Various Levels of Case Definition

When facing a new disease or when at the beginning of an investigation it is usual to design different level of case definition from very sensitive (suspect case) to very specific (laboratory confirmed case). Usually cases are temporarily classified as suspect until laboratory results are known.

During an outbreak of botulism in Italy in 2004 two levels of case definition were developed as follows:

A 'probable botulism case' was defined as:

- a person who had dined at restaurant A on February 22 or 24, 2004 and - had experienced diplopia, or blurred vision, or fixed/dilated pupils and - at least one of the following symptoms: dysphagia, dry mouth, dysarthria, upper/lower extremity weakness, dyspnoea, or severe constipation.

Those who met the probable case definition and had laboratory-confirmed botulism were considered definite cases.

With the sophistication of laboratory methods and availability of many diagnostic tests for a single disease some case definition may become lengthy and resemble more a complex decision tree as for example the WHO SARS case definition. The following is one of the WHO regularly updated case definition during the SARS epidemic in the west pacific WHO region in 2003.

Suspect SARS case: definition for public health surveillance
1. A person presenting after 1 February 2003 with a history of:
high fever (>38° C);
AND
cough, or breathing difficulty
AND one or more of the following exposures during the 10 days prior to onset of symptoms:
AND one or more of the following:
close contact* with a person who is a suspect or probable case of SARS
history of travel, to an area with recent local transmission of SARS **
residing in an area with recent local transmission of SARS
2. A person with an unexplained acute respiratory illness resulting in death after 1 November 2002, but on whom no autopsy has been performed AND one or more of the following 
exposures during to 10 days prior to onset of symptoms:
close contact, * with a person who is a suspect or probable case of SARS;
history of travel to an area with recent local transmission of SARS
residing in an area with recent local transmission of SARS
* Close contact means having cared for, having lived with, or had direct contact with respiratory secretions or body fluids of a suspect or probable case of SARS.
** Areas in which there are reported foci of transmission of SARS are updated on the WHO website (http://www.who.int/csr/sarsareas/en/). Current areas (1 April 2003) with reported 
foci of transmission are: