Complement Component C3
Complement component C3 is a protein in the complement system, part of your immune defenses. This test measures the amount of C3 in your blood.
Because the complement system helps clear infections and immune complexes, and its levels can shift in autoimmune and inflammatory conditions. C3 is often measured alongside C4 to help assess conditions such as lupus and certain types of kidney inflammation. Tracking C3 over time can help a clinician gauge disease activity.
A higher-than-normal C3 is usually less clinically important and can simply reflect general inflammation, since complement proteins can rise as part of the body's acute-phase response. It is typically the low values, not the high ones, that draw the most attention in autoimmune evaluation.
A low C3 can indicate that complement is being consumed during active autoimmune disease, such as a lupus flare or some forms of kidney inflammation, and it may prompt closer monitoring. Low levels can also occur with certain infections or inherited complement deficiencies, so your clinician interprets it in context.
A result within your lab's reference range is typical. Because low levels can signal complement consumption in active autoimmune disease, any low or changing result is best interpreted by your clinician.
Sources
Educational context only, not medical advice or a diagnosis. Reference ranges vary by lab, age and sex; always discuss your results with a clinician.