the crook of your arm to select the best vein.
  3. The skin will be cleansed with an alcohol swab or other cleaning method.
  4. A needle connected to a special tube or syringe will be inserted into the vein to collect blood. You may feel a little prick but generally not any undue pain.
  5. Once enough blood is obtained, the tourniquet is released and the needle is removed.
  6. Gentle pressure is applied with a piece of gauze to prevent further bleeding.
  7. The puncture wound is bandaged.

After the Test

After the test, you may be asked to sit for a minute or two to see whether you feel dizzy or faint. If you do, let the nurse or phlebotomist know. Otherwise, you are free to go and can drive yourself home.

There may be bruising and pain at the puncture site which usually resolve on their own without treatment. In some cases, a pocket of blood called a hematoma may form, which tends to look worse than it actually is.

To help ease pain and bruising, you can apply an ice pack for 15 to 20 minutes several times a day for the first 24 hours. For the next 24 hours, use a warm, moist compress for 15 to 20 minutes several times a day to improve blood flow and reduce inflammation.

Infections are extremely rare. Even so, call your healthcare provider immediately if you have signs of an infection, including fever, chills, a pus-like discharge, and increasing pain, redness, or swelling at the puncture site.

Interpreting the Results 

The C3 complement test is measured in milligram per deciliter (mg/dL). The results are based on a reference range of values. Anything above the reference range is considered high and anything below the reference range is considered low.

High values are sometimes marked on the lab report as "high" or "H." Similarly, low values may be marked as "low" or "L."

The normal reference range for a C3 complement test is between 88 and 201 mg/dL.

There are many different reasons why C3 protein values may be low or high.

Low C3 values can occur with:

High C3 values can occur with:

If C3 values are abnormally high or low, additional tests or procedures may be ordered to narrow or confirm the suspected cause.


A C3 complement test is used to measure a protein called C3 which is part of the body's immune response. High or low values may indicate medical problems ranging from autoimmune diseases and immune disorders to infections and chronic kidney or liver disease.

The test involves a simple blood draw, the results of which are interpreted with other tests to narrow the possible causes. If an abnormal result is returned, additional tests may be ordered.

A Word From Verywell

It is important to remember that a C3 complement test cannot diagnose any disease or condition on its own. If you receive an abnormally high or low value, don't panic or jump to any conclusions. Simply ask your healthcare provider what the results may mean and why other tests may be needed.

By understanding what the findings mean (and don't mean), you can make more informed judgments about your health.

Frequently Asked Questions

  • What is the function of C3?

    C3 enhances the action of antibodies and other immune cells that target and kill disease-causing organisms, but it doesn't work alone. C3 is one of more than 30 different proteins that make up the complementary immune system.

  • What causes low C3 complement levels?

    Many different diseases and medical conditions can cause C3 levels to drop, including autoimmune diseases, liver disease, kidney disease, and inherited disorders like hemolytic anemia and C3 deficiency.

2 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Nilsson B, Ekdahl KN. Complement diagnostics: concepts, indications, and practical guidelines. Clin Dev Immunol. 2012;2012:962702. doi:10.1155/2012/962702

  2. MedlinePlus. Complement component C3.

By Heidi Moawad, MD
Heidi Moawad is a neurologist and expert in the field of brain health and neurological disorders. Dr. Moawad regularly writes and edits health and career content for medical books and publications.