C3 Complement

C3 complement is a blood test that measures the amount of complement component 3, a key protein in your immune system. C3 helps your body recognize and clear germs, damaged cells, and immune complexes. It is produced mainly by the liver and circulates in the bloodstream as part of the complement cascade.

Clinicians often check C3 together with other complement proteins to understand how your immune system is functioning. Levels can change with inflammation, certain infections, autoimmune conditions, and rare inherited complement deficiencies.

C3 levels can help your clinician evaluate inflammation and immune activity. In autoimmune diseases such as lupus or certain kidney conditions, C3 may decrease when the immune system is highly active and consuming complement. Low C3 can also occur with rare genetic deficiencies, which may increase susceptibility to specific infections. Higher C3 levels may be seen with acute inflammation or metabolic conditions.

This test is commonly ordered when you have signs of autoimmune disease, unexplained kidney findings, recurrent infections, or abnormal results on related tests. Tracking C3 over time can help monitor disease activity and response to treatment when used alongside your symptoms, exam findings, and other laboratory tests.

Your C3 result is interpreted in the context of your history, exam, and other labs. A lower result can suggest complement consumption from active immune complex disease, infection, or a hereditary deficiency. A higher result can reflect an acute phase response or other inflammatory or metabolic states.

If results are unexpected, your clinician may repeat the test, check related markers such as C4 or total complement activity, and review medications and liver function. Follow-up can include urine and kidney assessments, autoimmune panels, or referral to a specialist. Discuss any concerns with your care team so results can be interpreted safely and accurately for your situation.