Question

A patient was suspected of having brucellosis. Serum sample was sent for a standard agglutination test. It came negative initially but after dilution of the serum sample, the test was positive. What could be the reason for the initial negative test?

• A. Prozone
• B. Postzone
• C. Complement inactivation
• D. Antigen antibody complexes

Hint:

When an agglutination test is negative at high antibody concentration but positive after dilution, recall which zone of the antigen-antibody reaction is responsible.

Answer 1

The Correct Option is A

Step 1: Understand the scenario. The agglutination test (Standard Agglutination Test / SAT for brucellosis) was negative at the undiluted concentration but became positive after dilution. This is a classic scenario of a false negative due to antibody excess.

Step 2: Define the Prozone phenomenon. The Prozone phenomenon (also called the Hook effect) occurs when there is an excess of antibodies relative to antigen. In this zone of antibody excess, each antigen is saturated by antibodies and there are insufficient antigens for antibody molecules to cross-link (bridge) between antigen particles, so no visible agglutination occurs. The result appears falsely negative.

Step 3: Explain how dilution corrects it. When the serum is diluted, antibody concentration decreases until an optimal antigen-antibody ratio (equivalence zone) is reached, allowing proper lattice formation and visible agglutination.

Step 4: Differentiate from Postzone. In the Postzone phenomenon, there is excess antigen (not antibody), leading to soluble immune complexes and also no visible agglutination. However, the initial sample here is undiluted serum (antibody-rich), so the initial false negative is due to Prozone.

Step 5: SAT for brucellosis. False negative SAT is seen in the Prozone phenomenon and in the presence of incomplete (blocking) antibodies.

Answer: Prozone