In the complement fixation test (CFT), a positive result (disease/antibody present) is indicated by:
- A Presence of hemolysis — complement is activated by Ag-Ab complexes to lyse sensitized RBCs
- B Precipitation of antigen-antibody complexes visible as a line in agar gel
- C Agglutination of sensitized sheep RBCs by complement-antibody complexes
- D Absence of hemolysis — complement is fixed by test Ag-Ab complexes and is unavailable to lyse RBCs ✓
Explanation
In CFT, serum (heat-inactivated to remove endogenous complement) is mixed with specific antigen and a known amount of complement; if specific antibody is present, Ag-Ab complexes form and FIX (consume) all complement via classical pathway; in the indicator system (sensitized sheep RBCs + amboceptor), no free complement remains to lyse the RBCs → NO hemolysis = POSITIVE result. If no antibody is present, complement remains free, lyses sensitized sheep RBCs → hemolysis = NEGATIVE result. The counterintuitive reading (absence of hemolysis = positive) is a classic exam point.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.