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:
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:
|
Probable SARS case:
definition for public health surveillance
A suspect
case with radiographic evidence of infiltrates consistent with
pneumonia or
respiratory distress syndrome (RDS) on chest X-ray (CXR)
OR
A suspect
case of SARS that is positive for SARS coronavirus by one or more
assays.
OR
A suspect
case with autopsy findings consistent with the pathology of RDS
without an
identifiable cause.
Exclusion criteria: A case should be excluded if an alternative
diagnosis can fully explain their illness.