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Also known as: C1; C1q; C2; C3; C4; CH50; CH100 (among others)
Formal name: Complement Activity; Complement Component; Total Complement; Total Hemolytic Complement Activity

The Test Sample

What is being tested?

The complement system is complex and composed of a set of more than 30 circulating blood proteins that work together to promote immune and inflammatory responses. Its principal role is to destroy foreign pathogens like bacteria and viruses. The complement system can also be activated when the body makes antibodies against its own tissues that it thinks are foreign (autoantibodies) as happens in autoimmune diseases. Complement tests measure the quantity or activity of complement proteins in the blood.

The complement system is part of the body's innate immune system. Unlike the acquired immune system, which produces antibodies that target and protect against specific threats, the innate immune system is non-specific and can quickly respond to foreign substances. It does not require previous exposure to an invading microorganism or substance and does not maintain a memory of previous encounters.

There are nine primary complement proteins that are designated C1 through C9. These components, in addition to the remaining proteins, work together in a cascade manner by activating, amplifying, breaking apart, and forming complexes that respond to infections, non-self tissues (transplants), dead cells (apoptosis), or inflammation.

Complement activation may be initiated in several different ways. These are termed classical, alternative or lectin pathways. However, the final product from all activation pathways is the same – the formation of the membrane attack complex (MAC). Complement activation causes several things to happen ("complement cascade"):

  • The MAC binds to the surface of each microorganism or abnormal cell that has been targeted for destruction. It creates a lesion (hole) in the membrane wall and causes lysis, which is destruction of the cell by letting the contents out – like piercing a water-filled balloon.
  • It increases the permeability of blood vessels, allowing infection-fighting white blood cells (WBCs) to move out of the bloodstream and into the tissues.
  • It attracts WBCs to the site of the infection.
  • It stimulates phagocytosis, a process in which microorganisms are engulfed by macrophages and neutrophils and killed.
  • It increases the solubility of the immune complexes and helps to clear them out of the blood.

Complement tests measure the quantity or the function (activity) of complement proteins in the blood. Complement components may be measured individually or together to determine whether the system is functioning normally. C3 and C4 are the most frequently measured complement proteins. Total complement activity (CH50 or CH100) can be measured if a health practitioner suspects a deficiency that is not measured by C3 or C4. CH50 measures the function of the complete classical complement pathway, C1 – C9. If this measurement is outside the normal range, then each of the nine different complement levels can be measured individually to look for hereditary or acquired deficiencies.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.