What is this test?
C4 is a protein in the complement system, part of the immune response that supports antibodies and inflammation.
Why might it be ordered?
- To assess activity in certain autoimmune diseases, especially when tracked over time.
- When a rare hereditary or acquired deficiency of complement components is suspected.
What sample is needed?
A healthcare professional collects a venous blood sample. Depending on the method, the laboratory uses serum or plasma.
How should you prepare?
Tell the laboratory about all medicines, hormone treatments, vitamins and supplements you take. Do not stop or change treatment unless your clinician tells you to. Follow any instructions about fasting and the time of collection.
How should you understand the result?
Low C4 may occur when complement is being consumed or produced in lower amounts; a high level may occur with inflammation. C3 and C4 are usually interpreted together. Your result is interpreted using the reference interval on the laboratory report and in the context of your symptoms, medical history and other test findings.
Sources and editorial status
This patient-facing text is an editorial draft. It should be approved by a laboratory physician before official publication.