Case definition. In order to gain understanding about the causes and outcomes of a disease, epidemiologic investigations are conducted. Definition of what constitutes a case is one of the first steps in an epidemiologic investigation. The purpose of case definition is to ensure that cases represent the same entity (Xxxxxxxx, 1997). In the setting of an “undefined” or “new” disease, the preliminary case definition is usually a list of the symptoms and signs shared by the patients, or a clinical syndrome. In other words, it might be said that case definition in the new disease setting is the description of a group of people who have homogenous disease characteristics. As new information about the disease is discovered, for example a pathogen that is associated with the collection of symptoms and signs, the case definition and other disease definitions (e.g. diagnostic methods) are modified accordingly. AIDS is a classic example of an evolving case definition (Xxxxx and Xxxxxxx, 1994; Tyler and Last, 1998). The preliminary case definition published by the Centers for Disease Control was a list of disease manifes- tations: lymphopenia, opportunistic infections with no apparent reason for immunosuppression, and others (CDC, 1982). The case definition was modified as researchers learned about the biology of the disease, most notably to include infection by the HIV virus (CDC, 1985).
Case definition
