The Venereal Disease Research Laboratory (VDRL) test for syphilis is classified as which type of reaction?

• A. Agglutination reaction
• B. Flocculation reaction
• C. Hemagglutination reaction
• D. Precipitation reaction

Answer: (B)

The VDRL test is a flocculation reaction. It uses cardiolipin as the antigen and patient serum containing reagin antibodies. When these antibodies bind the cardiolipin, they form loose, fine aggregates that appear as visible clumps or a cloudy flocculent suspension. This is distinct from true precipitation (which forms a solid lattice in a gel), and from agglutination or hemagglutination (which require particulate targets like latex beads or red cells). Because the reaction relies on soluble antigen (cardiolipin) forming suspended aggregates with antibodies, the outcome is flocculation rather than a solid precipitate or particulate clumping.