epidemiology Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population. It is a cornerstone of public health, and shapes policy decisions and evide ...

, a clinical case definition, a clinical definition, or simply a case definition lists the clinical criteria by which

public health Public health is "the science and art of preventing disease, prolonging life and promoting health through the organized efforts and informed choices of society, organizations, public and private, communities and individuals". Analyzing the det ...

professionals determine whether a person's illness is included as a ''case'' in an

outbreak In epidemiology, an outbreak is a sudden increase in occurrences of a disease when cases are in excess of normal expectancy for the location or season. It may affect a small and localized group or impact upon thousands of people across an entire ...

investigation—that is, whether a person is 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 (e.g., incidence and

prevalence In epidemiology, prevalence is the proportion of a particular population found to be affected by a medical condition (typically a disease or a risk factor such as smoking or seatbelt use) at a specific time. It is derived by comparing the number o ...

How are they used

A case definition defines a case by placing limits on time, person, place, and shared definition with data collection of the phenomenon being studied. 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. Shared definition of the phenomenon impacts the study methods and ensures terminology is used in a consistent manner. 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 A gold standard is a monetary system in which the standard economic unit of account is based on a fixed quantity of gold. The gold standard was the basis for the international monetary system from the 1870s to the early 1920s, and from the l ...

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 Specific may refer to: * Specificity (disambiguation) * Specific, a cure or therapy for a specific illness Law * Specific deterrence, focussed on an individual * Specific finding, intermediate verdict used by a jury in determining the fina ...

case definition, usually applied after the outbreak is considered more well understood, will exclude most non-cases, but will also exclude some actual cases.

Diagnostic criteria

The term ''diagnostic criteria'' designates a case definition with a specific combination of signs, symptoms, and test results that the clinician uses to attempt to determine the correct diagnosis. Some examples of diagnostic criteria are: * Amsterdam criteria for

hereditary nonpolyposis colorectal cancer Hereditary nonpolyposis colorectal cancer (HNPCC) or Lynch syndrome is an autosomal dominant genetic condition that is associated with a high risk of colon cancer as well as other cancers including endometrial cancer (second most common), ovary, ...

* McDonald criteria for multiple sclerosis * ACR criteria for systemic lupus erythematosus *

Centor criteria The Centor criteria are a set of criteria which may be used to identify the likelihood of a bacterial infection in adult patients complaining of a sore throat. They were developed as a method to quickly diagnose the presence of Group A streptococc ...

for

strep throat Streptococcal pharyngitis, also known as streptococcal sore throat (strep throat), is pharyngitis (an infection of the pharynx, the back of the throat) caused by ''Streptococcus pyogenes'' a gram-positive, group A streptococcus. Common sympto ...

Clinical definitions

When diagnostic criteria are universally accepted they can be considered a "clinical definition" because they define 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 A retrospective diagnosis (also retrodiagnosis or posthumous diagnosis) is the practice of identifying an illness after the death of the patient (sometimes in a historical figure) using modern knowledge, methods and disease classifications. Altern ...

allows to establish the

sensitivity and specificity ''Sensitivity'' and ''specificity'' mathematically describe the accuracy of a test which reports the presence or absence of a condition. Individuals for which the condition is satisfied are considered "positive" and those for which it is not are ...

of the clinical definitions.

References

{{reflist Epidemiology

How are they used

Diagnostic criteria

Clinical definitions

See also

References