A case definition from a broader sense encompassing it is used in more than just medical terms. A case is a request for a service for a subject - customer, employee, or citizen. The case has supporting information - documents, data, forms, etc. - as part of the request for service. The case is managed by a knowledge worker or subject matter expert - health services professional, law enforcement professional, government agent, human resources professional, financial services professional, etc. - who in context of the case information and set in their experience will resolve the outcome of the case.
A medical centric case definition is the method by which public health professionals define who is included as a case in an outbreak investigation, (i.e. a person considered directly affected by an outbreak) or in the surveillance of public health conditions. A case definition has the following characteristics, it defines a case in time, person and place. Time criteria may include all cases of a disease identified from, for example, January 1, 2008 to March 1, 2008. Person criteria may include age, gender, ethnicity, and clinical characteristics such as symptoms (e.g. cough and fever), clinical tests (e.g. pneumonia on chest X-ray). Place criteria will usually include a geographical entity such as a town, state, or country but may be as small as an institution, a school class, or a restaurant meal session. Case definitions may also be categorised into suspect, probable and confirmed cases. For example in the investigation of an outbreak of pneumococcal pneumonia in a nursing home the case definition may be specified as: Suspect Case: All residents of Nursing Home A with onset of cough and fever between January 1, 2008 and February 1, 2008. Probable Case: Meet the suspect case definition plus have pneumonia on chest X-ray. Confirmed Case: Meet the probable case definition plus have pneumococcal infection confirmed by blood culture or other isolation of pneumococci from normally sterile site.
By creating a case definition, public health professionals are better equipped to study an outbreak and determine possible causes. As investigations proceed, this definition may be expanded or narrowed. This is characteristic of the dynamic nature of outbreak investigations.
There are two types of case definition. One is sensitive and another one is specific. A sensitive case definition will capture all cases, but will include many non-cases. A specific case definition will exclude most non-cases, but will also exclude some actual cases.