A 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 created 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.
- ↑ http://www.atsdr.cdc.gov/HEC/CSEM/cluster/case_def.html Disease Clusters: An OverviewCase Definition from the United States Department of Health and Human Services