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Blood Test Tracker

Complement C3

The most abundant complement protein in your blood. Complement is part of the immune system, and C3 sits at its center.

AI-verified against cited sources. Awaiting expert reviewAlso called C3, C3cLOINC 4485-9

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What it is

Complement is a group of proteins that help the body clear germs and cell debris. C3 is the main one, made mostly by the liver. When the immune system is active, C3 is used up as it does its job. A C3 test measures how much is in your blood. It is often ordered with C4, which is part of the same system.

Why it matters

Clinicians use C3 to follow certain immune and kidney conditions. A low level can mean the complement system is being used up faster than the body makes it. It is not a test for any one disease, and C3 can also rise with inflammation, so it is read together with other results and symptoms.

If it’s low

A low C3 means complement may be getting used up faster than the body makes it, or the body may be making less. It is a clue clinicians use in certain immune and kidney conditions. By itself it does not say what is causing it.

Common causes

  • Normal variation, especially when the result is just under the lab's range
  • A sample that sat too long or was handled poorly before testing
  • Active lupus, especially when the kidneys are involved
  • Kidney inflammation after some infections (post-infectious glomerulonephritis) and other immune-related kidney conditions such as C3 glomerulopathy
  • Long-term infections or other conditions with immune complexes in the blood
  • Severe liver disease, since the liver makes most C3
  • Malnutrition
  • Severe infection (sepsis)
  • Rare inherited C3 deficiency

What can help

  • Ask your clinician which other tests, such as C4, urine and kidney tests, help explain it
  • If you are followed for lupus or a kidney condition, ask how often C3 should be rechecked
  • Bring a list of your conditions, symptoms and medicines to the visit

If it’s high

A high C3 most often reflects inflammation, because C3 is an acute-phase protein that rises when the body is responding to infection or injury. It is not usually a concern on its own.

Common causes

  • Infection or other inflammation, including a recent illness
  • Pregnancy
  • Higher body weight and related metabolic changes
  • Long-term inflammatory conditions, such as ulcerative colitis
  • Recovery from tissue injury
  • Some cancers, such as leukemia and non-Hodgkin lymphoma

What can help

  • Ask your clinician to read it with inflammation markers such as CRP and with how you feel
  • Ask whether repeating the test once any illness has passed 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 5 ranges

See C3 by age and sex, with the U.S. survey numbers where they exist.

What can affect your result

  • Inflammation and infection, which raise C3
  • Pregnancy, which can raise it
  • Body weight: levels tend to run higher with higher body weight
  • Liver function and nutrition, since the liver makes C3
  • Age: children's ranges differ slightly from adults'
  • Sample handling and the lab's method, which is why each lab prints its own range
  • Complement proteins are fragile, so handling matters. ARUP asks that serum be separated from cells within 2 hours, and it rejects samples that clotted in the refrigerator or were frozen and thawed repeatedly.
  • C3 is an acute-phase protein, so inflammation or infection around the time of the draw can raise it and may hide a drop caused by a long-term condition.
  • Assay methods differ (Mayo uses nephelometry, ARUP uses immunoturbidimetry), and so do the ranges. Compare results from the same lab when you can.

Can change within days to weeks as inflammation or the activity of an immune condition changes.

When to talk to a clinician

A low C3 on its own is not an emergency, but it is worth discussing with your clinician. Contact them soon if you also have swelling in your legs, hands or face, foamy or cola-colored urine, a new rash or joint pain, or an unexplained fever.

Questions you could ask

  • How often should C3 be checked in my situation?
  • Should my C3 be read together with my C4?
  • Could inflammation or a recent illness explain this?
  • Should we repeat it once I have recovered?
  • Which other tests, such as C4, urine or kidney tests, would help explain this?
  • Could a recent illness or a sample handling issue be part of it, and should we repeat it?

Questions people ask

Is C3 the same as the C3c on my report?

Some lab reports label the test C3c. We group it with C3 here, but compare it only with the range printed on your own report, because labs use different methods.

Why do labs print different C3 ranges?

Labs use different methods and reference groups. Mayo Clinic Laboratories lists 75 to 175 mg/dL and ARUP lists 90 to 180 mg/dL for adults. The range on your report is the one your lab used.

Does a low C3 mean I have lupus?

Not by itself. Low C3 can occur with several conditions, and many people with lupus have a normal C3. Your clinician reads it with your symptoms, C4, antibody tests and urine and kidney tests.

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 C3 unit converter

Is a change real?

C3 naturally varies about 4.6% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 14% 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

C3 is the central protein of the complement cascade, where the classical, lectin and alternative pathways converge. C3 convertases cleave it into C3a and C3b, and C3b is further processed to iC3b and C3c fragments. Serum C3 is measured by nephelometry or immunoturbidimetry. Some reports use the name C3c, which has its own LOINC code. Low C3 with normal C4 points to alternative-pathway activation (as in C3 glomerulopathy or post-infectious glomerulonephritis), while low C3 and low C4 points to classical-pathway consumption by immune complexes (as in active lupus). Because C3 is an acute-phase reactant, concentrations rise with inflammation and can mask consumption. The 2019 EULAR/ACR classification criteria for systemic lupus erythematosus count C3 or C4 below the lower limit of normal as one of its immunologic domains. Intervals vary by lab: Mayo Clinic Laboratories 75 to 175 mg/dL; ARUP 90 to 180 mg/dL for ages 18 and older.

LOINC 4485-9 (primary), 4491-7. Specimen: serum-or-plasma.

Sources

  1. Complement Component 3 (test 0050150), ARUP Laboratories
  2. Complement C3 and C4 with Anti-C1q Antibodies, Serum (test C341Q), Mayo Clinic Laboratories (Extended Catalog)
  3. Complement Blood Test, MedlinePlus (National Library of Medicine)
  4. 2019 European League Against Rheumatism/American College of Rheumatology Classification Criteria for Systemic Lupus Erythematosus, Arthritis & Rheumatology (2019)

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

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