Complement fixation test
The complement fixation test is an immunological test wherein the binding of the complement is observed to detect the presence of an antigen or antibody as a result of their interaction. The interaction between a particular antigen and a specific antibody may either be through immune complexes (i.e. classical pathway) or by particular surfaces (i.e. alternative pathway). Thus, it is used to diagnose diseases and infections. It is particularly useful in identifying bacterial pathogens that are difficult to grow in cultures. However, ELISA and PCR methods are now preferred over complement fixation test as serological method for clinical diagnosis.
The complement fixation test is a two-step process. The antibody from the patient serum and the antigen are mixed with fresh complement. Sensitized sheep cells are then added. When the patient antibody is absent, the complement will be able to bind to the antibody-coated sheep cells and cause hemolysis. But when the antibody is present, the antigen-antibody complex binds to the complement, and therefore, no hemolysis will occur. When there is no hemolysis, it indicates a positive reaction.1