In the complement fixation test (CFT) used for serodiagnosis, a NEGATIVE result (patient serum does NOT contain antibody to the antigen) is indicated by:

• A Hemolysis of sheep RBCs in the indicator system ✓
• B No hemolysis of sheep RBCs (button of intact RBCs at the bottom of the well)
• C Precipitation of antigen-antibody complexes visible as a visible line
• D Agglutination of sensitized latex particles

Explanation

The CFT has two stages. In the first stage, patient serum (possibly containing antibody) is incubated with specific antigen and a fixed amount of complement. If patient antibody is present, it forms immune complexes that fix (consume) complement. In the second (indicator) stage, sensitized sheep RBCs (coated with anti-SRBC antibody) are added. If complement was fixed in stage 1 (positive result), no free complement remains — RBCs are NOT lysed (no hemolysis = positive test). If patient antibody is absent (negative test), complement is NOT fixed and remains free to lyse the indicator RBCs — hemolysis occurs, indicating a negative result.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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Written and medically reviewed by the StethoPrep medical team.