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What Is the Complement Fixation Test?

By Emma Lloyd
Updated Mar 03, 2024
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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 illnesses, but these days the polymerase chain reaction, and other methods of DNA detection, is more commonly employed. The value of the complement fixation test now lies in its ability to diagnose autoimmune disorders, or to 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 results in the coating of those pathogens by proteins that allow immune cells to ingest and destroy the bacteria. The process, called opsonization, requires the presence of antibodies specific to proteins on the surface of the bacteria. Proteins in the complement cascade can also directly kill by coating bacteria in molecules that cause them to burst, something that may occur in the absence of specific antibodies.

A doctor will sometimes order a complement fixation test for a patient who is experiencing repeated infections. The test sample is a small amount of blood, typically taken from an arm vein, 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 has been allowed to coagulate. The complement fixation test is then carried out on the serum.

Before the test can be carried out, some additional processing, to destroy the patient’s own complement proteins, is required. This is necessary because the complement fixation test measures the rate at which the patient’s antibodies react to complement, and complement concentrations and activity levels vary from person to person. To achieve standardized results, the patient’s complement is destroyed, and a complement sample of known concentration and activity level is used instead.

At this stage, the complement sample is then 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 to autoimmune antibodies. Sheep red blood cells, bound to antibodies specific to the cells, are then added to the serum sample.

If the sample contains antibodies specific to the antigen being tested for, then the antibodies will react with the complement which was added. This will cause all the complement in the sample to be used up, and there will be nothing in the sample to react with the sheep cells; however, if the specific antibodies are not present, the complement will not be used up. In this case, the complement will cause the sheep red blood cells to burst, staining the test 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 for this test is in monitoring a person who has a known autoimmune disease. In this case, testing levels of antigen-specific complement activity can be a good way to measure the efficacy of a certain treatment. This is often carried out 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 infections of the central nervous system because the test does not require a sample of cerebrospinal fluid, which is much more difficult to obtain.

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