Complement Test: Types, Procedure, And Results - Healthline

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HealthlineSubscribeWhat to Know About a Complement Blood Test

Medically reviewed by Kelsey Nank, PA-C — Written by Ann Pietrangelo — Updated on February 23, 2026

What is it?
Purpose
Types
Preparation
Procedure
Risks
Results
Takeaway

Key takeaways

A complement blood test measures how well complement proteins in your blood support immune defense. The complement system helps antibodies fight germs and clear foreign substances.
Complement testing can help diagnose immune conditions and monitor treatment for autoimmune diseases such as systemic lupus (SLE) and rheumatoid arthritis (RA). Testing may include total hemolytic complement (CH50) plus targeted C2, C3, or C4 levels.
Results can be higher or lower than expected, depending on the condition and the level of inflammation. Contact a doctor right away after a blood draw if you have heavy bleeding, swelling, or fainting.

What is a complement test?

A complement test is a blood test that measures the activity of a group of proteins in the bloodstream. These proteins make up the complement system, which is one part of the immune system.

The complement system helps antibodies fight infections and destroy foreign substances in the body, such as viruses, bacteria, and other germs.

The complement system is also involved in the functioning of autoimmune diseases and other inflammatory conditions. When a person has an autoimmune disease, the body views its own tissues as foreign and makes antibodies against them.

There are nine major complement proteins, labeled C1 through C9. However, this system is very complex. Currently, dozens of known substances in the immune system combine with complement proteins when activated.

A total complement measurement checks the activity of the main complement components by gauging the total amount of complement protein in your blood. One of the more common tests is known as a total hemolytic complement, or a CH50 measurement.

Complement levels that are too low or too high can cause problems.

What is the purpose of a complement test?

A complement test is commonly used to diagnose autoimmune diseases or other immune function conditions. Certain diseases can have abnormal levels of a particular complement.

A doctor can use a complement test to monitor the progress of a person undergoing treatment for an autoimmune disease such as systemic lupus (SLE) or rheumatoid arthritis (RA). It can also gauge the effectiveness of ongoing treatments for autoimmune disorders and certain kidney conditions. The test can also identify individuals at high risk of complications in certain diseases.

What are the types of complement tests?

A total complement measurement checks how well the complement system is functioning.

A doctor often orders total complement tests for people with a family history of complement deficiency and those who have symptoms of:

RA
hemolytic uremic syndrome (HUS)
kidney disease
SLE
myasthenia gravis, a neuromuscular disorder
an infectious disease, such as bacterial meningitis
cryoglobulinemia, which is the presence of abnormal proteins in the blood

Specific complement tests, such as C2, C3, and C4 tests, may help evaluate the course of certain diseases. Depending on your symptoms and history, your doctor will order either a total complement measurement, one of the more targeted tests, or all three. A blood draw is all that’s necessary.

How do you prepare for a complement test?

A complement test requires a routine blood draw. No preparation or fasting is necessary.

How is a complement test performed?

A healthcare professional will follow these steps to perform the blood draw:

They disinfect an area of skin on your arm or hand.
They wrap an elastic band around your upper arm to allow more blood to fill the vein.
They insert a small needle into your vein and draw the blood into a small vial. You may feel a pricking or stinging sensation from the needle.
When the vial is full, they remove the elastic band and needle and place a small bandage over the puncture site.

There may be some soreness in the arm where the needle entered the skin. You may also experience some mild bruising or throbbing after the blood draw.

What are the risks of a complement test?

The blood draw carries few risks. Rare risks from a blood draw include:

excessive bleeding
swelling
lightheadedness
fainting
infection, which can happen any time the skin is broken

Notify your doctor right away if you have any of these symptoms.

What do the test results mean?

Results of a total complement measurement are usually expressed in units per milliliter. Tests that measure specific complement proteins, including C3 and C4, are typically reported in milligrams per deciliter (mg/dL).

According to Mayo Medical Laboratories, the following are typical complement readings for people ages 16 years and older. Values can vary between laboratories. Sex and age can also affect expected levels.

total blood complement: 30 to 75 units per mL (U/mL)
C2: 25 to 47 mg/dL
C3: 75 to 175 mg/dL
C4: 14 to 40 mg/dL

Higher-than-normal results

Values that are higher than usual may indicate a wide variety of conditions. Often, these are related to inflammation. Some conditions associated with elevated complement can include:

cancer
viral infections
metabolic dysfunction-associated steatotic liver disease (MASLD), formerly known as non-alcohol-related fatty liver disease (NAFLD)
metabolic syndrome
obesity
diabetes
heart disease
chronic skin conditions like psoriasis
ulcerative colitis (UC)

Complement activity in the bloodstream is characteristically low in people with active autoimmune diseases such as lupus. However, blood complement levels may be healthy or high with RA.

