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Noroviruses are believed to be the most common etiologic agent of foodborne outbreaks of gastroenteritis, yet diagnostic tests for these agents are not readily available in the United States. In the absence of assays to detect norovirus, several clinical and epidemiologic profiles—the criteria of Kaplan et al. (vomiting in >50% of patients, mean incubation period of 24–48 h, mean duration of illness of 12–60 h, and no bacterial pathogen) and the ratios of fever to vomiting and diarrhea to vomiting—have been used to distinguish foodborne outbreaks of gastroenteritis caused by noroviruses from those caused by bacteria.To examine how well clinical and epidemiological profiles discriminate between foodborne outbreaks of gastroenteritis due to noroviruses and those due to bacteria and to estimate the proportion of reported outbreaks that might be attributable to noroviruses, we reviewed subsets of the 4050 outbreaks reported from 1998 to 2000.The set of criteria of Kaplan et al. is highly specific (99%) and moderately sensitive (68%) in discriminating confirmed outbreaks due to bacteria from those due to norovirus and was the most useful diagnostic aid evaluated. Each individual component of the criteria, the fever-to-vomiting ratio, and the diarrhea-to-vomiting ratio were more sensitive, yet less specific, and therefore less useful, than the criteria of Kaplan et al. We estimated that, at a minimum, 28% of all the foodborne outbreaks reported to the Centers for Disease Control and Prevention may be attributed to norovirus on the basis of these criteria.Until norovirus diagnostic tests become widely available, the criteria of Kaplan et al. remain the most useful and discriminating diagnostic aid to identify foodborne outbreaks of gastroenteritis due to noroviruses.