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Also known as: C3; C4; Total complement (also known as CH50 or CH100); Total haemolytic complement activity
Formal name: Complement activity; Complement proteins, C1 - C9
Related tests: Antibody tests, ESR, CRP, RF, ANA

At a Glance

Why Get Tested?

To determine whether deficiencies or abnormalities in the proteins that are part of the complement system are contributing to increased infections or increased autoimmune activity; to monitor the activity of autoimmune diseases

When to Get Tested?

When you have recurrent microbial (usually bacterial) infections, unexplained inflammation or oedema, or symptoms related to an autoimmune disorder; to help monitor an acute or chronic condition that affects the complement system

Sample Required?

A blood sample drawn from a vein in your arm

The Test Sample

What is being tested?

The complement system is a set of circulating blood proteins that work together to promote immune and inflammatory responses. Their principal role is to destroy foreign substances like bacteria and viruses. The nine primary complement components are those designated as C1 through C9. They are assisted and regulated by several subcomponents and inhibitors.

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 advance exposure to an invading microorganism or substance and does not maintain a memory of previous encounters. As part of the innate immune system, the complement system has evolved to recognise antigen-antibody complexes (immune complexes) as well as certain structures and polysaccharides (complex carbohydrates) found on the outside membranes of microorganisms and other foreign cells.

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:

  • 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 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 serum.

Complement proteins both promote and regulate these activities. Inherited or acquired deficiencies or abnormalities in one or more of the complement components may adversely affect the integrity and function of the immune system. Deficiencies may also arise because of decreased production or increased use of one or more complement proteins.

These tests measure the quantity or the function (activity) of complement proteins in the blood. Complement components may be measured individually and 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 the doctor 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 9 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.


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

No test preparation is needed.

The Test

Common Questions

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.