What this test measures
Complement is a system of plasma proteins that amplifies the immune response. C3 is the central convergence point of all three complement pathways (classical, alternative, lectin) and is the most abundant complement protein. When complement is activated by immune complexes, certain infections, or autoantibodies, C3 is consumed faster than the liver can replace it — so serum C3 falls.
The test quantifies C3 in serum, reported in mg/dL. It is usually ordered with C4 (a more sensitive marker of classical-pathway activation in SLE) and sometimes CH50 (the total complement haemolytic activity).
Why it matters
Low C3 is one of the most useful real-world markers of autoimmune disease activity. In systemic lupus erythematosus (SLE), C3 and C4 fall during flares, especially with lupus nephritis — and rise as the flare settles. Serial C3 levels alongside anti-dsDNA help rheumatologists track disease activity and treatment response.
Low C3 with low C4 — "classical pathway consumption" — points to SLE, cryoglobulinaemic vasculitis, severe infection, or post-streptococcal glomerulonephritis. Low C3 with normal C4 — "alternative pathway consumption" — suggests membranoproliferative glomerulonephritis, C3 glomerulopathy, or atypical haemolytic uraemic syndrome (aHUS). Indian nephrologists use this pattern to narrow the differential in unexplained glomerular disease.
How to prepare
No fasting required. Disclose any recent infection (transient C3 drop with active infection), pregnancy (C3 rises modestly), and any anti-complement therapy (eculizumab, ravulizumab). Tell the lab if you are tested for SLE, glomerulonephritis, or recurrent infection — clinical context shapes interpretation.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Complement C3 (mg/dL)[1][2][3] | Adult 90 – 180 mg/dL (lab-specific) | < 90 mg/dL: complement consumption — active SLE (especially with lupus nephritis), post-streptococcal glomerulonephritis, membranoproliferative glomerulonephritis, C3 glomerulopathy, cryoglobulinaemic vasculitis, severe infection or sepsis, hereditary C3 deficiency (rare), severe liver failure (reduced synthesis). | > 180 mg/dL: acute-phase response — infection, inflammation, post-operative state, malignancy, IBD. Not usually clinically actionable; reflects nonspecific inflammation. |
C3 / C4 patterns and what they suggest
| C3 | C4 | Most likely cause |
|---|---|---|
| Low | Low | Active SLE (especially lupus nephritis), cryoglobulinaemic vasculitis, severe infection |
| Low | Normal | Post-streptococcal GN, MPGN, C3 glomerulopathy, atypical HUS |
| Normal | Low | Hereditary angioedema, early SLE, cryoglobulinaemia |
| Normal | Normal | Inactive disease or non-complement-mediated condition |
Frequently asked questions
Why is C3 low in lupus?
In active SLE the immune system makes antibodies against self-antigens. These form immune complexes that activate complement and consume C3 (and C4) faster than the liver can replace them. As the flare settles with treatment, C3 rises back to normal.
Can C3 monitor my lupus over time?
Yes. Serial C3 levels, ideally with anti-dsDNA and clinical assessment, are part of standard SLE monitoring. A falling C3 often precedes clinical flare, and a rising C3 with falling anti-dsDNA usually accompanies treatment response.
What does low C3 with normal C4 mean?
It points to alternative-pathway complement activation — most commonly membranoproliferative glomerulonephritis, C3 glomerulopathy, or atypical haemolytic uraemic syndrome. These are kidney-focused diagnoses needing nephrology input.
Is low C3 dangerous?
Low C3 itself does not cause symptoms — it is a marker of an underlying immune-complex or alternative-pathway process. The clinical concern is whatever is consuming the complement (active SLE, glomerulonephritis, severe infection).
Can low C3 cause infections?
Hereditary deficiency of C3 (extremely rare) causes recurrent severe bacterial infections. Acquired low C3 from active autoimmune disease rarely causes infection-prone state — the immunosuppressive treatment for the disease is usually the bigger infection-risk factor.
Do I need to fast?
No. C3 can be tested at any time of day with no fasting required. Avoid testing during an acute infection if the question is autoimmune activity — infection independently raises C3.
What is CH50?
CH50 measures the total functional activity of the classical complement pathway — the integrated activity of all complement proteins working together. A normal C3 and C4 with a low CH50 suggests deficiency of one of the other complement components. It is used in suspected primary complement deficiency.
Related Immunology tests
Tests commonly ordered alongside COMPLEMENT 3 (C3), or that help interpret an unexpected result.
Sources & references
- NIH MedlinePlus — Complement Test · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — Complement Deficiency · accessed 2026-05-30T00:00:00.000Z
- Mayo Clinic Labs — Complement C3 · accessed 2026-05-30T00:00:00.000Z
- EULAR — SLE Recommendations · accessed 2026-05-30T00:00:00.000Z
Book with Zelnoo
Get your COMPLEMENT 3 (C3) test done at home — transparent prices, NABL-accredited labs.
Zelnoo lets you compare diagnostic test prices across NABL-accredited labs in Mumbai & Thane, book a free home phlebotomist visit, and receive digital reports in 24–48 hours into a consent-first report vault. No subscriptions, no membership fees — pay only for the test you book.
Book COMPLEMENT 3 (C3) now