Lower-than-usual results

Certain complement levels that are lower than usual may occur with:

lupus
cirrhosis with severe liver damage or liver failure
glomerulonephritis, a type of kidney disease
hereditary angioedema, which is episodic swelling of the face, hands, feet, and some internal organs
malnutrition
a flare-up of an autoimmune disease
sepsis, an infection in the bloodstream
septic shock
fungal infection
some parasitic infections

In some people with infectious and autoimmune diseases, complement levels may be so low that they are undetectable.

People who lack certain complement proteins may be more prone to infections. Complement deficiency may also be a factor in the development of autoimmune diseases.

What happens after a complement test?

After the blood draw, your healthcare professional will send the blood sample to a laboratory for analysis.

Keep in mind that your total complement test results can be normal even if you’re deficient in several specific complement proteins.

Talk with your doctor about how the results apply to you. They may recommend more testing to make a final diagnosis.

How we reviewed this article:

SourcesHistoryHealthline has strict sourcing guidelines and relies on peer-reviewed studies, academic research institutions, and medical journals and associations. We only use quality, credible sources to ensure content accuracy and integrity. You can learn more about how we ensure our content is accurate and current by reading our editorial policy.

Bardhan M, et al. (2023). Physiology, complement cascade.https://www.ncbi.nlm.nih.gov/books/NBK551511/
Blood tests. (2022).https://www.cancerresearchuk.org/about-cancer/tests-and-scans/blood-tests
Complement blood test. (2024).https://medlineplus.gov/lab-tests/complement-blood-test/
Coss SL, et al. (2023). The complement system and human autoimmune diseases.https://www.sciencedirect.com/science/article/pii/S0896841122001871
Cserhalmi M, et al. (2019). Regulation of regulators: Role of the complement factor H-related proteins.https://www.sciencedirect.com/science/article/pii/S1044532319300247
Ghias MH, et al. (2020). Role of the complement pathway in inflammatory skin diseases: A focus on hidradenitis suppurativa.https://www.sciencedirect.com/science/article/pii/S0022202X19333019
Heggi MT, et al. (2024). Microbial evasion of the complement system: A continuous and evolving story.https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2023.1281096/full
Killeen RB, et al. (2025). Cryoglobulinemia.https://www.ncbi.nlm.nih.gov/books/NBK557606/
Kolev M, et al. (2022). Inside-out of complement in cancer.https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2022.931273/full
Mollah F, et al. (2023). Complement deficiency.https://www.ncbi.nlm.nih.gov/books/NBK557581/
Signs of hemolytic uremic syndrome. (2024).https://www.cdc.gov/ecoli/signs-symptoms/hus.html
Test ID: C1Q. Complement C1q, serum: Clinical and interpretive. (n.d.).https://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/8851
Test ID: C2FXN. C2 complement, functional, serum: Clinical and interpretive.https://www.mayocliniclabs.com/test-catalog/overview/32137#Clinical-and-Interpretive
Test ID: C3. Complement C3, serum: Clinical andinterpretive. (n.d.).https://www.mayocliniclabs.com/test-catalog/overview/8174#Clinical-and-Interpretive
Test ID: C4. Complement C4, serum: Clinical andinterpretive. (n.d.).https://www.mayocliniclabs.com/test-catalog/overview/8171#Clinical-and-Interpretive
Test ID: COM. Complement, total, serum: Clinicaland interpretive. (n.d.).https://www.mayocliniclabs.com/test-catalog/overview/8167#Clinical-and-Interpretive

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Medically reviewed by Kelsey Nank, PA-C — Written by Ann Pietrangelo — Updated on February 23, 2026

Complement Test: Types, Procedure, And Results - Healthline

Key takeaways

What is a complement test?

What is the purpose of a complement test?

What are the types of complement tests?

How do you prepare for a complement test?

How is a complement test performed?

What are the risks of a complement test?

What do the test results mean?

Higher-than-normal results

Lower-than-usual results

What happens after a complement test?

How we reviewed this article:

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