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Blood Test Tracker

Complement C4

A protein in the complement system, part of your immune defense. Low or high levels are read together with C3 and your symptoms.

AI-verified against cited sources. Awaiting expert reviewAlso called C4LOINC 4498-2

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What it is

Complement is a group of proteins in your blood that help antibodies and immune cells clear germs and damaged cells. C4 is one of the early proteins in the pathway that antibodies switch on. Much of it is made by the liver. This test measures how much C4 is in your blood, and it is usually ordered together with C3.

Why it matters

When many antibody-and-antigen clumps (immune complexes) form, complement proteins such as C4 get used up and the level falls. Clinicians follow C3 and C4 over time in conditions such as lupus. C4 is also part of the work-up for repeated, unexplained swelling episodes (hereditary angioedema), although it is not used alone for that. Some healthy people have a lower C4 because of how many C4 genes they inherited.

If it’s low

A low C4 can mean complement is being used up faster than the body makes it, or that the body makes less of it. It is also common for a person to have a naturally low level. Many other things can cause this, and your clinician can put it in context.

Common causes

  • Fewer copies of the C4 genes, which gives a naturally lower level that can be healthy for you
  • Healthy infants, whose C4 is often low in the first year
  • Active lupus, where immune complexes use up complement
  • Cryoglobulinemic vasculitis, a blood-vessel inflammation that is often linked to hepatitis C
  • Some kidney conditions caused by immune complexes
  • Hereditary angioedema types 1 and 2 (C4 is usually low, but it is not used alone to confirm the condition)
  • Rheumatoid arthritis, or repeated bacterial infections
  • Cirrhosis or malnutrition, which reduce how much the liver makes

What can help

  • Ask whether C3, a CH50 test and urine tests would help put it in context
  • Ask whether a repeat test with careful sample handling is worthwhile
  • If swelling episodes are the concern, ask whether a C1 inhibitor test is needed

If it’s high

A high C4 usually reflects inflammation, since the body makes more complement during acute and long-lasting inflammation. A high result on its own is rarely the main concern. Many other things can cause this, and your clinician can put it in context.

Common causes

  • Acute or long-lasting (chronic) inflammation
  • Ulcerative colitis
  • Some blood and lymph cancers, such as leukemia or non-Hodgkin lymphoma
  • Recovery on treatment for lupus or another autoimmune condition (levels can rise as the condition settles)

What can help

  • Ask whether an inflammation test such as CRP would help put the result in context
  • Ask whether a repeat test after you recover from an illness makes sense

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

The usual range depends on age. Choose an age to see the one that applies.

Children and teens 13 ranges

See C4 by age and sex, with the U.S. survey numbers where they exist.

What can affect your result

  • Age: infants have lower C4 than adults, and lab intervals change through childhood (ARUP)
  • Sample handling: complement breaks down in samples that wait too long or are thawed more than once
  • Genes: the number of C4A and C4B gene copies you inherited sets part of your usual level
  • Inflammation tends to raise C4, while use by immune complexes lowers it, so the result depends on which effect is stronger
  • Pregnancy: complement-related tests for hereditary angioedema can be misleading and are repeated after delivery (WAO/EAACI 2021)
  • Complement proteins break down in a sample that sits too long or is thawed and refrozen. ARUP asks for serum to be separated within 2 hours and frozen, and rejects samples that clotted in the refrigerator or went through repeated freeze-thaw cycles.
  • Results can differ between immunoturbidimetry (ARUP) and nephelometry (Mayo) methods, so compare your own results using the same lab when you can.
  • Infants have lower complement than adults, and cord blood is lower still, so adult ranges don't apply to young babies (WAO/EAACI 2021).

Changes over days to weeks as disease activity or treatment changes.

When to talk to a clinician

Talk with your clinician about a result below your lab's range, especially with joint pain, a facial rash, mouth sores, foamy or dark urine, or swelling in your legs or face. Get emergency help right away for swelling of the lips, tongue or throat, or trouble breathing, whatever your C4 result.

