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Complement Fixation Test

Complement Fixation Test. Lab 10. Introduction. The complement fixation test is an immunological medical test that can be used to detect the presence of either specific antibody or specific antigen in a patient's serum.

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Complement Fixation Test

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  1. Complement Fixation Test Lab 10

  2. Introduction • The complement fixation test is an immunological medical test that can be used to detect the presence of either specific antibody or specific antigen in a patient's serum. • It was widely used to diagnose infections, particularly with microbes that are not easily detected by culture methods. • However, in clinical diagnostics labs it has been largely superseded by other serological methods such as ELISA and by DNA-based methods of pathogen detection, particularly PCR.

  3. Process • Serum is isolated from the patient. • Patients naturally have different levels of complement proteins in their serum. To negate any effects this might have on the test, the complement proteins in the patient's serum must be destroyed • A known amount of standardized complement proteins added • The antigen of interest is added to the serum. • Sheep Red Blood Cells (sRBCs) which have been pre-bound to anti-sRBC antibodies are added to the serum.

  4. Process • If the patient's serum contains antibodies against the antigen of interest, they will bind to the antigen in step 3 to form antigen-antibody complexes. • The complement proteins will react with these complexes and be depleted. Thus when the sRBC-antibody complexes are added in step 4, there will be no complement left in the serum. • However, if no antibodies against the antigen of interest are present, the complement will not be depleted and it will react with the sRBC-antibody complexes added in step 4, lysing the sRBCs and spilling their contents into the solution, thereby turning the solution pink

  5. Test outcomes The test is considered negative if the solution turns pink at this point and positive otherwise.

  6. Testing for antigen • While detection of antibodies is the more common test format, it is equally possible to test for the presence of antigen. In this case, the patient's serum is supplemented with specific antibody to induce formation of complexes; addition of complement and indicator sRBC is performed as before.

  7. Test Evaluation • Advantages of CFT • Ability to screen against a large number of viral and bacterial infections at the same time. • Cheap • Disadvantages of CFT • Not sensitive - cannot be used for immunity screening • Time consuming and labor intensive • Often non-specific

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