Step 3: Establish a Case Definition; Identify Cases

Case Definitions

By a case definition we mean the standard criteria for categorizing an individual as a case. Establishing a case definition (the criteria that need to be met in order to be considered "a case") can be tricky, particularly in the initial phases of the investigation. You want your definition to specific enough to identify true cases of disease, but you also want it to be broad enough and sensitive enough that it will identify most, if not all of the cases. As a result, the case definition may change during the investigation. In the earliest stages, it might be broader and less specific in order to make sure you identify all of the potential cases ("possible" cases), but later on, it might include more specific clinical or laboratory criteria that enable you to categorize individuals as "probable" or "confirmed" cases.

Case definitions may include four types of information:

  1. clinical information such as symptoms or lab results, e.g. the presence of fever >101o F and jaundice for hepatitis A or the presence of elevated IgM anti-HAV antibodies in an outbreak of hepatitis A
  2. personal characteristics of the cases, e.g., individuals in a certain age group
  3. limits with respect to the location of the case (e.g., residing or working on the South Shore of Massachusetts) 
  4. a specified time period for this particular outbreak (e.g., during February and March 2009 or among people who attended a specific  wedding)

The CDC also makes well established case definitions available:

Example #1: CDC Case Definition for Giardiasis

Clinical description

An illness caused by the protozoan Giardia lamblia and characterized by diarrhea, abdominal cramps, bloating, weight loss, or malabsorption. Infected persons may be asymptomatic.

Laboratory criteria for diagnosis

Case classification

Example #2: CDC Case Definitions for Viral Hepatitis

Clinical case definition

An illness with a) discrete onset of symptoms and b) jaundice or elevated serum aminotransferase levels

Laboratory criteria for diagnosis

Case classification

Comment: A serologic test for IgG antibody to the recently described hepatitis C virus is available, and many cases of non-A, non-B hepatitis may be demonstrated to be due to infection with the hepatitis C virus. With this assay, however, a prolonged interval between onset of disease and detection of antibody may occur. Until a more specific test for acute hepatitis C becomes available, these cases should be reported as non-A, non-B hepatitis. Chronic carriage or chronic hepatitis should not be reported.

Clinical criteria for a Case Definition

These should be simple, objective, and discriminating (i.e. able to distinguish between people with disease and those without disease. For example,

Also, case definitions should not include risk factors that you may want to evaluate, since all of the cases would have the risk factor, and this would be misleading. A case definition is not the same as a clinical diagnosis. Case definitions are an aid to conducting an epidemiologic investigation, whereas a clinical diagnosis is used to make treatment decisions for individual patients.


Sometimes investigators will use a loose definition early on to help them identity the extent of the outbreak. However, once the investigation progresses to the stage of conducting analytic studies to test hypotheses, a more specific definition should be used in order to reduce misclassification which would bias the results.

Categories of Cases: Confirmed, Probable, and Possible Cases

Case Finding

Once a case definition has been established, there should be a concerted effort to identify as many cases as possible in order to accurately establish the magnitude and scope of the outbreak. The cases that are reported to the state and local health departments may represent only a small fraction of the total cases for the outbreak. Therefore, in addition to cases identified via passive surveillance (i.e., cases that self-report or are reported to the state and local health department by physicians' offices, clinics, hospitals, and laboratories) it is often fruitful to conduct active surveillance by calling hospitals, laboratories, clinics, and physicians offices in order to identify potential cases that otherwise would have gone unreported. As cases are identified, it can also be useful to ask them if they know of others who are similarly affected, e.g., family members and acquaintances. Occasionally, investigators will try to identify cases by posting notices in the media. These serve the dual purpose of alerting the public about potential hazards and identifying possible cases that have already become ill. For more information on case finding see Case Finding and Line Listing: A Guide for Investigators.