C3 blood test: what should you know?

Complement proteins help the immune system respond to infection and clear damaged cells. C3 may be checked when a clinician is investigating inflammation, repeated infections, or an immune-related illness. A result outside the laboratory range is a clue, not a diagnosis. Lower C3 can occur when complement is being used during an inflammatory process, while a higher result may accompany general inflammation. The meaning depends on your history and on tests such as C4, blood counts, urine checks, or kidney assessment. Related pages include [Chronic Kidney Disease](/conditions/chronic-kidney-disease/) and [Coeliac Disease](/conditions/coeliac-disease/).

A blood sample is taken from a vein in your arm. Ask whether you need to fast; preparation depends on the other tests ordered at the same visit. Tell the clinician about medicines, supplements, recent infections, and any established immune condition. Do not change treatment because of a result on its own. Your clinician may repeat the test or compare it with C4 and other findings before deciding whether further assessment is needed.

The test itself does not usually require urgent action. Seek prompt care for severe breathing difficulty, facial swelling, confusion, a rapidly spreading rash, or signs of serious infection such as marked weakness with a high temperature. If you have swelling, dark urine, much less urine, or new severe pain, contact your clinician because these symptoms may need assessment alongside the blood result.

Why was C3 requested for me? Was C4 tested as well? Is the result low, high, or within this laboratory's range? Could a recent infection or medicine affect it? Do I need urine or kidney tests? What symptoms should prompt a review?