Complement Blood Test
Reviewed
Also called: complement antigen, compliment activity C3, C4, CH50, CH100, C1 qC2 test
Measures the levels and/or activity of complement proteins (C1–C9, often C3, C4, or CH50/CH100) to help diagnose or monitor autoimmune diseases like lupus and rheumatoid arthritis, and to help diagnose certain infections.
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
No special preparation is needed.
Blood sample from a vein in the arm
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
A complement blood test measures the levels and/or activity of complement proteins. The results can show decreased or increased levels/activity.
- Lower than normal complement levels or activity: Lower than normal complement levels or activity Limitation: Decreased levels or activity of complement proteins may mean your body is trying to get rid of foreign substances or attacking its own tissues. This can be a sign of conditions such as: lupus, rheumatoid arthritis, cirrhosis, certain kidney diseases, hereditary angioedema, malnutrition, or recurring infections. Next step: Your provider will correlate these findings with your symptoms and other tests.
- Higher than normal complement levels or activity: Higher than normal complement levels or activity Limitation: Increased levels or activity of complement proteins may indicate acute or chronic inflammation. This can be a sign of conditions such as: certain cancers (e.g., leukemia, non-Hodgkin lymphoma), ulcerative colitis, or if you are being treated for lupus or another autoimmune disease, it may mean your treatment is working. Next step: Your provider will interpret these results along with your clinical picture and other lab tests.
- 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.
- 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?