Agency for Toxic Substances and Disease Registry Case Studies in Environmental Medicine (CSEM)
Disease Clusters: An Overview
Case Definition
The case definition is based on that which is most unusual about the disease cluster under investigation. The choice of a case definition depends on the information at hand. Often, several differing case definitions exist that might be selected in light of what is initially known about a presumed cluster.
Case definitions can be narrow or expanded. A narrow case definition focuses on the most unusual or most coherent group of diseases reported to affect the population of concern. An expanded case definition would likely include a larger number of diseases that were each likely to be related to one another by a common cause. For example, if the public health department were evaluating a possible cluster of an apparently infectious disease that resembled measles, they might choose either a narrow case definition (such as children with fever, cough, and morbilliform rash) or an expanded case definition (such as all people with fever). In the case of the teacher with breast cancer, evaluation of what seems unusual (the breast cancer cases) is most appropriate.
Narrow definitions tend to exclude some cases that might be related to the cluster. By contrast, expanded definitions often decrease the possibility of finding an explanation for the cluster. It is more difficult to hypothesize a unique cause for a variety of less-related diseases than for a single disease or a homogeneous group of diseases.
Case definitions can group together diseases that might share common causes: for example, the childhood cancer cases in Toms River Township, New Jersey, with drinking water exposure, exposure to ionizing radiation, or exposure to cigarette smoke. A case definition that encompassed all of the outcomes known to be highly associated with those exposures could be a rational choice in some cluster settings (when the cause of a disease outbreak is known in advance, the disease outbreak is more properly called a sentinel event, not a cluster). As a general rule, expanded case definitions generally work best when consistent scientific information exists about the presumed causal exposure (as for cigarettes or radiation).
Case definition has two applications:
for epidemiologic surveillance studies relating to the prevalence of the disease.
for diagnostic purposes using applicable diagnostic features, causes, and pathophysiology.
