The false sero-negativity of brucella standard agglutination test: Prozone phenomenon

Abstract

Objectives: We aimed to assess prozone phenomenon that is quite rare and causes false negativity in serological diagnosis of brucellosis with standard dilution titers. Materials and methods: In this study the tests of four cases that have false negative serological results were evaluated. Blood cultures were obtained from all cases while cerebrospinal fluid cultures were studied in the two cases. Standard agglutination test (SAT) and Coombs test were performed to all patients. Results: SAT and Coombs test was negative in titers up to 1/640 in all cases. The SAT and Coombs tests in cerebrospinal fluid (CSF) of the two cases with neurobrucellosis diagnosis were negative, as well. Since the clinical and laboratory findings suggested the brucellosis, the serums were restudied by diluting up to 1/10240 titer and we saw that the first 3 cases became positive at a titer of 1/1280. The fourth case remained negative and therefore, we applied high dilution Coombs test. This time the test gave a positive result at 1/10240 titer beginning from 1/2560 titer. B.melitensis was isolated from two cases. Conclusion: SAT and Coombs’ test must be diluted to titers 1/2560 or more in order to exclude false sero-negativity in cases with clinical and laboratory findings suggesting brucellosis. J Microbiol Infect Dis 2011; 1(3):110-113