Case classifications: What are suspected, probable, and confirmed cases?

There is a lot of information circulating about the swine flu and its pandemic potential. Some of this information may be confusing and difficult to interpret. Additionally, rumors, inflated or sensational reporting, and the use of jargon serve to further complicate the picture. As you read updates on the swine flu situation, you may come across the terms suspected case, probable case, and confirmed case. Understanding what these terms mean and when they are used may help you interpret the information presented in the media.

A case classification system is something epidemiologists and other members of the public health community use for tracking cases. Cases are typically classified as one of the following: suspected, probable, or confirmed.

A suspected case is a case that meets clinical case definition; meaning the signs and symptoms a person has or presents with are consistent or compatible with a particular disease. The suspected case definition is often used for reporting purposes, so a case is reported to public health authorities for further investigation. For instance, a San Diego resident presenting to a health clinic with fever, cough, sore throat, muscle aches, tiredness, and headache may be reported by the clinic staff to public health authorities as a suspected case of swine flu. Health authorities may then require the clinic to obtain a clinical specimen from the patient in order to determine whether the suspected case meets probable or confirmed case definition.

A probable case is a case that meets clinical case definition and has supportive or presumptive laboratory results that are consistent with the diagnosis, yet do not meet the criteria for laboratory confirmation. For instance, with regard to swine flu, a probable case may be a case that meets clinical case definition (fever, cough, sore throat, muscle aches, tiredness, and headache) and provided a clinical specimen that tested positive for influenza A, but was untypable at a certain laboratory. While this patient has supportive laboratory results, as evidenced by a positive influenza A result, the specimen likely has not been strain typed; therefore, the case cannot be considered confirmed.

A confirmed case can either be laboratory confirmed or epidemiologically linked.

Laboratory confirmed: A confirmed case is laboratory confirmed if the patient’s clinical specimen meets the diagnostic criteria of a specified laboratory method. Clinical specimens are often forwarded to a reference laboratory for ascertainment or confirmation of laboratory results. National laboratories such as those at the Centers for Disease Control and Prevention and the Public Health Agency of Canada have been testing samples for the swine flu strain causing recent epidemic levels of human infection.

Epidemiologically linked: A confirmed case is 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
b) transmission of the disease-causing pathogen (in this case influenza virus) by the usual modes of transmission are 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. This means, for instance, if a child in a household is a laboratory confirmed case of swine flu and the child’s parent becomes ill with symptoms consistent with swine flu within a time period consistent with the amount of time it would take to develop symptoms after being infected by the child, the parent would be considered an epidemiologically linked confirmed case.

As you read media releases, please keep in mind that the numbers of suspected cases will be higher than the numbers of probable and confirmed cases. This makes sense because the criteria for meeting a suspected case classification are broader or more sensitive than probable and confirmed case classifications. Often times, information sources may use suspected case counts to sensationalize the situation; other times, suspected case counts are used because it takes time and resources to confirm cases and demand for instantaneous information is high.