What this test measures
Complement C4 is an early component of the classical complement pathway. It is consumed when immune complexes (antibody bound to antigen) trigger classical-pathway activation — making serum C4 one of the most sensitive markers of classical-pathway activity. The test quantifies C4 in serum, in mg/dL, and is usually ordered with C3.
C4 has two genetic variants (C4A and C4B) and varying copy numbers in the population, so the normal range is wide and partial deficiency is relatively common in healthy adults. Absolute zero C4 is rare and significant.
Why it matters
C4 is the more sensitive of the two main complement proteins for tracking SLE activity — it falls earlier than C3 during a lupus flare and rises again with successful treatment. Rheumatologists use serial C4 alongside C3 and anti-dsDNA to monitor disease.
C4 also drops in cryoglobulinaemic vasculitis (often disproportionately low compared to C3), early SLE before classical-pathway exhaustion, and hereditary angioedema (HAE) — a rare but serious condition where C4 is persistently low because of C1-inhibitor deficiency. A patient with recurrent unexplained angioedema and low C4 should have C1-inhibitor level and function measured to confirm HAE.
How to prepare
No fasting required. Disclose pregnancy, recent infection, and any anti-complement medication. For suspected hereditary angioedema, the test is best done when not in an acute attack — C4 stays low between attacks too, but the baseline level is what is most informative.
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 C4 (mg/dL)[1][2][3] | Adult 10 – 40 mg/dL (lab-specific) | < 10 mg/dL: classical-pathway activation — active SLE (often before C3 falls), cryoglobulinaemic vasculitis, hereditary or acquired angioedema (low C1 inhibitor), severe infection, partial genetic C4 deficiency (common in healthy people). Persistent very low C4 with recurrent angioedema = check C1 inhibitor. | > 40 mg/dL: acute-phase response — infection, inflammation, post-operative state, malignancy. Not usually clinically actionable on its own. |
C4 patterns and what they suggest
| C4 | C3 | Most likely cause |
|---|---|---|
| Low | Low | Active SLE, cryoglobulinaemia, severe infection |
| Low (isolated) | Normal | Early SLE, hereditary angioedema, partial C4 gene deficiency |
| Normal | Low | Alternative-pathway disease (MPGN, C3 glomerulopathy, atypical HUS) |
| Normal | Normal | Inactive disease or non-complement-mediated condition |
Frequently asked questions
Why might C4 be low in a healthy person?
Partial C4 gene deficiency is common (up to 10–15% of healthy adults have one missing copy of C4A or C4B). These people have lower baseline C4 but usually no clinical problems. Repeat at a healthy baseline and interpret in clinical context.
How is C4 different from C3 in tracking lupus?
C4 is more sensitive — it falls earlier in a flare and stays low slightly longer than C3. Both are usually checked together; combined falls support active classical-pathway disease.
What is hereditary angioedema (HAE)?
A rare genetic disease in which deficiency or dysfunction of C1 inhibitor causes uncontrolled classical-pathway activation. Patients have recurrent, painful, non-itchy swelling of skin, gut, and (dangerously) the airway. C4 is persistently low. Confirm with C1 inhibitor level and function.
Should I worry about low C4 without symptoms?
Probably not. Mild persistent low C4 without symptoms is often partial genetic deficiency. Without recurrent angioedema, recurrent infection, or autoimmune symptoms it is rarely clinically actionable. Discuss with your doctor.
Do oral contraceptives affect C4?
They can raise C4 modestly via oestrogen effects on the liver. Disclose contraception when the test is interpreted.
Can C4 normalise on treatment?
In SLE, yes — C4 typically returns to normal as the immune-complex burden falls with treatment. In HAE, baseline C4 stays low even when symptoms are controlled; the diagnosis is genetic and lifelong.
Do I need both C3 and C4?
Yes, almost always. The pattern of the two together (both low vs C3-only vs C4-only) narrows the differential better than either alone.
Related Immunology tests
Tests commonly ordered alongside COMPLEMENT 4 (C4), 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 C4 · accessed 2026-05-30T00:00:00.000Z
- US Hereditary Angioedema Association · accessed 2026-05-30T00:00:00.000Z
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