Clinical case definition
In epidemiology, a clinical case definition, a clinical definition, or simply a case definition lists the clinical criteria by which public health professionals determine whether an individual is included as a case in an outbreak investigation, i.e. is a person considered directly affected by an outbreak. Absent an outbreak, case definitions are used in the surveillance of public health in order to categorize those conditions present in a population.
How are they used
A case definition defines a case by placing limits on 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) and the results of clinical tests (e.g. pneumonia on chest X-ray). Place criteria will usually specify 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 are often used to label individuals as suspect, probable, or 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, a case definition may be expanded or narrowed, a characteristic of the dynamic nature of outbreak investigations. At any given time, the case definition is supposed to be the gold standard to diagnose a given disease. A sensitive case definition, often applied early in an outbreak, will capture all cases, but will include many non-cases. A specific case definition, usually applied after the outbreak is considered more well-understood, will exclude most non-cases, but will also exclude some actual cases.
Some examples of diagnostic criteria are:
- Amsterdam criteria for hereditary nonpolyposis colorectal cancer
- McDonald criteria for multiple sclerosis
- ACR criteria for systemic lupus erythematosus
- Centor criteria for strep throat
When diagnostic criteria are universally accepted they can be considered a "clinical definition" because it defines the limits of the affected population, determining which patients are inside and outside of the set.
A clinical definition should be regarded as a statistical analysis tool, and not a substitute for a pathological definition when this is required. Posthumous diagnosis allows to stablish the sensitivity and specificity of the clinical definitions
The free dictionary defines a clinical disease as "A stage in the history of a pathologic condition that begins with anatomic or physiologic changes that are sufficient to produce recognizable signs and symptoms of a disease".
It is in some way a similar concept to the one of syndrome.
- Keou FX1, Bélec L, Esunge PM, Cancre N, Gresenguet G. World Health Organization clinical case definition for AIDS in Africa: an analysis of evaluations. East Afr Med J. 1992 Oct;69(10):550-3.
- http://www.atsdr.cdc.gov/HEC/CSEM/cluster/case_def.html Disease Clusters: An Overview Case Definition from the United States Department of Health and Human Services
- José Roberto Lambertucci, Revisiting the concept of hepatosplenic schistosomiasis and its challenges using traditional and new tools, Rev. Soc. Bras. Med. Trop. vol.47 no.2 Uberaba Mar./Apr. 2014 Epub Apr 11, 2014, http://dx.doi.org/10.1590/0037-8682-0186-2013
- R Saracci, Is necropsy a valid monitor of clinical diagnosis performance? BMJ 1991;303:898, doi: http://dx.doi.org/10.1136/bmj.303.6807.898
- The Free Dictionary, Clinical Disease