Case definitions

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The purpose of case definitions is to ensure that cases are counted in a systematic and consistent manner to enable comparison and analysis of trends. Case definitions are essential to an effective outbreak investigation.

In outbreak investigations, case definitions are used for counting outbreak cases only, they are not used for establishing clinical diagnoses or making healthcare decisions for an individual patient. Existing guidelines, algorithms, and protocols should be followed as per local jurisdictional policies for appropriate clinical case management.

To build case definitions for an outbreak, it is often helpful to consider what is known about the outbreak (e.g., disease/pathogen, descriptive epidemiology of ill persons), and how ill persons are likely to be identified (e.g., testing practices, surveillance systems).

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Creating case definitions

Case definitions should be simple and practical. There are two components that should be included in case definitions:

1. Clinical and/or laboratory criteria to assess if an individual has the illness of interest

The clinical features should be significant or hallmark signs of the illness that are simple to measure and objective. If the pathogen causing illness is known, then the appropriate laboratory criteria (e.g., pathogen type, subtype) should be included.

 2. Restrictions by person, place, and time

Characteristics of the people affected

Consider what is known about the reported cases and how this might be used to restrict the case definition. For example, the case definition may be restricted by age group, or to individuals who attended a common event or gathering (e.g. attendees of a particular wedding or banquet).

Information about the place or location

The case definition may restrict individuals to those who are residents of or visitors to a particular community, or employees of a particular organization. The geographic scope of the outbreak may influence the information included about location or place.

Timeframe of the outbreak

Consider the time frame in which illnesses associated with the outbreak are likely to have occurred. This may be based on what is known about the outbreak (e.g., earliest onset date of reported cases, date of suspected exposure), while allowing for some degree of uncertainty (e.g., first reported case may not represent the earliest case in the outbreak, thus, expanding the timeframe may be necessary).

Some additional examples of criteria and restrictions included in case definitions are provided in the following table:

Element Descriptive Feature Example
 Laboratory  Pathogen, serotype  E. coli O157:H7
 PulseNet Canada cluster code  2005NEWWGS-1MP
 Clinical symptoms  Acute gastrointestinal illness  3 or more loose stools in a 24 hour period
 Person  Age group  children under 5 years old
 Sex  females
 Occupation  health care workers at hospital A
 Exclusion criteria  persons with chronic diarrhea
 Place  Geographic location  resident or visitor to city B
 Water source  residents connected to water line C
 Time  Illness onset  onset of illness on or after January 1, 2014 

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Sensitivity and specificity

Developing a case definition requires a suitable balance between identifying all possible cases (sensitivity) and correctly excluding non-outbreak-related illnesses (specificity). In the early stages of an investigation, when case finding is important, it is often helpful to cast a wide net to capture as many cases as possible by using a sensitive case definition.

As the outbreak evolves and more information becomes available, case definitions can be refined to be more specific using additional laboratory or epidemiologic restrictions. These restrictions help to avoid misclassification and are useful for hypothesis testing.

The table below compares a sensitive versus a specific case definition. In this example, the sensitive case definition focuses on clinical criteria to identify a case, in contrast to the use of laboratory diagnoses for the specific case definition.

Sensitive case definition Specific case definition

 Three or more loose stools in a 24 hour period


Resident or visitor to Province A


Symptom onset on or after March 1, 2014

 Laboratory confirmed case of E. coli O157:H7


Resident or visitor to Province A


Symptom onset on or after March 1, 2014

There are no set rules for how sensitive or specific a case definition should be. However, it is useful to take into consideration potential advantages and disadvantages of adopting a more sensitive or more specific definition.

  Sensitive Specific
 Advantages Increase chances of finding cases (“true positives”); able to find as much information as possible on true cases early on, rather than having to go back and find cases later (which may have implications for food recall). Improve case classification (i.e., minimize “false positives”); strengthen subsequent analytical tests or studies; may be more resource efficient.
 Disadvantages  May capture individuals who are not actually part of the outbreak (“false positives”); may be resource intensive. May miss cases (“true positives”).

