Complement Blood Test
Reviewed
Also called: complement antigen, compliment activity C3, C4, CH50, CH100, C1 qC2 test
A complement blood test measures the levels and/or activity of a group of proteins that are part of the complement system.
Specimen: Blood
Why this test may be ordered
- To diagnose or monitor autoimmune diseases such as lupus and rheumatoid arthritis.
- To help diagnose certain bacterial, viral, or fungal infections.
- To check if treatment for an autoimmune disease is working (e.g., lupus).
Preparation and collection
You don't need any special preparations for a complement blood test.
A health care professional will take a blood sample from a vein in your arm, using a small needle. This usually takes less than five minutes.
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.
Understanding results
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.
- Lower than normal complement levels or activity: 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. Limitation: Increased or decreased complement levels can also be a sign of a condition affecting your immune system. Next step: 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.
- Higher than normal complement levels or activity: If your results show increased levels or activity of complement proteins, it may mean that your body has acute or chronic inflammation. Limitation: If you are being treated for lupus or another autoimmune disease, increased levels or activity of complement proteins may mean your treatment is working. Next step: If you have questions about your results, talk to your provider.
- Results are not diagnostic by themselves; your provider will consider your symptoms, medical history, and other tests.
- The test does not specify which condition you have; further testing may be needed.
What to do next
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.
- Why might I need this test?
- What do the results mean?
- How should I prepare for the test?
- Are there any risks to the test?
Evidence source
Source: U.S. National Library of Medicine (MedlinePlus). Practices and reference ranges may differ in your country.