Complement Blood Test

What is a complement blood test?

A complement blood test measures the amount or activity of complement proteins in the blood. Complement proteins are part of the complement system. This system is made up of a group of proteins that work with the immune system to identify and fight disease-causing substances like viruses and bacteria.

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) measures the amount and activity of all the major complement proteins.

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 serious health problem.

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

What is it used for?

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

• Lupus, a chronic disease affecting multiple parts of the body, including the joints, blood vessels, kidneys, and brain
• Rheumatoid arthritis, a condition that causes pain and swelling of the joints, mostly in the 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?

You may need a complement blood test if you have symptoms of an autoimmune disorder, especially lupus. Symptoms of lupus include:

• A butterfly-shaped rash across your nose and cheeks
• Fatigue
• Mouth sores
• Hair loss
• Sensitivity to sunlight
• Swollen lymph nodes
• Chest pain when breathing deeply
• Joint pain
• Fever

You may also need this test if you are being treated for lupus or other autoimmune disorder. The test can show how well the treatment is working.

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 lower than normal amounts or decreased activity of complement proteins, it may mean you have one of the following conditions:

• Lupus
• Rheumatoid arthritis
• Cirrhosis
• 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 recurrent infection (usually bacterial)

If your results show higher than normal amounts or increased activity of complement proteins, it may mean you have one 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 amounts or activity of complement proteins may mean your treatment is working.

If you have questions about your results, talk to your health care provider.

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