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Complement is a blood test that measures the activity of certain proteins in the liquid portion of your blood.

The complement system is a group of proteins that move freely through your bloodstream. The proteins work with your immune system. They also play a role in the development of inflammation.

There are nine major complement proteins. They are labeled C1 through C9.

How the Test is Performed

A blood sample is needed. This is most often taken through a vein. The procedure is called a venipuncture.

How to Prepare for the Test

There is no special preparation.

How the Test will Feel

When the needle is inserted to draw blood, some people feel slight pain, while others feel only a prick or sting. Afterward, there may be some throbbing.

Why the Test is Performed

Total complement activity (CH50, CH100) looks at the overall activity of the complement system. In most cases, other tests that are more specific for the suspected disease are done first. C3 and C4 are the complement components measured most often.

A complement test may be used to monitor people with an autoimmune disorder. It is also used to see if treatment for their condition is working. For example, people with active lupus erythematosus may have lower-than-normal levels of the complement proteins C3 and C4.

Complement activity varies throughout the body. For example, in people with rheumatoid arthritis, complement activity in the blood may be normal or higher-than-normal, but much lower-than-normal in the joint fluid.

People with gram negative septicemia and shock often have very low C3 and components of what's known as the alternative pathway. C3 is often also low in fungal infections and some parasitic infections such as malaria.

Normal Results

  • Total blood complement level: 41 to 90 hemolytic units
  • C1 level: 16 to 33 mg/dL
  • C3 levels: 88 to 252 mg/dL in males; 88 to 206 mg/dL in females
  • C4 levels: 12 to 72 mg/dL in males; 13 to 75 mg/dL in females

Note: mg/dL = milligrams per deciliter.

Note: Normal value ranges may vary slightly among different laboratories. Talk to your health care provider about the meaning of your specific test results.

The examples above show the common measurements for results for these tests. Some laboratories use different measurements or may test different specimens.

What Abnormal Results Mean

Increased complement activity may be seen in:

Decreased complement activity may be seen in:


Risks associated with having blood drawn are slight but may include:

  • Excessive bleeding
  • Fainting or feeling light-headed
  • Hematoma (blood accumulating under the skin)
  • Infection (a slight risk any time the skin is broken)


The "complement cascade" is a series of reactions that take place in the blood. The cascade activates the complement proteins. The result is an attack unit that creates holes in the membrane of bacteria, killing them.

Alternative Names

Complement assay; Complement proteins



Sullivan KE, Grumach AS. The complement system. In: Adkinson NF, Bochner BS, Burks AW, et al, eds. Middleton's Allergy: Principles and Practice. 8th ed. Philadelphia, PA: Elsevier Saunders; 2013:chap 8.

Update Date 1/20/2015

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