Complement C3
A protein of the immune system's complement cascade. In autoimmune disease its useful signal is a LOW level — the proteins get used up when immune complexes form — so a falling C3 often tracks active lupus, especially in the kidneys.
What it measures
C3 is a central protein of the complement system, a cascade of proteins that helps the immune system attack microbes and clear debris. In everyday illness it behaves as an acute-phase reactant and can rise with inflammation. But its most useful clinical role is the opposite: in certain autoimmune diseases, immune complexes activate and 'consume' complement faster than the body replaces it, so the level falls. A low C3 (usually checked with C4) is therefore a marker of active immune-complex disease — most importantly systemic lupus, where a falling C3 with a rising anti-dsDNA often signals a flare, particularly lupus nephritis. It is also low in some kidney diseases (certain types of glomerulonephritis) and in rare inherited complement deficiencies that predispose to infection.
What a high value can mean
- Inflammation / infection — C3 is an acute-phase protein and rises non-specifically.
- Generally not a clinically worrying direction — a high C3 is rarely the point of the test.
What a low value can mean
- Active lupus (SLE), especially lupus nephritis — complement consumed by immune complexes; a falling C3 tracks a flare.
- Other immune-complex diseases — certain types of glomerulonephritis, cryoglobulinaemia.
- Inherited complement deficiency — rare; predisposes to recurrent infections.
- Severe liver disease or malnutrition — reduced production.
When to discuss with a doctor
C3 is most useful in the context of autoimmune disease, where the meaningful result is a low or falling level. In known lupus, C3 (with C4 and anti-dsDNA) is monitored because a drop can precede or accompany a flare, especially kidney involvement, prompting closer review. A low C3 with kidney signs (protein or blood in the urine) points toward an immune-mediated kidney process needing specialist assessment. It is interpreted with the other autoimmune and kidney markers, not alone. Mediora.AI shows C3 alongside anti-dsDNA, ANA and kidney function so the lupus-activity pattern is visible; the interpretation belongs with rheumatology or nephrology.