By a case definition we mean the standard criteria for categorizing an individual as a case. Establishing a case definition (the criteria that need to be met in order to be considered "a case") can be tricky, particularly in the initial phases of the investigation. You want your definition to specific enough to identify true cases of disease, but you also want it to be broad enough and sensitive enough that it will identify most, if not all of the cases. As a result, the case definition may change during the investigation. In the earliest stages, it might be broader and less specific in order to make sure you identify all of the potential cases ("possible" cases), but later on, it might include more specific clinical or laboratory criteria that enable you to categorize individuals as "probable" or "confirmed" cases.
Case definitions may include four types of information:
- clinical information such as symptoms or lab results, e.g. the presence of fever >101o F and jaundice for hepatitis A or the presence of elevated IgM anti-HAV antibodies in an outbreak of hepatitis A
- personal characteristics of the cases, e.g., individuals in a certain age group
- limits with respect to the location of the case (e.g., residing or working on the South Shore of Massachusetts)
- a specified time period for this particular outbreak (e.g., during February and March 2009 or among people who attended a specific wedding)