Autoimmune Panel

Complement C4 (C4)

Most people meet complement C4 as a lupus marker. A value that stays low can point somewhere almost nobody connects it to.

Part of the Autoimmune Panel — see all 16 values together, including Anti-Centromere Antibodies, Anti-Cyclic Citrullinated Peptide, Anti-Double Stranded DNA.

A low complement C4 usually points at one disease; stay low, and it quietly flags a different one most people never connect. Complement component 4, written C4 on a lab report, is one protein in the complement system, the cascade of blood proteins that helps antibodies clear infections and clear away cellular debris. When a doctor orders C4, they are nearly always thinking about lupus, where the complement system gets consumed by autoimmune activity and the level drops.

A low C4 behaves a little like a faint gas smell that reads as burnt toast in most kitchens but warns of a leak in one. In most patients the smell is exactly what everyone assumes: autoimmune activity, the usual suspect. In a smaller group, the same low reading is a leak no one looks for, hereditary angioedema, a condition many people and quite a few of their doctors never tie back to this single number.

C4 is reported in mg/dL, the same measurement as g/L scaled differently, so a result near 0.15 g/L is the same as 15 mg/dL. MedlinePlus puts the usual reference window around 15 to 45 mg/dL (0.15 to 0.45 g/L), and some labs run a little wider, which is why your report's own range matters more than any single textbook boundary. If a low number has you searching, our guide to reading flagged values walks through what a single out-of-range result does and doesn't tell you.

What a C4 result usually means

mg/dL
Below most lab floors < 15

Values like 8, 10, 12, or 14 mg/dL fall under the usual floor, and a 16 sits right at the borderline. This is where both the lupus story and the hereditary angioedema story begin, so a low reading is a prompt to look further rather than an answer on its own.

Inside the usual window 15–45

Reassuring in most people. One catch to remember: C4 is an acute-phase reactant, so infection or inflammation can lift a borderline-low result back into range and mask a deficiency.

Above the usual window > 45

Less discussed. MedlinePlus links a high C4 to cancer and ulcerative colitis, and the level can also rise with acute inflammation or in response to treatment.

What does a low C4 mean?

Start with the expected reading. In active systemic lupus erythematosus, the immune system burns through complement faster than the body replaces it, and both C3 and C4 tend to fall together. These two are the most commonly measured complement proteins, and clinicians watch them to gauge how active the disease is and whether treatment is working. A low C4 alongside a low complement C3 in someone with lupus symptoms is a familiar pattern, often checked together with ANA and anti-dsDNA antibodies. Low C4 carries the most meaning next to the rest of a lupus workup, including the anti-Smith antibody and anti-dsDNA.

Now the twist. A C4 that keeps coming back low, especially when C3 is normal, is the classic screening clue for hereditary angioedema. This is a genetic condition that causes unexplained swelling attacks, often swollen lips and throat that come and go, and sometimes abdominal pain mistaken for other problems. The WAO guideline names C4 the single best screening test for it. The detail that makes C4 so useful here is that the level can stay low even between attacks, when a person feels fine and nothing else on a panel looks off.

That same usefulness comes with a hard limit. C4 alone cannot confirm or rule out hereditary angioedema, because neither its sensitivity nor its specificity is absolute, and the 2021 WAO and EAACI guidance does not accept C4 as the sole parameter. A diagnosis needs C4 measured together with the C1-inhibitor protein level and a C1-inhibitor function test. The acute-phase behavior makes the caveat sharper: an infection running at the time of the draw can pull a deficient C4 up into the normal range and hide the very problem the test was meant to catch.

What can push C4 low

  • Active lupus

    Autoimmune activity consumes complement; C3 and C4 usually fall together.

  • Lupus nephritis

    Kidney involvement in lupus often tracks with low complement.

  • Hereditary angioedema

    A persistently low C4 is the classic screening clue, confirmed only with C1-inhibitor testing.

  • Glomerulonephritis

    Several kidney inflammations consume complement.

  • Liver disease

    Hepatitis and cirrhosis can lower production.

  • Inherited complement deficiency

  • Systemic vasculitis

  • Some bacterial infections

There is one more reason two people with an identical C4 number may not carry the same risk. People inherit different numbers of C4 genes, and the total copy count across both chromosomes ranges from 2 to 6, with about 4 being the most common. Research in lupus found that carrying only 2 copies was tied to markedly higher disease risk, while having 5 or more copies looked protective. Because baseline C4 protein differs from person to person, a value that is ordinary for one body may be relatively low for another.

