—from the Epidemiological Bulletin, Vol. 20 No. 1, March 1999—
Norms and Standards in Epidemiology: Case Definitions
The use of case definitions is very important in epidemiology in order to standardize criteria for identification of cases. All case definition must include the three classical dimensions of epidemiological variables: time, place and person. It is of foremost importance to precisely define what will be considered as a case, in order to accurately monitor the trends of reported diseases, to detect their unusual occurrences and, consequently, to evaluate the effectiveness of intervention. Thus, the usefulness of public health surveillance data depends on its uniformity, simplicity and timeliness.
According to the Dictionary of Epidemiology, edited for the International Epidemiological Association by John M. Last, a case in epidemiology, is a person in the population or study group identified as having the particular disease, health disorder, or condition under investigation. A variety of criteria may be used to identify cases, e.g. individual physician’s diagnoses, registries and notifications, abstracts of clinical records, surveys of the general population, and population screening, among others. The epidemiological definition of a case is not necessarily the same as the ordinary clinical definition.
In the United States, requirements for reporting diseases are mandated by state laws or regulations, even though the list of reportable diseases in each state differs. The Centers for Disease Control (CDC) have established a policy that requires state health departments to report cases of selected diseases to the CDC’s National Notifiable Diseases Surveillance System (NNDSS). However, before 1990, the usefulness of such data was limited by the lack of uniform case definitions for public health surveillance. There was no explicit criteria for identifying cases for public health surveillance purposes.
In October 1990, CDC published Case Definitions for Public Health Surveillance, which, for the first time, provided uniform criteria for reporting cases to increase the specificity of reporting and improve the comparability of diseases reported from different geographic areas.
In 1996, the CDC revised the list of diseases under epidemiological and public health surveillance, and published it as "Case Definitions for Infectious Conditions Under Public Health Surveillance" (MMWR 1997;46). The definition of terms used in this list for case classification is established as follows:
Clinically compatible case: a clinical syndrome generally compatible with the disease, as described in the clinical description.
Confirmed case: a case that is classified as confirmed for reporting purposes.
Epidemiologically linked case: a case in which (a) the patient has had contact with one or more persons who either have/had the disease or have been exposed to a point source of infection (i.e., a single source of infection, such as an event leading to a foodborne-disease outbreak, to which all confirmed case-patients were exposed) and (b) transmission of the agent by the usual modes of transmission is plausible. A case may be considered epidemiologically linked to a laboratory-confirmed case if at least one case in the chain of transmission is laboratory confirmed.
Laboratory-confirmed case: a case that is confirmed by one or more of the laboratory methods listed in the case definition under Laboratory Criteria for Diagnosis. Although other laboratory methods can be used in clinical diagnosis, only those listed are accepted as laboratory confirmation for national reporting purposes.
Probable case: a case that is classified as probable for reporting purposes.
Supportive or presumptive laboratory results: specified laboratory results that are consistent with the diagnosis, yet do not meet the criteria for laboratory confirmation.
Suspected case: a case that is classified as suspected for reporting purposes.
Since case definition is an important component of epidemiological surveillance, the Epidemiological Bulletin will publish periodically the CDC’s case definitions. This issue presents cholera, plague, and yellow fever, the three international quarantine diseases established by the International Health Regulations (1969), third annotated edition of 1983, which was updated and printed in 1992 by WHO. These diseases are also part of WHO’s epidemiological surveillance system.
CHOLERA (Revised September 1996)
Clinical Description: an illness characterized by diarrhea and/or vomiting; severity is variable.
Laboratory Criteria for Diagnosis
• Isolation of toxigenic (i.e., cholera toxin-producing) Vibrio cholerae O1 or O139 from stool or vomits, or
• Serologic evidence of recent infection
Case Classification
Confirmed: a clinically compatible illness that is laboratory confirmed
Comment: illnesses caused by strains of V. cholerae other than toxigenic V. cholerae O1 or O139 should not be reported as cases of cholera. The etiologic agent of a case of cholera should be reported as either V. cholerae O1 or V. cholerae O139. Only confirmed cases should be reported to NNDSS by state health departments.
PLAGUE (Revised September 1996)
Clinical description: plague is transmitted to humans by fleas or by direct exposure to infected tissues or respiratory droplets; the disease is characterized by fever, chills, headache, malaise, prostration, and leukocytosis that manifests in one or more of the following principal clinical forms:
• Regional lymphadenitis (bubonic plague);
• Septicemia without an evident bubo (septicemic plague);
• Plague pneumonia, resulting from hematogenous spread in bubonic or septicemic cases (secondary pneumonic plague), or inhalation of infectious droplets (primary pneumonic plague);
• Pharyngitis and cervical lymphadenitis resulting from exposure to larger infectious droplets or ingestion of infected tissues (pharyngeal plague).
Laboratory criteria for diagnosis
Presumptive:
• Elevated serum antibody titer(s) to Yersinia pestis
fraction 1 (F1) antigen (without documented fourfold
or greater change) in a patient with no history of plague vaccination, or
• Detection of F1 antigen in a clinical specimen by fluorescent assay.
Confirmatory:
• Isolation of Y. pestis from a clinical specimen, or
• Fourfold or greater change in serum antibody titer to Y. pestis F1 antigen.
Case classification
Suspected: a clinically compatible case without presumptive or confirmatory laboratory results
Probable: a clinically compatible case with presumptive laboratory results.
Confirmed: a clinically compatible case with confirmatory laboratory results.
YELLOW FEVER
Clinical description: a mosquito-borne viral illness characterized by acute onset and constitutional symptoms followed by a brief remission and a recurrence of fever, hepatitis, albuminuria, and symptoms and, in some instances, renal failure, shock, and generalized hemorrhages.
Laboratory criteria for diagnosis:
• Fourfold or greater rise in yellow fever antibody titer in a patient who has no history of recent yellow fever vaccination and cross-reactions to other flaviviruses have been excluded, or
• Demonstration of yellow fever virus, antigen, or genome in tissue, blood, or other body fluid.
Case classification
Probable: a clinically compatible case with supportive serology (stable elevated antibody titer to yellow fever virus, e.g., greater than or equal to 32 by complement fixation, greater than or equal to 256 by immunofluorescence assay, greater than or equal to 320 by hemagglutination inhibition, greater than or equal to 160 by neutralization, or a positive serologic result by immunoglobulin M-capture enzyme immunoassay). Cross-reactive serologic reactions to other flaviviruses must be excluded, and the patient must not have a history of yellow fever vaccination.
Confirmed: a clinically compatible case that is laboratory confirmed.
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Epidemiological Bulletin, Vol. 20 No. 1, March 1999