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

Answer 1

The Correct Option is A

In microbiology, the scenario presented involves a standard agglutination test initially showing a negative result but turning positive upon dilution of the serum sample. This paradoxical finding is typically attributed to the "prozone effect." Let's delve into this phenomenon:

Concept	Explanation
Prozone Effect	A high concentration of antibodies in the serum can interfere with the formation of antigen-antibody complexes necessary for agglutination. Instead of observable clumping, these excess antibodies saturate the antigen sites and prevent visible reactions.

To better understand this:

• During agglutination tests, antigen-antibody interactions lead to visible clumping.
• However, when antibodies are present in extremely high concentrations, they can bind excessively to antigens and block proper lattice formation, preventing visible clumping.

After diluting the sample, the concentration of antibodies is reduced, allowing for adequate binding and lattice formation, which results in a positive agglutination reaction. This check confirms the presence of the prozone effect. As a result, the initial negative test was due to this high antibody concentration phenomenon, not because of the absence of antibodies.