Questions you could ask

  • How does my C4 fit with my symptoms and my other lab results?
  • Could a recent illness or inflammation explain a higher C4?
  • Should we recheck it after I recover?
  • Is my C3 also low, and what does the pattern of both suggest?
  • Could my C4 be naturally low, and is a repeat test with careful handling worthwhile?
  • Do I need other tests, such as CH50, urine tests or a C1 inhibitor test?

Questions people ask

Why is C4 usually checked with C3?

C3 and C4 are the two most commonly tested complement proteins, and the pattern of both together says more than either alone. For example, a low C4 with a normal C3 points to a different set of explanations than both being low.

Can I have a low C4 and be healthy?

Yes. How much C4 you make depends partly on how many C4 genes you inherited, so some healthy people run low. Healthy infants also often have a low C4. Your clinician looks at your symptoms and your other tests.

How do I convert g/L to mg/dL?

Multiply g/L by 100. For example, 0.25 g/L is 25 mg/dL. To go the other way, divide mg/dL by 100.

Show the science and sources

Units

  • 1 mg/dL = 1 mg/dL
  • 1 mg/dL = 0.01 g/L
  • 1 mg/dL = 10 mg/L

Open the C4 unit converter

Is a change real?

C4 naturally varies about 6.9% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 20% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: The European Biological Variation Study (EuBIVAS): a summary report.

For clinicians and researchers

C4 exists as two isotypes, C4A and C4B, encoded in the MHC class III region with wide copy number variation; in one lupus cohort C4 serum concentration tracked gene copy number, with a mean of 0.110 g/L at two copies and 0.256 g/L at five or six copies, and homozygous C4A deficiency was the strongest genetic risk factor for lupus (Jüptner 2018). It is measured by immunoturbidimetry (ARUP) or nephelometry (Mayo). Adult intervals run from 10 to 48 mg/dL across ARUP (10–40), Labcorp (12–38), Mayo (14–40) and ABIM (14–48). The 2019 EULAR/ACR classification criteria for lupus count a C3 or C4 below the lower limit of normal, with extra weight when both are low; these are classification criteria for research, not a diagnostic test (Aringer 2019). In hereditary angioedema types 1 and 2, the WAO/EAACI guideline uses C1 inhibitor function, C1 inhibitor protein and C4 together; C4 is usually low but has limited sensitivity and specificity, is not recommended alone for screening or diagnosis, can have its sensitivity raised by drawing blood during an emerging attack, is frequently low in healthy infants under 12 months, and C3 is expected to be normal (WAO/EAACI 2021). In suspected mixed cryoglobulinemia, a low C4 and a high rheumatoid factor can be helpful clues (2026 cryoglobulinemia review). Preanalytical handling matters because complement is labile (ARUP).

LOINC 4498-2 (primary). Specimen: serum-or-plasma.

Sources

  1. Pediatric Testing Reference Ranges (includes adult intervals for Labcorp tests 001776, 001784, 001792 and 001834), Labcorp Diagnostics
  2. Complement Component 4 (test 0050155), ARUP Laboratories
  3. Complement C3 and C4 with Anti-C1q Antibodies, Serum (test C341Q), Mayo Clinic Laboratories (via testcatalog.org)
  4. ABIM Laboratory Test Reference Ranges, January 2026, American Board of Internal Medicine (2026)
  5. Complement Blood Test, MedlinePlus (National Library of Medicine) (2024)
  6. The international WAO/EAACI guideline for the management of hereditary angioedema - The 2021 revision and update, World Allergy Organization Journal (2022)
  7. 2019 European League Against Rheumatism/American College of Rheumatology Classification Criteria for Systemic Lupus Erythematosus, Arthritis & Rheumatology (2019)
  8. Low copy numbers of complement C4 and homozygous deficiency of C4A may predispose to severe disease and earlier disease onset in patients with systemic lupus erythematosus, Lupus (2018)
  9. Cryoglobulinemia: An update on classification, pathophysiology, clinical presentation, and management, Journal of Internal Medicine (2026)
  10. LOINC code lookup (NLM Clinical Tables), National Library of Medicine

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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