Complement Fixation Test

There are three diagnostic methods used to identify viruses; the direct, the indirect and serological techniques. Complement fixation test is one of the serological tests used in virology to identify the presence of specific antibodies or antigens. In this context, complements are proteins in series form within the patient's serum which act in response to an antibody antigen complex. In case the reaction takes place on a membrane, pores are produced causing damage to the cell.

• Commercially produced complements from guinea pig
• Patient's serum
• Red blood cells preferably from sheep

1. In complement fixation test, venous blood is collected from the patient and serum isolated.

2. It is natural for individuals to differ in the quantity of complement in serum. With the purpose of eliminating any negative effects these complements might have on the test results, the serum complements are substituted with standardized complement proteins. The complements are destroyed through heating, but care is taken to preserve the antibodies. This can be achieved because complements are vulnerable to heat than antibodies. Thereafter, the standard complements area added into the serum.

3. Into the serum, the antigens of concern are added and the preparation incubated.

4. Red blood cells are added into the preparation

In complement fixation test, incubation is necessary as it gives complements a period to fix itself onto antibody antigen complex, that is, if it is present. If this happens, complements will not be available to act on the red blood cells and cause hemolysis. As a result, a positive test is designated by the presence of the red blood cells. The cells are observed at the bottom of the beaker. On the contrary, a negative complement fixation test is indicated by the lysis of the red blood cells. The red color of hemoglobin is observed as it spreads in the beaker due to lysis.

Testing for antigens:
It is a common practice in serology to test for antibodies in complement fixation test. Nevertheless, it is feasible to test for antigens. In this case, the test serum is supplemented with antibodies which must be specific in nature. This induces the formation of complement.

Advantages and disadvantages:
The test is cheap as it does not call for expensive equipments and reagents. Again, a large number of viral infections can be screened using complement fixation test. However, it should be noted that the test is labor intensive and frequently non specific.

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