Brucellosis is an important public health problem that is seen commonly in our country and the world. Although the absolute diagnosis of brucellosis requires isolation of the bacterium from blood or bone marrow, serological methods are used frequently because of antibiotic use before the diagnosis and the fact that culture results may not become available for a long time. False negative results due to the prozone phenomenon may be seen in the serum agglutination test that is commonly used in the serological diagnosis of brucellosis. The inhibition of agglutination at low dilutions due to an excess of antibodies or to nonspecific serum factors is called the prozone phenomenon. This case is reported to draw attention to the prozone phenomenon that should be recognized when there is a strong suspicion of brucellosis and the importance of clinical suspicion in the diagnosis of this disorder.

Keywords: Agglutination test, Brucellosis, Serum