BACKGROUND: Case-control study designs are often used for both investigation of foodborne outbreaks and to identify risks of routine enteric illnesses. However, this methodology requires intensive time and resources and is prone to selection and recall biases. Case-case studies are more recently being utilized as potentially valid alternatives, particularly to generate hypotheses. These designs can be conducted in less time, with far fewer resources and, given the highly selective nature of foodborne case reporting, both selection and recall bias are greatly reduced. There is, however, considerable variation in the literature on how these designs are implemented and how effect measures are interpreted.
METHODS: An extensive literature review of investigations of foodborne diseases was used to outline the strengths and limitations of case-case and case-control study designs. This led to a set of proposed standardized procedures for selection of comparison cases, utilization of common terminology and potential future uses.
RESULTS: Advantages and limitations of each study design are compared based on epidemiologic standards such as selection bias, recall bias and generalizability, as well as more applied standards such as time, cost and effort. Standard approaches for the selection of comparison cases are delineated in several ways. Case selection should be based on whether the aim of study is investigate an outbreak of foodborne disease or to study trends in routine transmission. For outbreak associated cases, linkage through genetic testing such as PFGE must also be taken into account with recommendations given. Interpretation of the resulting odds ratio, which may vary from a case-control study, is also given with examples.
CONCLUSIONS: This standardization of study design based on the case status and type will assist both public health agencies and researchers in the use of this design. This will allow more comparable results and, potentially, provide more efficient and timely identification of the cause of a foodborne outbreak.