A Case of Brucellosis Presenting as High Titer Negative Result by Standard Tube Agglutination Test

Abstract

Since prozone is a well known phenomenon in the serologic diagnosis of Brucella infections, it is necessary to prepare higher serum dilutions in the standard tube agglutination (STA) test for the brucellosis suspected patients. However, due to limited economical support, the serum dilutions generally last at 1/320-1/640 titers in some laboratories in Turkey. In this report, a brucellosis case whose STA test was found negative until the titer of 1/1280, has been presented. A 36-year-old female was admitted to our hospital with the complaints of fever, sweating, fatigue, generalized arthralgia and weight loss, lasting for 45 days. Hepatosplenomegaly was detected in the physical examination, and laboratory tests yielded anemia, leucopenia, elevated erythrocyte sedimentation rate and high C-reactive protein levels. Although brucellosis was suspected, Brucella STA test was found negative at 1/640 titer. On the sixth day of admission, Brucella melitensis was isolated from her blood culture. Since a positive result at 1/40 titer was detected in Brucella STA test with the use of Coombs antiserum, the patient's serum was retested at higher dilutions than 1/640, and positive result was obtained starting from 1/1280 dilution and extended to 1/5120 titer. The patient was treated with rifampin and doxycyline and discharged with complete cure. In conclusion, in countries endemic for brucellosis, STA test should be performed at 1/1280 or higher titrations in suspected patients especially in the presence of negative culture results, for the prevention of false negative results due to prozone phenomenon.