C3 and C4 blood test: what should you know?

C3 and C4 are part of the complement system, which helps identify and respond to unwanted material in the body. Testing them together gives more context than looking at either result alone. Reduced levels can occur when complement is being consumed during some inflammatory or immune processes; increased levels can accompany inflammation. Results can also be affected by the timing of illness and by the laboratory method. They do not identify a condition by themselves. Your clinician may compare them with blood counts, urine findings, kidney tests, or specific antibody tests. See related information on [Chronic Kidney Disease](/conditions/chronic-kidney-disease/) and [Anaemia](/conditions/anaemia/).

The sample is collected from a vein, usually during a routine blood appointment. Check whether fasting is needed because other tests may be collected at the same time. Bring a list of medicines and supplements, and mention recent infections, rashes, joint symptoms, or kidney problems. Wait for the clinician to interpret the pattern rather than reacting to one flagged result. Further testing, repeat blood work, or observation may be appropriate depending on your symptoms.

Arrange medical review for persistent fever, unexplained swelling, new joint inflammation, unusual bruising, or changes in urine. Seek emergency help for trouble breathing, swelling of the lips or tongue, collapse, confusion, or rapidly worsening illness. These symptoms need assessment regardless of whether your complement results are available.

What prompted both tests? Are C3 and C4 low, high, or within the local range? Do the results fit my symptoms? Could another illness or medicine explain the pattern? Which follow-up tests are needed, and when should I repeat the blood test?