One of the core tasks that epidemiologists do is to measure disease occurrence in a population, for example within a surveillance system. The second major task is to compare those occurrences between sub groups of the population, analyse and interpret those differences (e.g. in outbreak investigations or other field investigations).

To measure disease occurrence we need to first count patients with a specific disease. To do so epidemiologists first define the disease. This is the case definition. It is a set of standard criteria used to decide whether a person can be counted as having a particular disease or not. By using a standard case definition we make sure that all counted cases of the same disease have been identified the same way regardless of whom has identified the case. We should emphasize here that the epidemiological meaning of a case definition may differ from the clinical and biological meaning. The case definition is a tool to count cases. It is not a tool to make a diagnosis and treat a patient. A standard case definition implies that some people with the disease will not comply with the case definition criteria and that some without the disease may be counted as cases.

Learning Objectives

After reading this chapter, you will be better able to:

  • understand how to define a case in the context of public health investigations
  • understand the difference between a case definition in public health and a clinical diagnosis in the health care setting
  • apply various levels of case definitions (e.g. possible, probable,confirmed)