General principles for the application of the case definitions

Clinical criteria
Clinical criteria include common and relevant signs and symptoms of the disease which either individually or in combination constitute a clear or indicative clinical picture of the disease. The clinical criteria give the general outline of the disease
and do not necessarily indicate all the features needed for individual clinical diagnosis.

Laboratory criteria
Laboratory criteria include a list of laboratory methods that are used to confirm a case. Usually only one of the listed tests will be enough to confirm the case. If a combination of methods is needed to meet the laboratory confirmation, this is specified. The type of specimen to be collected for the laboratory tests is only specified when only certain specimen types are considered relevant for the confirmation of a diagnosis. For some agreed exceptions, laboratory criteria for a probable case are included. This is a list of laboratory methods which can be used to support the diagnosis of a case but which are not confirmatory.

Epidemiological criteria
An epidemiological link, during the incubation period, is defined as one of the
following six:

  • Human to human transmission: Any person who has had contact with a laboratory confirmed human case in such a way as to have had the opportunity to acquire the infection
  • Animal to human transmission: Any person who has had contact with an animal with a laboratory confirmed infection/colonisation in such a way as to have had the opportunity to acquire the infection
  • Exposure to a common source: Any person who has been exposed to the same common source or vehicle of infection, as a confirmed human case
  • Exposure to contaminated food/drinking water: Any person who has consumed food or drinking water with a laboratory confirmed contamination or a person who has consumed potentially contaminated products from an animal with a laboratory confirmed infection/colonisation
  • Environmental exposure: Any person who has bathed in water or has had contact with a contaminated environmental source that has been laboratory confirmed
  • Laboratory exposure: Any person working in a laboratory where there is a potential for exposure.

A person may be considered epidemiologically linked to a confirmed case if at least one case in the chain of transmission is laboratory confirmed. In case of an outbreak of faeco-oral or airborne transmitted infections, the chain of transmission does not necessarily need to be established to consider a case epidemiologically linked.

Case classification
Cases will be classified as “possible”, “probable” and “confirmed”.

A possible case is usually a case with the clinical criteria as described in the case definition without epidemiological or laboratory evidence of the disease in question. The definition of a possible case has high sensitivity and low specificity. It allows for detection of most cases but some false positives cases will be included into this category.

A probable case is usually a case with clinical criteria and an epidemiological link as described in the case definition. Laboratory tests for probable cases are specified only for some diseases. 

A confirmed case should be laboratory confirmed and may fulfil the clinical criteria or not as described in the case definition. The definition of a confirmed case is highly specific and less sensitive; therefore most of the collected cases will be true cases although some will be missed.

NA means “not applicable”.