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Past Approaches to Communicable Disease Surveillance

Surveillance Terms

Information from surveillance is used to make decisions about the best ways to prevent and control the disease. The term ‘surveillance’ implies information for action. We will review some basic surveillance terms.

Universal case reporting - A surveillance system in which all cases of a disease are reported.
Sentinel surveillance - A surveillance system in which reports are obtained from certain selected facilities or populations. Sentinel surveillance can apply both to reports of cases of disease or to periodic surveys, such as antenatal HIV surveys.
Laboratory-based reporting - A surveillance system in which the reports of cases come from clinical laboratories instead of physicians, other healthcare practitioners or hospitals.
Case definition - The clinical and laboratory characteristics that a patient must have to be counted as a case for surveillance purposes.
Prevalence - The proportion of persons in a population who have a disease or condition at a given point in time.
Incidence - The number of persons who develop a disease or condition within a specified time period. Incidence is expressed as a rate with the time period in the denominator.
Passive surveillance - A passive system refers to data generated without solicitation, intervention or contact by the health agency carrying out the surveillance. Other agencies initiate reporting. Example: normal disease case reporting by health facilities.

Active surveillance - The organisation conducting surveillance initiates procedures to obtain reports. Example: making telephone calls or visits to health facilities to obtain information.

Relationship

between

disease and

case definition

There is a relationship between disease and case definition. Look at table 3.1 and the four terms after it.
Table 3.1 Relationship between disease and case definition.

	True disease
Case definition	Present	Absent	Total
Definition met	a	b	a+b
Definition not met	c	d	c+d
Total	a+c	b+d	N

Sensitivity – Referring to table 3.1 above, the ability of a case definition or laboratory test to predict true disease (a/(a+c)).
Specificity - The ability of a case definition or laboratory test to predict absence of true disease (d/(b+d)).
Positive predictive value - The proportion of persons meeting a case definition and having a positive laboratory test who have true disease (a/(a+b)).
Negative predictive value - The proportion of persons not meeting a case definition, and having a negative laboratory test who do not have true disease (d/(c+d)).
Discussing

the table

Examine the table and definitions above and answer the following questions:

For a given case definition, a = 10, b = 10, c = 30 and d = 150. Determine the specificity of this case definition.

Using these numbers, what is the negative predictive value of the case definition? What does this figure represent?

Past Approaches to Communicable Disease Surveillance

Generally, limitations of communicable disease surveillance systems in some Caribbean countries are as follows:

Duplication of effort: Vertical or categorical surveillance systems established to report a single disease as a component of specific disease intervention programmes. This results in duplication of effort and resources.

Delay in reporting: Health workers fail to report index cases of epidemic-prone diseases in a timely manner. This delay in reporting the earliest suspected cases significantly slows identification of outbreaks and impedes the effectiveness of response.

Inadequate data collection, analysis, use and dissemination. Collection, analysis, utilisation and dissemination of surveillance data are inadequate. Usually, surveillance data are passed from facilities through to national level without adequate analysis. Feedback is also generally inadequate.

Lack of integrated training. Little attention has been given to combining surveillance training activities to increase efficiency. As a result, each programme organises programme-specific training courses (including surveillance) for the same health personnel.

Lack of evaluation. Inadequate attention has been given to the evaluation of programmes using surveillance data. Many resources are invested in interventions that are not adequately evaluated.

Lack of laboratory involvement and co-ordination. Involvement of laboratories in the surveillance system is inadequate. Neither national nor inter-country laboratory networks have been established to fulfil important public health functions, including the confirmation of cases and outbreaks when the specificity of clinical diagnosis is low.

Lack of supervision. Supervisory support, completeness and timeliness of reporting are generally inadequate.

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