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E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
Case Definitions
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
Objectives of this lecture
To identify: • The elements that should be included in a case
definition, • The type of case definitions, and • The balance between sensitive and specific
case definitions.
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
Case Definition
• Essential tool in a surveillance system
• A case definition for each health event
• Different health events
– Mutually exclusive case definitions
• Consistency in reporting
• Surveillance ≠ patient management
Definition
Elements
Characteristics
SE vs SP
PPV
Post outbreakSARS
WHO standards
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
Elements of a case definition
• Clinical
• Laboratory diagnosis (if relevant)
• Criteria for place
• Criteria for person
• Epidemiological link (if relevant)
Definition
Elements
Characteristics
SE vs SP
PPV
Post outbreakSARS
WHO standards
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
Types of case definitions
Health events:– disease (rabies)
– syndrome (AFP)
– public health issue (AMR)
– environment (animal vector)
Clinical / Laboratory
Multilevels– Suspected
– Probable
– Confirmed
Specific / Sensitive
Definition
Elements
Characteristics
SE vs SP
PPV
Post outbreakSARS
WHO standards
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
Case definitions should be• Simple / Clear
• Adapted to – Staff qualification– Available diagnostic
• Stable
• Standardized
• Available at all reporting levels– Health posts / health centres– Hospitals– Laboratories
Definition
Elements
Characteristics
SE vs SP
PPV
Post outbreakSARS
WHO standards
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
Levels• Suspected case
– Clinical signs and symptoms
– No laboratory evidence
• Probable case– Suspect case + supportive lab (eg: single high titre)
– Suspect case + epi link (eg: contact with confirmed case)
• Confirmed case– Laboratory evidence
Cholera
Suspected: Acute watery diarrhoea
Confirmed: Isolation of Vibrio cholerae
Cholera
Suspected: Acute watery diarrhoea
Confirmed: Isolation of Vibrio cholerae
Definition
Elements
Characteristics
SE vs SP
PPV
Post outbreakSARS
WHO standards
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
Ministryof Health
WHO
Peripheral level
Intermediate level
Central level
Regional/International level
Clinical
(suspected)
+ Supportive laboratory data + epi
link (probable)
Diagnostic Laboratory (confirmed)
Regional reference laboratory
Definition
Elements
Characteristics
SE vs SP
PPV
Post outbreakSARS
WHO standards
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
Exposed
Clinical specimen
Symptoms
Pos. specimen
Infected
Seek medical attention
Report
Sensitivity
Proportion of true cases detected
=
reported true cases
total true cases
Definition
Elements
Characteristics
SE vs SP
PPV
Post outbreakSARS
WHO standards
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
Sensitivity (SE) vs Specificity (SP)
Higher sensitivity vs lower sensitivity depends on the objective
SE
SP
Definition
Elements
Characteristics
SE vs SP
PPV
Post outbreakSARS
WHO standards
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
Positive predictive value
Sick
No
tifi
edYes
Yes No
Total not notified
True -
False +
False -
True + Total notified
No
PPV = True+ / Total notified
Total sick Total not sick
Proportion of reported cases that have the health event under
surveillance
Definition
Elements
Characteristics
SE vs SP
PPV
Post outbreakSARS
WHO standards
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
• PPV depends on
- sensitivity
- specificity
- prevalence of the condition in the population
• Consequence of a low PPV– frequent "false-positive" report
- inappropriate follow-up of non-cases
- incorrect identification of epidemics (artifacts)
- wastage of resources
- inappropriate public concern (credibility)
Definition
Elements
Characteristics
SE vs SP
PPV
Post outbreakSARS
WHO standards
Positive predictive value
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
Definition
Elements
Characteristics
SE vs SP
PPV
Post outbreakSARS
WHO standards
SARS Re-emergenceRisk assessment
Potential zone of re-
emergence of SARS-CoV
Nodal areas
Low risk areas
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
SARS Syndromic Surveillance, 2003
Underlying assumptions
Definition
Elements
Characteristics
SE vs SP
PPV
Post outbreakSARS
WHO standards
The first cases or clusters of SARS may be missed
Diffuse-progressive nature of the health event.
