Complement tests, most commonly C3 and C4, are used to determine whether deficiencies or abnormalities in the complement system are causing, or contributing to, a person's disease or condition. Total complement activity (CH50 or CH100) may be ordered to look at the integrity of the entire classical complement pathway. Other complement components are ordered as needed to look for deficiencies.
Complement testing may be used to:
Help diagnose the cause of recurrent microbial infections (such as Streptococcus pneumoniae, Neisseria meningitides, Neisseria gonorrhea), angioedema, or inflammation.
Complement testing may be ordered when a person has unexplained inflammation or edema or symptoms of an autoimmune disorder such as SLE. It may also be ordered when a health practitioner suspects that someone may have an immune complex-related condition and wants to check the status of the person's complement system.
Individual complement components may be ordered when the total complement activity (CH50, sometimes called CH100) is abnormal to help determine which of the components are deficient or abnormal. C3 and C4 levels are the most frequently ordered, but others, such as C1 inhibitor, may be ordered when other deficiencies are suspected. C3 and C4 are often ordered together as the relative levels are often important.
When an acute or chronic condition has been diagnosed, complement testing may be used to help give a rough idea of the severity of the condition with the assumption that the severity is linked to the decrease in complement levels. Complement testing may also be ordered occasionally when a health practitioner wants to monitor the current activity of a condition.
Complement levels may be decreased due to increased consumption or, more rarely, a hereditary deficiency. Hereditary deficiency in one of the complement proteins will usually lead to a high frequency of recurrent microbial infections. Decreased complement levels also are associated with an increased risk of developing an autoimmune disease. Both C3 and C4 levels are typically depressed in SLE while C3 alone is low in septicemia and infections caused by fungi or parasites such as malaria.
If the deficiency is due to an underlying acute or chronic condition, complement levels will usually return to normal if the underlying condition can be resolved.
Complement protein levels are usually increased, along with other unrelated proteins called acute phase reactants, during acute or chronic inflammation. These all usually return to normal when the underlying condition is resolved. However, complement proteins are rarely measured in these conditions, compared to the widely ordered C-reactive protein (CRP), and the relevance of their measurement in these situations is not reviewed here.
Increased complement activity may also be seen with:
This article was last reviewed on March 31, 2014. | This article was last modified on September 15, 2014.
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