Complement
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 nearly 60 proteins that are in blood plasma or on the surface of some cells. The proteins work with your immune system and play a role to protect the body from infections, and to remove dead cells and foreign material. Rarely, people may inherit deficiency of some complement proteins. These people are prone to certain infections or autoimmune disorders.
There are nine major complement proteins. They are labeled C1 through C9. This article describes the test that measures total complement activity.
How the Test is Performed
A blood sample is needed. This is most often taken through a vein. The procedure is called a venipuncture.
Venipuncture
Venipuncture is the collection of blood from a vein. It is most often done for laboratory testing.
Read Article Now Book Mark ArticleHow 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. Others may 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.
Autoimmune disorder
An autoimmune disorder occurs when the body's immune system attacks and destroys healthy body tissue by mistake. There are more than 80 autoimmune d...
Read Article Now Book Mark ArticleLupus erythematosus
Systemic lupus erythematosus (SLE) is an autoimmune disease. In this disease, the immune system of the body mistakenly attacks healthy tissue. It c...
Read Article Now Book Mark ArticleC3
Complement C3 is a blood test that measures the activity of a certain protein. This protein is part of the complement system. The complement system ...
Read Article Now Book Mark ArticleC4
Complement component 4 is a blood test that measures the activity of a certain protein. This protein is part of the complement system. The compleme...
Read Article Now Book Mark ArticleComplement 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.
Rheumatoid arthritis
Rheumatoid arthritis (RA) is a disease that leads to inflammation of the joints and surrounding tissues. It is a long-term disease. It can also aff...
Read Article Now Book Mark ArticlePeople with some bacterial blood infections 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.
Shock
Shock is a life-threatening condition that occurs when the body is not getting enough blood flow. Lack of blood flow means the cells and organs do n...
Read Article Now Book Mark ArticleMalaria
Malaria is a parasitic disease that involves high fevers, shaking chills, flu-like symptoms, and anemia.
Read Article Now Book Mark ArticleNormal Results
The normal results for this test are:
- Total blood complement level: 41 to 90 hemolytic units
- C1 level: 14.9 to 22.1 mg/dL
- C3 levels: 88 to 201 mg/dL
- C4 levels: 15 to 45 mg/dL
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:
- Cancer
- Certain infections
- Ulcerative colitis
Ulcerative colitis
Ulcerative colitis is a condition in which the lining of the large intestine (colon) and rectum become inflamed. It is a form of inflammatory bowel ...
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Decreased complement activity may be seen in:
- Cirrhosis
Cirrhosis
Cirrhosis is scarring of the liver and poor liver function. It is the last stage of chronic liver disease.
Read Article Now Book Mark Article - Glomerulonephritis
Glomerulonephritis
Glomerulonephritis is a type of kidney disease in which the part of your kidneys that helps filter waste and fluids from the blood is damaged....
Read Article Now Book Mark Article - Hereditary angioedema
Hereditary angioedema
Hereditary angioedema is a rare but serious problem with the immune system. The problem is passed down through families. It causes swelling, partic...
Read Article Now Book Mark Article - Hepatitis
- Kidney transplant rejection
Kidney transplant
A kidney transplant is surgery to place a healthy kidney into a person with kidney failure.
Read Article Now Book Mark Article - Lupus nephritis
Lupus nephritis
Lupus nephritis, which is a kidney disorder, is a complication of systemic lupus erythematosus.
Read Article Now Book Mark Article - Malnutrition
- Systemic lupus erythematosus
- Rare inherited complement deficiencies
Risks
Risks associated with having blood drawn are slight, but may include:
- Excessive bleeding
- Fainting or feeling lightheaded
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
Considerations
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.
Reviewed By
Neil J. Gonter, MD, Assistant Professor of Medicine, Columbia University, NY and private practice specializing in Rheumatology at Rheumatology Associates of North Jersey, Teaneck, NJ. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.
Bean KV, Massey D, Gupta G. Mediators of inflammation: complement. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 47.
Ram S. Complement and deficiencies. In: Bennett JE, Dolin R, Blaser MJ, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 9th ed. Philadelphia, PA: Elsevier; 2020:chap 9.
Sullivan KE, Grumach AS. The complement system. In: Burks AW, Holgate ST, O'Hehir RE, Broide DH, et al. eds. Middleton's Allergy: Principles and Practice. 9th ed. Philadelphia, PA: Elsevier; 2020:chap 8.