Outbreak Case Definitions 

Purpose of a case definition: 

Development of a clear case definition is critical to effective investigation of an outbreak. 

Use of a common case definition allows for standardization of the cases of interest both 

within an ongoing outbreak investigation and possibly between outbreak investigations 

that differ over time or geographic location. 

Note: Additional agents, such as Arcanobacterium haemolyticus, Neisseria meningitidis, 

and Staphylococcus aureus, may cause respiratory syndromes but have not been 

associated with outbreaks. 

Developing outbreak case definitions: 

A case definition includes criteria for person, place, time, and clinical features. These 

should be specific to the outbreak under investigation. 

"Person" describes key characteristics the patients share in common. For example, this 

description may include: age, sex, race, occupation and exclusion criteria (e.g., “persons 

with no history of X disease”). 

"Place" typically describes a specific geographic location (state, county) or facility 

associated with the outbreak (X nursing home, Y high school). 

"Time" is used to delineate a period of time associated with illness onset for the cases 

under investigation. Limiting the time period enables exclusion of similar illnesses which 

are unrelated to the outbreak of interest. 

Initially, "clinical features" should be simple and objective (e.g., sudden onset of fever 

and cough). The clinical criteria may later be characterized by the presence of specific 

laboratory findings. 

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Table: Common components and examples of an outbreak case definition 

Element* Descriptive features Examples 

Person Age group “children under the age of 5 years” 

Sex “males” 

Occupation “health care workers at hospital X” 

Exclusion criteria “persons with no previous history of chronic cough 

or asthma” 

Race 

Place Geographic location “resident of Y county or state” 

Facility “living in X nursing home”; “student at A high 

school” 

Time Illness onset “onset of illness between May 4 and August 31, 

2007” 

Clinical 

features 

Pneumonia “clinical or radiographically confirmed pneumonia” 

“shortness of breath and fever” 

Laboratory 

criteria 

Cultures; serology Pneumococcus isolated from blood; rapid influenza 

test positive 

*Please note components of an outbreak case definition vary for each outbreak. 

Examples of a case definition: 

"Student attending X High School who has onset of fever and cough between 

January 4 and 24, 2007." 

“A resident of, or visitor to, Rapid City, South Dakota who was diagnosed by a 

physician, either clinically or radiographically, with community-acquired 

pneumonia (CAP) with symptom onset after May 1, 2005 and who had laboratory 

confirmation of Legionnaires’ disease by culture of Legionella, by urinary antigen 

test for Legionella pneumophila serogroup 1 (Lp1), by a four-fold or greater rise 

in serum antibody titer to Lp1, or detection of specific Legionella antigen by 

direct fluorescent antibody staining.” 

Case definitions are often further categorized by the degree of certainty regarding the 

diagnosis as "suspected", "probable", or "confirmed". 

Further examples: 

“A clinical case was defined as an acute cough illness lasting >2 weeks with onset 

during September 2004--February 2005 and without other apparent cause in a 

person living in the Amish community in Kent County. A confirmed case was 

defined as a clinical case of pertussis that 1) was laboratory confirmed by 

polymerase chain reaction (PCR) for Bordetella pertussis DNA or 2) had a direct 

epidemiologic link to a laboratory-confirmed case through a common household 

residence. All other clinical cases were considered probable.” 

References: 

1. Gregg, M.B. Field Epidemiology. New York: Oxford University Press, 2002. 

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