Complement fixation test (CFT)
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The complement fixation test is a traditional test used to detect the presence of specific antigens or antibodies. It involves incubating a patient's serum sample with a known antigen, then checking if any complement was activated and "fixed" or bound by the formation of antigen-antibody complexes. If complexes formed, the complement is fixed and will not react with indicator cells, showing a positive result. If no complexes formed, free complement will react with the indicators, showing a negative result. While economical for screening multiple infections, it is not very sensitive and can produce non-specific results.
Complement fixation test (CFT)
- 1. Dr. J. P. Saranraj COMPLEMENT FIXATION TEST
- 2. COMPLEMENT FIXATION TEST The Complement system is a part of the immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promotes inflammation, and attacks the pathogen's cell membrane. It is part of the Innate immune system. The Complement fixation test (CFT) is one of the major traditional tests for the demonstration of presence of specific antigens or antibodies. CFT was extensively used in Syphilis serology after being introduced by Wasserman in 1909. CFT consist of 2 steps.
- 3. First step – Complement fixation stage A known antigen and inactivated patient’s serum are incubated with a standardized, limited amount of Complement (Patient’s serum is heated at 56 °C for 30 minutes to inactivate endogenous complement which may disturb the test calibration). If the serum contains specific, complement activating antibody the complement will be activated or fixed by the Antigen-antibody complex. If there is no antibody in the patient’s serum, there will be no formation of antigen-antibody complex, and therefore complement will not be
- 4. Second step – Indicator Stage The second step detects whether complement has been utilized in the first step or not. This is done by adding the Indicator system. If the Complement is fixed in the first step due to the presence of antibody in the patient’s serum there will be no complement left to fix to the indicator system. If there is no specific antibody in the patient’s serum, there will be no antigen-antibody complex, and therefore, complement will be present free or unfixed in the mixture. This unfixed complement will now react with the Antibody - coated sheep RBCs to bring about
- 6. Results Positive test : The available complement is fixed by Ag-Ab complex and No hemolysis of sheep RBCs occurs. So the test is positive for presence of antibodies. Negative test: No Ag-Ab reaction occurs and the complement is free. This free complement binds to the complex of sheep RBC and it’s antibody to cause Hemolysis, causing the development of pink color.
- 7. Advantages Ability to screen against a large number of viral and bacterial infections at the same time. Economical. Disadvantages Not sensitive - cannot be used for immunity screening. Time consuming and labor intensive. Often non-specific e.g. Cross - reactivity between Herpes Simplex Virus and Varicella Zoster Virus.