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Purpose of the test

  1. A complement test may be used to monitor patients with an autoimmune disorder and to see if treatment for that condition is working. For example, patients with active lupus erythematosus may have lower-than-normal levels of the complement proteins C3 and C4.
  2. To Diagnose angioedema.
  3. This is done to monitor the activity of the disease in a patient with Systemic lupus erythematosus (S L E), Nephritis, membranoproliferative and post-streptococcal nephritis.


  1. If the sample is left at room temperature even for one hour then the value will be low.
  2. After getting the blood tries to separate serum as soon as possible and can freeze it for storage.


  1. A complement is a group of globulin proteins and these act as enzymes.
    1. These enzymes facilitate the immune system and give rise to the inflammatory response.
  2. The complement system is important to destroy the foreign body which may be even cells (Antigens).
  3. Complement consists of a group of 25 proteins given numerical names as C1 to C9 and sub-units.
    1. These are present in the serum and give a nonspecific inflammatory response to various antigens.
  4. Classical pathway: Compliment may be activated by classical pathway where immunoglobulin IgM and IgG activate complement C1 and ultimately C8 and C9.
  5. Alternative pathways: where there is direct activation of a C3 and the same chain reaction leading to activate C8 and C9.


  1. Complement is part of the immune system and particularly C3 is very significant to prevent infections and gives rise to the inflammatory response.
    1. C 3 is considered the heart of the system.
  2. Complement C3 is a blood test that measures the activity of a certain protein that is part of the complement system.
  3. This occupies the position where Classical and alternative pathways join and from there common pathway or terminal pathway starts.
  4. This test is performed to find out the deficiency of Complement C 3.
  5. Activated complement will lead to:
    1. Increased vascular permeability.
    2. Attracts White blood cell to the site of antigen and antibody.
    3. They act as a chemotactic agent.
    4. They increase phagocytosis phenomenon.
    5. They increase adherence of antibody to the antigen.
  6. C3 is made in the liver and less amount in the spleen, and other lymphoid organs.
  7. C4 is made in the lung and bone.

Complement pathway

  1. Classical pathway: The antigen form Antibody IgM or IgG which binds to C1q and then enzymatically give rise to chain reaction activating C4, C2 C3 and ultimately C8 and C9.
  2. Alternate pathway There is direct activation of C3 and there is a bypass of factor 1,2 and 4.
  3. These two complements (C8 and 9) cause damage to the membrane of antigens and destroy it. These basically act like drill machine.
  4. The activated C3 attracts inflammatory cells to the site ( chemotaxis). and also increase vascular permeability.

The normal range

Source 1

Sample Normal value mg/dL
Cord serum 65 to 111.8
Children 6 months 86.6 to 135.8
Adult 83 to 177

Another source =  Adult value =  76 to 142 mg/dL

Source 2

Decreased complement activity may be seen in:

  1. Hereditary angioedema.
  2. Bacterial infections (especially Neisseria).
  3. Cirrhosis.
  4. Poststreptococcal Glomerulonephritis (there is extremely low level).
  5. Hepatitis.
  6. Lupus nephritis.
  7. Malnutrition.
  8. Kidney transplant rejection.
  9. Systemic lupus erythematosus.
  10. Congenital C3 deficiency:
    1. C3 deficiency associated with recurrent pyogenic infection.
    2. C3 may be decreased due to congenital deficiency of control proteins, factor H, and factor I.
  11. Acquired C3 deficiency:
    1. This may be seen decreased in various types of infections and inflammatory conditions.
    2. The primary cause is an immune complex disease like SLE, Rheumatoid arthritis, Subacute bacterial endocarditis.
    3. Viremia, parasitemia, and bacterial sepsis.
    4. In membranous glomerulitis.

Increased C3 may be seen in:

  1. Acute phase protein in Rheumatic fever, Viral Hepatitis, myocardial infarction, cancers, diabetes, and pregnancy.
  2. in Sarcoidosis and Amyloidosis.
  3. Thyroiditis.
  4. Inflammatory bowel disease (Ulcerative colitis).
  5. Typhoid fever.
  6. Pneumococcal pneumonia.


Possible References Used

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