Case Definition

A case definition is a set of criteria used in making a decision as to whether an individual has a disease or health event of interest. Given that one of the goals of epidemiology is to quantify the magnitude of disease in a population, a case definition is an imperative step in this process because it establishes what criteria constitute a case of the disease. Case definitions are used both in ongoing public health surveillance and during outbreak investigations in field epidemiology.

A case definition should include several key characteristics. It should be clear, simple, and concise so that it can be easily applied to the population of interest. A case definition should be applied equally to all individuals being investigated. By applying the case definition in such a standardized way, the possibility of misclassification bias is minimized. Typically, a case definition includes both clinical and laboratory characteristics. These are ascertained by one or many methods that might include diagnosis by a physician, completion of a survey, and/or routine population screening methods. Individuals meeting a case definition can be categorized as ‘confirmed,’ ‘probable,’ or ‘suspected.’

Case definitions are used in ongoing public health surveillance to track the occurrence and distribution of disease within a given jurisdiction. In the United States, the Centers for Disease Control and Prevention have published a list of uniform case definitions for the mandatory reporting of several diseases called ‘Case Definitions for Infectious Conditions Under Public Health Surveillance.’ This list provides explicit case definitions for diseases of interest so that they can be reported by clinicians to public health authorities in a standard and uniform way across geographic locations. This is particularly useful for studies that compare the prevalence of disease across regions as they can use the same case definitions and, therefore, obtain a more accurate picture of disease.

The establishment and application of case definitions are also critical components of outbreak investigations in field epidemiology. A case definition is developed at an early stage of the outbreak investigation so that individual cases can then be identified. While the same criteria apply for developing a case definition in routine public health surveillance, in an outbreak investigation a case definition may also include information regarding person, place, and time, in addition to clinical and laboratory characteristics. For example, a case definition developed for a foodborne outbreak may include only those individuals who ate at a certain restaurant during a specified period of time. Furthermore, a case definition may initially be more broadly defined in an outbreak investigation scenario. This is done to increase sensitivity and therefore capture as many true cases as possible and minimize missing true cases. As the investigation continues and more knowledge is gained about the nature of the cases, the definition may be narrowed and therefore more specific. This is particularly important for a newly emerging disease where a standard case definition does not yet exist. An example of this was the outbreak of Severe Acute Respiratory Syndrome in Toronto in 2003 where a new case definition had to be developed as this was a new disease.

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