Complement Blood Test

What is a complement blood test?

A complement blood test measures the levels and/or activity of a group of proteins that are part of the complement system. This system "complements" or enhances your body's defenses against infections and other diseases. Your immune system makes antibodies, which are proteins to fight disease-causing substances like viruses and bacteria. The proteins in the complement system will also help fight the infection or disease. These proteins work together in a sequence, with one protein activating the next.

There are nine major complement proteins. They are labeled C1 through C9. Complement proteins may be measured individually or together:

• C3 and C4 proteins are the most commonly tested individual complement proteins.
• A CH50 test (sometimes called CH100 or a total complement test) measures the total activity and levels of all the major complement proteins.
• If the CH50 level is not normal, each of the nine complement levels can be measured individually to find out which one is abnormal.

If the test shows that your complement protein levels are not normal or that the proteins aren't working with the immune system as well as they should, it can be a sign of an autoimmune disease or other health condition. If you have an autoimmune disease your immune system attacks the healthy cells of your organs and tissues by mistake.

Other names: complement antigen, compliment activity C3, C4, CH50, CH100, C1 C1q, C2, total complement test

What is it used for?

A complement blood test is most often used to diagnose or monitor autoimmune diseases such as:

• Lupus, a chronic (long-lasting) disease that affects many parts of the body, including the joints, blood vessels, kidneys, and brain
• Rheumatoid arthritis, a condition that causes pain and swelling of the joints, often in the wrist, hands, and feet

It may also be used to help diagnose certain bacterial, viral, or fungal infections.

Why do I need a complement blood test?

Your health care provider may order complement blood test if you have an autoimmune disease such as lupus. The test can show how well the treatment is working.

Your provider may also order this test if you have the symptoms of an autoimmune disease. The symptoms of autoimmune disease can depend on the part of your body that's affected. The symptoms can come and go and may include:

• Infections
• Unexplained inflammation, such as redness, swelling, heat and pain
• Fatigue
• Joint pain or swelling
• Muscle pain

What happens during a complement blood test?

A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

Will I need to do anything to prepare for the test?

You don't need any special preparations for a complement blood test.

Are there any risks to a complement blood test?

There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

What do the results mean?

If your results show decreased levels or activity of complement proteins, it may mean that your body is trying to get rid of foreign substances or attacking its own tissues.

Decreased levels may be a sign of the following conditions:

• Lupus.
• Rheumatoid arthritis.
• Cirrhosis, scarring of the liver.
• Certain types of kidney disease.
• Hereditary angioedema, a rare but serious disorder of the immune system. It can cause swelling of the face and airways.
• Malnutrition.
• A recurring infection (usually bacterial).

If your results show increased levels or activity of complement proteins, it may mean that your body has acute or chronic inflammation.

Increased levels may be a sign of the following conditions:

• Certain types of cancer, such as leukemia or non-Hodgkin lymphoma
• Ulcerative colitis, a condition in which the lining of the large intestine and rectum become inflamed

If you are being treated for lupus or another autoimmune disease, increased levels or activity of complement proteins may mean your treatment is working.

Increased or decreased complement levels can also be a sign of a condition affecting your immune system. If you have questions about your results, talk to your provider. They will consider your symptoms, medical history, and the results of other blood tests to determine if additional tests are needed to make a diagnosis.

Learn more about laboratory tests, reference ranges, and understanding results.