What’s the Complement Fixation Test?

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The complement fixation test is an immunological medical test that detects specific antibodies in a person’s blood. This test was once used as a means of diagnosing infectious disease, but nowadays polymerase chain reaction and other DNA detection methods are more commonly employed. The value of the complement fixation test now lies in its ability to diagnose autoimmune diseases or monitor certain chronic conditions.

Complement proteins are those involved in an immunological reaction called the complement cascade. This reaction occurs in the presence of bacterial pathogens and causes those pathogens to be coated with proteins that allow immune cells to ingest and destroy the bacteria. The process, called opsonization, requires the presence of protein-specific antibodies on the surface of the bacteria. Proteins in the complement cascade can also kill directly by coating bacteria in molecules that cause them to burst, which can occur in the absence of specific antibodies.

A doctor sometimes orders a complement fixation test for a patient who is experiencing repeated infections. The test sample is a small amount of blood, usually taken from a vein in the arm, with no special preparation required for the patient. After the sample has been taken, it is processed to separate the liquid serum from the blood that has been allowed to clot. The complement fixation test is then performed on the serum.

Before the test can be performed, further processing is required, to destroy the patient’s complement proteins. This is necessary because the complement fixation test measures how quickly the patient’s antibodies react to complement, and the concentrations and activity levels of complement vary from person to person. To obtain standardized results, the patient’s complement is destroyed and a sample of complement of known concentration and activity level is used instead.

At this point, the complement sample is ready for testing and the antigen of interest is added to the test sample. The antigen is specific to a particular species of pathogen or autoimmune antibody. Sheep red blood cells, bound to cell-specific antibodies, are then added to the serum sample.

If the sample contains antibodies specific to the antigen being tested, the antibodies will react with the complement that has been added. This will deplete all complement in the sample and there will be nothing in the sample that will react with the sheep cells; however, if specific antibodies are not present, the complement will not be used. In this case, the complement will cause sheep red blood cells to burst, coloring the sample pink. Therefore, if the sample turns pink, it is a negative result and if the sample remains clear, it is a positive result.

An additional use of this test is to monitor a person who has a known autoimmune disease. In this case, testing the activity levels of the complement antigen-specific can be a good way to measure the effectiveness of a given treatment. This is often done for people with systemic lupus erythematosus (SLE), an autoimmune disease that develops when the body’s immune system reacts against proteins in cell nuclei. The complement fixation test is also useful for diagnosing central nervous system infections because the test does not require a CSF sample, which is much more difficult to obtain.

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