© 2019 Refik Saydam National Public Health Agency (RSNPHA).Brucellosis, which is common worldwide, especially the Middle East and the Mediterranean countries, is an zoonotic infectious disease. Our country also takes place a part of the endemic region. Although the incidence decreases with the measures taken over the years, the public health aspect of the disease continues. Consumption of dairy products that were obtained without heat treatment from ill animals is the most common transmission source. The clinical of the disease is highly variable and can be seen in a wide and different clinical spectrum from asymptomatic to serious complications that may lead to death. The diagnosis is based on the laboratory diagnosis as well as the clinical presentation, as in many infectious diseases. Serological tests still are the most common methods for diagnosis as well as culture and molecular tests. As a part of serological tests, Brucella tube agglutination tests are applied frequently for diagnosis. Brucella canis infection, prozone phenomenon, presence of blocking antibody, agammaglobulinemia, acute infection period in which antibody production is not sufficient yet are common diagnostic faults. Diagnosis with appropriate interventions such as culture, molecular methods, use of specific antigens, use of Coombs' antibody, increasing dilution is necessary approaches to prevent false serological evaluations. In our country, when the number of cases decreases with the eradication programs of brucellosis in farm animals carried out by the Ministry of Agriculture and Forestry, it is expected that the occupational disease character of the disease will be obvious, and Brucella canis related Brucellosis will increase relatively. For this reason, it is very crucial to use the diagnostic tests appropriately and to pay attention to the details, to keep away from the misdiagnoses, to avoid wasting time and examination, and to be careful in the early diagnosis in cases suspected of brucellosis clinically. In this review, the causes of seronegativity in brucellosis and issues that should be taken into consideration in terms of diagnosis for these causes.