Suppose you are asked to estimate the population prevalence of attention deficit hypersensitivity disorder (ADHD) among U.S. school-age children. How will you identify the children who should be counted as cases having ADHD? What defines a ‘case’?

The definition of a ‘case’ is critical in planning an epidemiologic investigation. The case definition must be carefully formulated to meet the objectives of the investigation, while also permitting valid comparisons with results from other studies. In this example, it may be of interest to consider whether the proportion of school-age children with ADHD has changed over a period of time. If the case definition changes significantly from one time period to the next, comparisons with previous years are problematic.
Suppose instead of estimating prevalence, the task is to define cases for a case-control study that is examining the risk from exposure. If the case definition is broad, it will be easier to include prospective cases, hastening the enrollment of study participants. However, variability among the cases will be greater than if the case definition was more narrow. A narrow case definition can slow the identification of sufficient numbers of cases but has the potential to reduce false positives.

Just as a clinical diagnosis for an individual requires meeting specific clinical and laboratory criteria, measuring disease frequency in populations requires the prior stipulation of which clinical, laboratory, epidemiologic, or quantitative criteria indicate the presence of the disease. Case definitions can include a degree of certainty (e.g. probable or confirmed, etc.) or specify the method to be used in assessing whether or not criteria are met.

In the United States, disease surveillance is not a responsibility directly given to the federal government. This means each state in the US may establish its own requirements for reporting diseases. Can you imagine the confusion if all 50 states had used their own definitions for different diseases? To assist the states, the U.S. Centers for Disease Control and Prevention (CDC) and the Council of State and Territorial Epidemiologists have published a set of uniform criteria for reporting cases of  Surveillance Case Definitions for Current and Historical Conditions.

Cases of disease can be categorized as follows:

1. Clinically compatible case

   A clinical syndrome is generally compatible with the disease, as described in the clinical description. A general clinical impression is that this is a case of disease.
2. Confirmed case

   A case that is classified as confirmed for reporting purposes. The case meets established criteria.
3. Epidemiologically linked case

   A case in which…

   1. the patient has had contact with one or more persons who either have/had the disease or have been exposed to a point source of infection (i.e., a single source of infection, such as an event leading to a foodborne disease outbreak, to which all confirmed case-patients were exposed) and
   2. transmission of the agent by the usual modes of transmission is plausible. A case may be considered epidemiologically linked to a laboratory-confirmed case if at least one case in the chain of transmission is laboratory confirmed
4. Laboratory-confirmed case

   A case that is confirmed by one or more of the laboratory methods listed in the case definition under laboratory criteria for diagnosis. Although other laboratory methods can be used in clinical diagnosis, only those listed are accepted as laboratory confirmation for national reporting purposes
5. Probable case

   A case that is classified as probable for reporting purposes. Supportive or presumptive laboratory results: specified laboratory results that are consistent with the diagnosis, yet do not meet the criteria for laboratory confirmation
6. Suspected case

   A case that has a lower certainty; is classified as suspected for reporting purposes
7. Clinically compatible case

   A clinical syndrome is generally compatible with the disease, as described in the clinical description. A general clinical impression is that this is a case of disease.

Each type has utility in different settings. To investigate a highly infectious, transmissible, or serious and deadly disease, casting a broad net will capture all suspected and probable cases. On the other hand, if declaring an individual to be a 'case' is likely to result in imposing severe restrictions, such as closing schools or removing a product from the market, the case definition should be more stringent. For example, a series of suspected cases of disease would not be sufficient to support a product recall. The epidemiologist would prefer to have a confirmed case or a laboratory-confirmed case to justify such action.