Clinical utility of a quantitative Rose Bengal slide agglutination test in the diagnosis of human brucellosis in an endemic region.

BACKGROUND: Brucellosis currently ranks as the most important zoonotic disease in the world. Brucellosis is difficult to diagnose because patients often have nonspecific clinical symptoms that can be attributed to a number of disease agents prevalent in the area. Thus, this has necessitated the dependency of clinicians on microbiological confirmation, very often by sero diagnostic methods. Early and accurate detection of brucellosis is important if specific antibiotic treatment is to be effective for the patients. The use of RBST as a qualitative means of diagnosis is quiet common. However, to date, there are only a handful of reports of the application of RBST as a quantitative diagnostic method in medical literature. The potential usefulness of quantitative Rose Bengal slide agglutination test (RBST) for suspected brucellosis was evaluated as a simple, inexpensive diagnostic tool to be used in clinical practice in an endemic region.
METHODS: 200 consecutive patients who reported to Belgaum Institute of Medical Sciences (BIMS) Hospital, Belgaum, Karnataka (India) between June 2009 and December 2011 were studied. Standard RBST, quantitative RBST, standard tube agglutination test (SAT), 2-mercaptoethanol test (2-ME), and blood cultures were carried out on all patients. The case was confirmed as positive for brucellosis if any one of the tests was positive and the data was compared to the quantitative RBST considering blood culture result as gold standard.
RESULTS: B. melitensis was cultured in only 28% of the patients in this study. In patients with negative blood cultures, serology was used for diagnosis. The sensitivities were 88.9% (standard RBST), 92.6% (SAT), and 57.4% (2ME). The specificities were found to be 87.7% (standard RBST), 86.2% (SAT), and 95.7% (2ME). RBST titers > or = 1:8 were detected in a majority of patients (50, 74%) with bacteriologically proven brucellosis thereby guiding clinician for prompt therapy. Prozone reaction with RBST observed in 4 patients was an interesting finding and these four true cases would have been underdiagnosed and denied therapy on the basis of qualitative/standard RBST alone. The possibility of prozone in patient's serum with high RBST antibody titers can be avoided by testing several dilutions.
CONCLUSIONS: This technique has an immense value particularly for use in resource poor settings seen in rural areas. It can deliver definitive diagnosis in < 10 minutes to the clinician, which may in turn result in the early initiation of specific treatment and could be applied thus as a bedside methodology. It is not technically demanding and easy to interpret, does not involve heavy capital outlay, or trained personnel and, thus, is potentially useful in resource poor laboratories, particularly in developing regions. In addition, quantitative RBST demonstrates sensitivity and specificity equivalent to that achievable by performing SAT. It can readily be extended to screen a vast number of blood samples particularly in areas where brucellosis is hyperendemic. Quantitative RBST and 2ME have been noted to be of great value in therapeutic monitoring. Our data suggest that RBST titers in a range of 1:8 and 1:16 can undoubtedly be considered diagnostic of brucellosis in conjunction with compatible clinical and epidemiological evidence for the patients residing in areas endemic for the disease. Quantitative RBST is, therefore, recommended for routine use in clinical microbiology laboratories as an accurate and speedy diagnostic assay.