What Is Complement C3 Test?

Medically Reviewed by Carol DerSarkissian, MD on February 12, 2024
3 min read

Complement component 3, often called C3, is an essential protein of the immune system. This protein plays a vital role in the body’s innate immune system, providing: the first line of defense against diseases. The complement C3 blood test measures the activity of this protein in your blood. 

Your doctor will order this test to diagnose and treat autoimmune diseases. The test is carried out to keep track of the progress of the disease, as well.

The complement system consists of 60 proteins that are present in the blood's plasma.

These proteins help the immune system protect your body from various infections. They also help in removing foreign material and dead cells.

When people inherit a deficiency of complement proteins, they are more likely to develop certain disorders and infections because their body is not efficiently killing bacteria before they cause infections. 

A health care professional will draw blood from a vein on the back of the hand or the inner side of the elbow.

Here is the testing procedure:

  • An elastic band is wrapped around the arm.
  • The vein swells with blood.
  • The area is cleaned with antiseptic.
  • The needle is inserted into the vein.
  • Blood is collected in a tube.
  • The puncture site is covered with a bandage.

Your doctor will order a complement C3 test if they think you have an autoimmune disorder. An autoimmune disease is a malfunction of the body's immune system. It means that your body attacks its own tissues and healthy cells.

Lupus is an example of an autoimmune disease that might be detected with a complement C3 test.‌

Autoimmune diseases have various symptoms, ranging from mild to serious and affecting specific organs or the whole body. 

‌A complement C3 test will help your doctors track the progress and the condition of the disease.

Your doctor may also suggest the complement C3 test if you get repeated episodes of bacterial infections. This is done for the following conditions:

The complement C3 test results vary depending on the person's health history, age, and sex. The normal range for the complement C3 test is 80 to 160 milligrams per deciliter or 0.8 to 1.6 grams per liter.

The complement levels in the blood rise after an injury or injection, but when the complement system responds to a disease, your complement levels go down.

Low Complement Levels

If your complement components are normal but your C3 complement levels are low, you have an inherited component C3 deficiency. If a person inherits component deficiency, they are more likely to develop certain autoimmune diseases.

However, if your test results show that you have low levels of other complement components, this results from acquired disease. For example, lower levels of C3 and C4 are a sign of lupus. Decreased complement activity may also be due to the following conditions:

Elevated C3 Complement Levels 

High C3 complement levels indicate inflammation in the body. The inflammation could be due to ulcerative colitis or other acute diseases like sarcoma, leukemia, or Hodgkin's Lymphoma.

These are all different types of cancer. However, increased levels of C3 are not the main indication of cancer.

Elevated C3 complement levels are also observed in people with diabetes. Therefore, other tests are necessary to confirm the diagnosis.

If you get treatment for a disease such as lupus, high levels of C3 indicate that the treatment works. 

Blood tests don't usually pose a threat, but some risks are associated with withdrawing blood. These include the following:

  • Lightheadedness
  • Hematoma (blood clot)
  • Infection at the injection site 
  • Excessive bleeding

If you experience any of these, call your doctor right away.