SARS has aspecific clinical features and no rapid diagnostic test is available
–Clinical features of the disease
–Diagnostic capacity
Seasonal occurrence of other respiratory diseases (influenza, other CoV)
Epidemiological pattern of diseases with similar clinical characteristics in the region.
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
Definition of a SARS “Alert” for EWAR
• 2 or > Health Care Workers
– in same Health care unit (HCU)
– fulfilling clinical case definition of SARS
– with onset of illness in the same 10-day
period
OR
• Hospital acquired illness in 3 or >
persons
– in same HCU
– fulfilling clinical case definition of SARS
– with onset of illness in the same 10-day
period
• 2 or > Health Care Workers
– in same Health care unit (HCU)
– fulfilling clinical case definition of SARS
– with onset of illness in the same 10-day
period
OR
• Hospital acquired illness in 3 or >
persons
– in same HCU
– fulfilling clinical case definition of SARS
– with onset of illness in the same 10-day
period
Definition
Elements
Characteristics
SE vs SP
PPV
Post outbreakSARS
WHO standards
A person presenting after 1 November 2002 with history of: - high fever (>38 °C) AND - One or more symptoms of lower respiratory tract illness AND Radiographic or autoptic evidence consistent with pneumonia or RDS ANDNo alternative diagnosis can fully explain the illness
A person presenting after 1 November 2002 with history of: - high fever (>38 °C) AND - One or more symptoms of lower respiratory tract illness AND Radiographic or autoptic evidence consistent with pneumonia or RDS ANDNo alternative diagnosis can fully explain the illness
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
SARS Clinical case definitionFever (38°C) AND
– One or more symptoms of lower respiratory tract illness (cough, difficulty of breathing, shortness of breath)
AND– Radiographic evidence of
lung infiltrates consistent with pneumonia or RDS
– OR autopsy findings consistent with the pathology of pneumonia or RDS without an identifiable cause
AND– No alternative diagnosis can
fully explain the illness
Definition
Elements
Characteristics
SE vs SP
PPV
Post outbreakSARS
WHO standards
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
Post-outbreakCase definition
Post-Outbreak Case-Definitions
Outbreak case definition
Definition
Elements
Characteristics
SE vs SP
PPV
Post outbreakSARS
WHO standards
Clinical SARSAtypical pneumonia
Suspect
Probable
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
SARS Laboratory case definition
• PCR positive for SARS-CoV ( 2 different clinical specimens OR sequential sampling from the same site OR 2 assays or repeat PCR using a new RNA extract from the original clinical sample on each occasion of testing)
OR
• Seroconversion or 4-fold or greater rise in titre between acute and convalescent phase sera by ELISA or IFA
OR
• Virus isolation with PCR validation
Definition
Elements
Characteristics
SE vs SP
PPV
Post outbreakSARS
WHO standards
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
Aggregatednotification
Syndromic SurveillanceLink with SARS Surveillance
CRF notification
Definition
Elements
Characteristics
SE vs SP
PPV
Post outbreakSARS
WHO standards
Fever + Respiratory
Fever + Respiratory+ chest X-ray
Clinical SARSAtypical pneumonia
Suspect
Probable
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
WHO standards• 48 CD syndromes,
conditions
– same format • Available
english, french, spanish and arabic
• To be adapted to country needs
www.who.int/csr/resources/publications/surveillance/WHO_CDS_CSR_ISR_99_2_EN/en/
Definition
Elements
Characteristics
SE vs SP
PPV
Post outbreakSARS
WHO standards
E P I D E M I C A L E R T A N D R E S P O N S E SEARO –CSR Early Warning and Surveillance System Module
Question Time
1. What are the objectives of a case definition?2. How would you approach the design a case
definition for typhoid fever?