Clinical criteria include common and relevant signs and symptoms of the disease which either individually or in combination constitutes a clear or indicative clinical picture of the disease. They give the general outline of the disease and do not necessarily indicate all the features needed for individual clinical diagnosis.
Laboratory criteria are 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. Laboratory criteria for a probable case are included for some agreed exceptional cases. Those laboratory criteria consist of a list of laboratory methods which can be used to support the diagnosis of a case but which are not confirmatory.
Epidemiological criteria are deemed to have been met when an epidemiological link can be established.
Epidemiological link, during the incubation period, means one of the following six:
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the fact that a person has had contact with a laboratory confirmed human case in such a way as to have had the opportunity to acquire the infection;
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the fact that a person has had contact with an animal with a laboratory confirmed infection/colonization in such a way as to have had the opportunity to acquire the infection;
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the fact that a person has been exposed to the same common source or vehicle of infection, as a confirmed human case;
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the fact that a person has consumed food or drinking water with a laboratory confirmed contamination or has consumed potentially contaminated products from an animal with a laboratory confirmed infection/colonization;
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the fact that a person has bathed in water or has had contact with a contaminated environmental source that has been laboratory confirmed;
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the fact that a person has worked 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.
Transmission may occur by one or more of the following routes:
Airborne: by projection of aerosol from an infected person onto the mucous membranes while coughing, spitting, singing or talking, or when microbial aerosols dispersed into the atmosphere are inhaled by others;
Contact: direct contact with an infected person (faecal-oral, respiratory droplets, skin or sexual exposure) or animal (for example, biting, touching) or indirect contact to infected materials or objects (infected fomites, body fluids, blood);
Vertical: from mother to child, often in utero, or as a result of the incidental exchange of body fluids usually during the perinatal period;
Vector transmission: transmission by infected mosquitoes, ticks, mites, flies and other insects which transmit disease to humans through their bites;
Food or water: consumption of potentially contaminated food or drinking water.
Cases are classified as ‘possible’, ‘probable’ and ‘confirmed’. The incubation periods for diseases are given in the additional information to facilitate the assessment of the epidemiological link.
A possible case means a case classified as possible for reporting purposes. It is usually a case meeting the clinical criteria as described in the case definition without epidemiological or laboratory evidence of the disease in question. The definition of a case as possible 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 means a case classified as probable for reporting purposes. It 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 means a case classified as confirmed for reporting purposes. Confirmed cases are laboratory confirmed and may or may not fulfil the clinical criteria as described in the case definition. The definition of a case as confirmed is highly specific and less sensitive; therefore most of the collected cases will be true cases although some will be missed.
The clinical criteria of some diseases do not allude to the fact that many acute cases are asymptomatic (for example, hepatitis A, B and C, campylobacteriosis, salmonellosis) although these cases may still be important from a public health perspective on national level.
Confirmed cases fall in one of the three subcategories listed below. They will be assigned to one of those subcategories during the analysis of data using the variables collected within the context of the case information.
The case meets the laboratory criteria for case confirmation and the clinical criteria included in the case definition.
The case meets the laboratory criteria for case confirmation but there is no information available regarding the clinical criteria (for example,only laboratory report).
The case meets the laboratory criteria for case confirmation but doesn't meet the clinical criteria in the case definition or is asymptomatic.
Note: For some of the conditions under surveillance the structure of the case definitions does not follow the typical structure of the case definition such as in the cases of Creutzfeldt-Jakob disease (CJD), healthcare associated infections and antimicrobial resistance.