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Biochemistry
Complement is a system of around sixty proteins that work alongside your immune system, tagging invaders for destruction and clearing away dead cells and debris. C3 is one of the two complement proteins most often measured directly, and it sits at a point in the complement cascade where several activation pathways converge, part of why it falls in a wide range of unrelated conditions rather than one specific disease. It's ordered most often to track lupus, since C3 typically drops during an active flare as the protein gets used up faster than the liver can replace it, and it's also checked when unexplained kidney inflammation or an unusually severe infection raises the question of complement involvement.
The test reports C3 concentration in a blood sample, in milligrams per decilitre; one commonly cited reference range lists normal C3 as roughly 88 to 201 mg/dL, though this varies by lab. A low result suggests the protein is being consumed faster than it's replaced, as happens during active complement activation, while a high result is less specific and has been linked to conditions including cancer, some infections, and ulcerative colitis. It's typically ordered together with C4, since the pattern across both proteins carries more information than either alone.
C3 sits near the centre of the complement cascade, the point where multiple different triggers, antibodies binding to a target, certain bacterial surfaces recognised directly, or other immune signals, all funnel into a shared next step. That central position is part of why a low C3 can show up in such a wide range of situations, since almost any strong trigger of the complement system passes through it.
In active systemic lupus erythematosus, immune complexes trigger ongoing complement activation, using up C3 faster than the liver can produce more. Research following lupus patients has found C3 and C4 levels move in the opposite direction to disease activity, falling as symptoms like joint pain and rash worsen and recovering as the disease settles under treatment, which is why the test is used mainly for monitoring an already-diagnosed case rather than for diagnosing a new one.
Beyond lupus, low C3 turns up in certain kidney diseases where the kidney's own filtering structures become a target of complement activity, in some severe bacterial or fungal infections where the immune system is fighting hard, in some forms of liver disease, and, rarely, in an inherited complement deficiency present from birth. Because the list is broad, a low C3 result is a prompt for more specific follow-up testing, not a diagnosis by itself.
Active lupus triggers ongoing complement activation, which uses up C3 faster than the liver can replace it, so levels fall as disease activity rises and tend to recover as it settles.
It's typically ordered together with C4, since the pattern across both proteins gives more information about complement activity than either result alone.
It can also point toward certain kidney diseases, severe infections, some liver conditions, or, rarely, an inherited complement deficiency, so further tests are usually needed to narrow it down.
Yes. Some severe bacterial or fungal infections can lower C3 as the complement system is actively engaged in fighting them.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
It's less specifically linked to a single condition, but has been associated with cancer, certain infections, and ulcerative colitis in published findings.
Yes. A rising C3 level generally tracks with lupus activity settling down under treatment, which is why it's used for ongoing monitoring, not just at diagnosis.