Question
Complement fixation test.
Answer
The Complement Fixation Test (CFT) is a classical serological technique used to detect specific antibody (or antigen) by exploiting the fact that complement is consumed (“fixed”) when antigen-antibody complexes form, using this consumption as an indirect indicator of a specific reaction.
Principle: the test uses two systems —
- Test system: known antigen + patient’s serum (heat-inactivated to destroy the patient’s own complement) + a measured amount of guinea pig complement. If the serum contains specific antibody, antigen-antibody complexes form and fix (consume) the complement.
- Indicator system: sheep red blood cells (SRBC) pre-sensitized with anti-SRBC antibody (haemolysin), added after the test system has been allowed to react.
Interpretation: if specific antibody was present in the patient’s serum, complement was already used up in the test system, leaving none available to lyse the indicator SRBCs — the indicator system remains unlysed (no haemolysis), read as a positive result. If the patient’s serum lacked specific antibody, no antigen-antibody complex formed, complement remains free, and it is available to lyse the sensitized SRBCs when the indicator system is added — visible haemolysis, read as a negative result.
(Note the counter-intuitive reading: absence of haemolysis = positive test; presence of haemolysis = negative test.)
Applications: historically used for diagnosis of various infections including syphilis (Wassermann reaction, an early CFT), viral infections, and rickettsial/chlamydial diseases (e.g., lymphogranuloma venereum); largely superseded in routine practice by simpler, more sensitive techniques (ELISA), but remains conceptually important and is still used in some reference/research settings.
Limitations: technically demanding (requires careful titration of complement and controls), time-consuming, and requires fresh/properly stored complement.