What does a high C4 mean?

High C4 draws less attention but is worth naming. MedlinePlus connects an elevated C4 to cancer and ulcerative colitis, and because C4 climbs with inflammation, a high reading can simply reflect an acute-phase response or recovery during treatment. As with the low side, the number is a starting point for a conversation, not a verdict.

What to do about a low C4

  1. 1

    Bring the result to your doctor

    A single low C4 is a screening signal. Your doctor reads it against your symptoms and the rest of the panel, not in isolation.

  2. 2

    Ask whether timing skewed it

    Because C4 rises with inflammation, clinicians may repeat the test once an infection or flare has settled so a real deficiency isn't masked.

  3. 3

    Mention any swelling

    If you have unexplained swelling attacks, raise hereditary angioedema. The workup adds the C1-inhibitor protein level and a C1-inhibitor function test, since C4 alone cannot confirm it.

  4. 4

    Pair it for the lupus question

    C4 is usually read with C3, ANA, and anti-dsDNA antibodies to judge whether lupus is active.

  5. 5

    Don't fast on its own account

    A C4 test does not require fasting; follow whatever instructions your lab gives for the wider draw.

Reading C4 in context

C4 rarely answers a question by itself. It does its real work as part of the broader autoimmune panel, read alongside C3 and antibody tests, and sometimes next to antiphospholipid antibodies and anticardiolipin when clotting risk is also in question. For how these results fit together on one report, our guide to reading an autoimmune panel lays out the combinations.

Because complement levels track disease activity, a single C4 says less than a series of them. A value that drifts down over months, or one that stays stubbornly low between symptom-free stretches, often carries more meaning than any one reading pulled from a single morning's blood draw.

Sources

Written and reviewed by BloodSight Editorial Team · Last updated

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Normal ranges

Group Range Unit
Adult Male 12–54 mg/dL
Adult Female 12–54 mg/dL

Reference ranges may vary by laboratory and individual factors.

Complement C4 — Common Questions

If my C4 is low but I don't have lupus, what else could it mean?
Low C4 has a long list of causes beyond lupus. MedlinePlus links it to hereditary angioedema, glomerulonephritis, hepatitis, cirrhosis, inherited complement deficiencies, systemic vasculitis, kidney transplant rejection, and some bacterial infections. The pattern that surprises people most is hereditary angioedema, where a persistently low C4 is the classic screening clue. A low value on its own is a prompt to look further with a doctor, not a diagnosis.
Can a low C4 mean hereditary angioedema even between swelling attacks?
Yes. One reason C4 is useful for hereditary angioedema is that it can stay low even between attacks, when nothing else looks wrong. The WAO guideline calls C4 the single best screening test for this condition. Repeating the test during an attack improves the odds of catching a low value, but a low reading between episodes can still be the first clue.
Is a low C4 enough to diagnose hereditary angioedema?
No. C4 screens, it does not confirm. Its sensitivity and specificity are both limited, so the WAO and EAACI guidance does not treat C4 as the sole parameter. Diagnosis pairs C4 with the C1-inhibitor protein level and a C1-inhibitor function test. All three together tell the story that C4 alone cannot.
Can an infection make a low C4 look normal and hide a problem?
It can. C4 is an acute-phase reactant, so infection or inflammation can lift the level into the normal range. A borderline-low value measured during an illness may read as reassuring while masking an underlying deficiency. That is why a doctor may repeat the test once any acute illness has settled.
Why is my C4 low when my C3 is normal?
C3 and C4 sit at different points in the complement cascade, so they don't always move together. In active lupus both tend to fall. An isolated low C4 with a normal C3 is one of the patterns that nudges clinicians to think about hereditary angioedema or an inherited C4 deficiency rather than lupus alone.
Do two people with the same C4 number have the same risk?
Not necessarily. People inherit different numbers of C4 genes, and total copy number ranges from 2 to 6, with about 4 being most common. Research in lupus found that having only 2 copies carried markedly higher disease risk while 5 or more copies was protective. Baseline C4 protein differs from person to person, so the same number can mean different things in different bodies.

Disclaimer

This content is for informational and educational purposes only. It is not intended as medical advice, diagnosis, or treatment recommendation. Reference ranges may vary by laboratory. Always discuss your results with a qualified healthcare professional.

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