The complement fixation test (CFT) is used for serological diagnosis of several infections. In a positive CFT, what is the expected result when sheep red blood cells and hemolysin (sensitizing antibody) are added as an indicator system?

• A Hemolysis occurs (positive result) because complement was not fixed by the antigen-antibody complex
• B No hemolysis occurs (positive result) because complement was fixed (consumed) by the patient's antigen-antibody complex, leaving none to lyse the indicator RBCs ✓
• C Hemolysis occurs only in the control well, not the patient well
• D The red cells agglutinate but do not lyse, indicating a positive CFT

Explanation

In CFT, a two-step reaction occurs: (1) patient serum + antigen + fixed amount of complement are incubated — if specific antibody is present in patient serum, immune complexes form and fix (consume) all complement; (2) sensitized sheep RBCs (coated with anti-sheep RBC hemolysin) are added as indicator. If complement was consumed (positive test), there is no free complement to lyse indicator RBCs → NO hemolysis = positive result (tubes remain cloudy/red). If the patient has no antibody, complement remains free → HEMOLYSIS = negative result (tubes clear). This counterintuitive logic requires careful interpretation.

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

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