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Case definition categories

Case definition categories are often used to group cases during outbreak investigations to reflect the degree of certainty that the case is part of the outbreak.  Confirmed, probable and suspect case categories are commonly used, although different terminology may be found (e.g., “persons under investigation”).

Case categories can be used to help:

  • Track potential cases, particularly if case information (e.g., laboratory confirmation) is pending or not available
  • Identify individuals that warrant further investigation in a timely manner (e.g., administer case interview) and require public health measures (e.g., infection control advice)
  • Gauge the epidemiologic evidence as the outbreak continues to evolve
  • Improve data analysis and ability to detect meaningful associations

Confirmed case definitions represent a high degree of certainty that the ill individuals are related to the outbreak (high specificity), while probable and suspect cases have a lower degree of certainty (higher sensitivity).

The following table highlights some general features for the different case categories.  Restrictions on person, place, and time may be consistent across all case categories (e.g., time frame of interest).

Case category General features
 Confirmed  Laboratory confirmation of agent

 Typical clinical features of illness


Partial laboratory results (confirmation pending)

OR Epidemiologic link to a laboratory-confirmed case


 Typical clinical features of illness


Missing laboratory and epi information

An individual’s case classification can change throughout the course of the investigation.  An individual may be first classified as a suspect case, but be re-classified as a confirmed case as additional laboratory information becomes available.

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Exposure in case definitions

In general, a case definition should not include characteristics related to a possible or suspect source of illness (e.g., a food product); otherwise, it will not be possible to measure and statistically test the relationship between exposure and illness.

Sometimes, however, this general rule is breached and “exposure” is included in the probable case definition to increase specificity and reduce follow-up time. However, inclusion of “exposure” in a case definition is not considered best practice.

Primary versus secondary cases

It is important to distinguish between primary and secondary cases. Primary cases are directly exposed to the outbreak source, while secondary cases are defined as individuals who contracted the illness through exposure to a primary case, rather than the outbreak source itself (e.g., household contacts who become infected). Secondary cases should be included in defining the scope of the outbreak, but are not included in an analytic study to identify the source of the outbreak; only primary cases would be included in a study.

  • Example of a secondary case definition: A symptomatic individual or laboratory confirmation of E coli O157:H7 with onset 1-10 days after close contact with a primary case

Changing case definitions

Case definitions will often change over the course of an investigation as the outbreak evolves and more information becomes available. The following are examples of circumstances in an outbreak that may lead to a refinement of the case definition:

  • The available laboratory data becomes more specific (e.g., refining from laboratory-confirmed pathogen to laboratory-confirmed pathogen with a specific serotype, PulseNet Canada cluster code, or other subtyping method)
  • Additional serotypes are identified in food samples
  • The exposure becomes more focused (e.g., narrowing the focus from resident of/or visitor to Community A to exposure to Facility B)

Changing the case definition can have a considerable impact on the data collected and the interpretation. For example, during the Severe Acute Respiratory Syndrome (SARS) outbreak in 2003, the case definition changed from a clinical-based case definition (limited information at the outset) to the addition of a laboratory component.  The result was a large decrease in the number of reported confirmed cases, as expected with a more specific case definition.

This highlights the need to analyze and interpret the outbreak data in the context of the change in case definition.  As this can often create confusion, clear communication of the rationale for implementing the change and the expected impact is critical. It is also important to communicate changes to the entire outbreak team and ensure everyone is using the same definition and applying it in the same way. 

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Toolkit case definitions example table

  • This table in Microsoft Word summarizes case definitions used in a variety of enteric outbreaks.

ECDC case definitions example table

  • This PDF document includes a table summarizing examples of case definitions from published food- and waterborne outbreaks. 

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Gregg, M.B (ed.). 2002. Field Epidemiology, 2nd Edition. Oxford University Press, Oxford, England.

MacDonald, P.D.M. 2012. Methods in Field Epidemiology. Jones & Bartlett Learning, Burlington, MA.

World Health Organization. 2008. Foodborne Disease Outbreaks: Guidelines for Investigation and Control. WHO Press, Geneva, Switzerland.

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