To determine whether deficiencies or abnormalities in complement system proteins are contributing to increased infections or increased autoimmune activity; to help monitor the activity and treatment of autoimmune diseases and immune complex-related diseases (Complement deficiencies may comprise between 1 and 10% of all primary immunodeficiencies.)
Complement
Complement
Also Known As
C1
C1q
C2
C3
C4
CH50
CH100
Total Complement
Formal Name
Complement (Antigen, Functional or Activity)
This article was last reviewed
on
This article waslast modified
on March 25, 2021.
At a Glance
Why Get Tested?
When To Get Tested?
When you have recurrent microbial (usually bacterial) infections, unexplained inflammation or edema, or symptoms related to an autoimmune disorder; periodically to help monitor a known acute or chronic condition that affects the complement system
Sample Required?
A blood sample drawn from a vein in your arm
Test Preparation Needed?
None
Accordion Title
Common Questions
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How is it used?
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) 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
- Help diagnose and monitor the activity and treatment of acute or chronic autoimmune diseases such as lupus (systemic lupus erythematosus or SLE) and rheumatoid arthritis
- Monitor immune complex-related diseases and conditions such as glomerulonephritis (a kidney disorder), serum sickness, and vasculitis
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When is it ordered?
Complement testing may be ordered when a person has unexplained inflammation or edema or symptoms of an autoimmune disorder such as lupus. It may also be ordered when a healthcare 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 healthcare practitioner wants to monitor the current activity of a condition.
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What does the test result mean?
Complement levels may be decreased due to increased consumption (because of increased activation) 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 lupus 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.
Decreased complement activity may be seen with:
- Recurrent microbial infections (usually bacterial)
- Autoimmune diseases, including lupus and rheumatoid arthritis
- Hereditary angioedema
- Acquired angioedema
- Various types of kidney disease, including glomerulonephritis, lupus nephritis, membranous nephritis, and IgA nephropathy as well as kidney transplant rejection
- Cirrhosis
- Hepatitis
- Malnutrition
- Septicemia, shock
- Serum sickness (immune complex disease)
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:
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Is there anything else I should know?
Increased and decreased complement levels will not tell a healthcare practitioner what is wrong with a patient, but they can give an indication that the immune system is involved with a condition.
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What are the other parts of the innate immune system?
It includes:
- The phagocytic system (white blood cells, including neutrophils and monocytes/macrophages), whose function is to ingest and digest invading microbes
- Inflammatory mediators produced by various cells, including basophils, mast cells, and eosinophils
- Natural killer (NK), B and T cells that are specialized lymphocytes that kill some tumors cells, microorganisms, and cells that have been infected by viruses
- Acute phase reactants and cytokines, which are a group of soluble proteins that can cause changes in the growth of many cells, including the white blood cells that produce them