Complement Fixation
Assay

In it's simplest form the test is used to detect a patient serum antibody, so an ANTIGEN that is recognized by that antibody is the first reagent shown. If the antibody is present in the patient's serum it binds to the antigen, and the complement reagent is completely consumed in the reaction. (The test can also be used to look for antigen in the serum by modifying the reagents used).

The complement fixation assay indicator system uses sheep red blood cells (SRBC) and anti-SRBC antibody.
If the antibody specific for the antigen in the assay is present in the patient's serum, then complement is completely consumed in the reaction and there is none left to bind to the SRBC/anti-SRBC complexes.

A Test Positive For Ab =
NO HEMOLYSIS

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Negative Test Results

Complement Fixation
Assay

If there is NO ANTIBODY PRESENT in the patient's serum the antigen is not bound, and the complement reagent does not have immune complexes with which to react.

Complement is still present in the indicator reaction and binds strongly to the SRBC/anti-SRBC complexes. This causes the SRBCs to burst in a process called hemolysis.

A Test Negative For Ab = LOTS OF HEMOLYSIS

Biomedical Laboratory Information

Last modified 11-21-03.