In describing a surveillance system, three desirable qualitative
attributes should be addressed: simplicity, flexibility, and acceptability.
Simplicity of a surveillance system refers both to its structure
and to its ease of operation. STD surveillance systems should be as simple
as possible, while still meeting their objectives. This task is more difficult
with STDs because of the complexity of case definitions (e.g., latent and congenital
syphilis) and the multiple levels of reporting. It may be useful to think of
the simplicity of a surveillance system from two perspectives: the design of
the system and the size of the system. The following measures might be considered
in evaluating the simplicity of a system:
Amount and type of information necessary to establish a STD diagnosis
Number and type of STD reporting sources
Methods of transmitting STD case information and data
Number of staff needed to efficiently handle workload
Type and extent of data analysis
Amount of computerization
Methods of distributing reports
Amount of time spent operating the system
The cost estimates for a system are also an indirect indicator
of simplicity. Simple systems usually cost less than those that are more complex.
Flexibility of a STD surveillance system refers to its ability
to adapt to changing information needs (such as the addition of new conditions
or data- collection elements) or operating conditions with little additional
cost in time, staff, or allocated funds. STD prevention programs have often
been challenged to quickly adapt to such emerging priorities as resistant gonorrhea
and AIDS. Generally, simpler systems will be more flexible—fewer components
will need to be modified when adapting the system for use with another disease;
therefore, the system should be able to track and analyze trends of other STDs
and new pathogens. All systems should have the ability to easily share data
sets with other systems (STD-MIS, HARS, TIMS) to determine co-infection rates,
similar populations at risk, commonalities between affected populations, etc.
This should be done while maintaining patient confidentiality.
Acceptability reflects the willingness of individuals and organizations
on whom the system depends to participate in the STD surveillance system. This
attribute refers to the acceptability of the system to health department staff
and to those individuals outside the sponsoring agency (e.g., doctors or laboratory
staff) who are asked to report STDs. To assess acceptability, programs should
carefully review the points of interaction between the system and its participants.
Measurable indicators of acceptability include:
Subject or agency participation rates
Time required to generate acceptable participation of key providers or agencies
Interview completion rates and question refusal rates, if the system involves case interviews
Completeness of report forms
Physician, laboratory, or hospital or facility reporting rates
Timeliness of reporting
Time and degree of complexity required of local health departments to capture positive case reports in the system
Quantitative Attributes
The four quantitative attributes of a surveillance system include
sensitivity, predictive value positive, representativeness, and timeliness.
Often difficult to measure precisely, even indirect estimates can be useful
in improving the efficiency of a system and in comparing it to other systems.
Sensitivity of a STD surveillance system can be considered
on two levels. First, at the level of STD case reporting, sensitivity refers
to the ability of the system to identify the completeness of reporting from
the community. That is, how many cases were reported in relation to the number
of actual cases in the community. Another aspect of completeness of reporting
concerns the completeness of each individual report. That is, how complete
is the information gathered for each report. Second, the system can be evaluated
for its ability to detect epidemics. The sensitivity of a STD surveillance
system is affected by the likelihood that:
Persons with certain STDs seek medical care.
The STD is correctly diagnosed, which reflects
the skill of care providers and the accuracy of the diagnostic tests.
The case will be reported to the system
once it has been diagnosed.
The measurement of sensitivity in a STD surveillance system requires
a) validation of information collected by the system and b) collection of information
external to the system to determine the frequency of STD in the community.
Practically speaking, the primary emphasis in assessing sensitivity is to estimate
the proportion of the total number of cases of STD in the community being detected
by the system. A surveillance system that does not have a high degree of sensitivity
can still be useful in monitoring trends, as long as the level of sensitivity
remains reasonably constant. Questions concerning sensitivity most commonly
arise when changes in the disease occurrence are noted. These changes can be
precipitated by such events as increased awareness of a disease, introduction
of new diagnostic tests, increasing morbidity, or changes in the method of
conducting surveillance.
Quality of reported data is an important element in any STD surveillance
system. This may be influenced by the clarity of surveillance reports, the
quality of training and supervision of persons who complete surveillance forms,
the care exercised in data entry and data management, and the regularity with
which the system is reviewed.
Predictive Value Positive (PVP) is the proportion of persons identified as being infected with STD who actually do have the disease under surveillance. With STDs, the PVP for a positive syphilis screening test (e.g., RPR) is considerably lower than that for a positive chlamydia or gonorrhea test. Therefore, the effect on available public health resources to confirm syphilis is different from that for chlamydia or gonorrhea. A record of the number of case investigations and the proportion of persons who actually have the condition under surveillance allows the calculation of the PVP at the level of case detection. When assessing PVP, primary emphasis is laced on the confirmation of cases reported through the surveillance system. Its effect on the use of public health resources can be considered on two levels: the ability to detect a single infection; and potential epidemics. At the individual case level, PVP affects those resources required for investigation of cases. A STD surveillance system with low PVP, and therefore with frequent false-positive case reports, will require a heavy expenditure of program resources to identify very few new cases. False positives or negatives result in disruption of patients' lives and create negative impressions of the system. With regard to potential "outbreak" situations, a high rate of erroneous case reports over the short term might trigger an inappropriate and costly response. A low PVP means that (a) non- ases are being investigated, and (b) there may be mistaken reports of epidemics. False-positive reports to surveillance systems lead to unnecessary interventions, and falsely detected "epidemics" lead to costly investigations. A surveillance system with high PVP will minimize unnecessary and inappropriate expenditure of resources. Understanding and properly applying PVP can help programs to make the most appropriate and cost-effective use of available resources.
Representativeness of a STD surveillance system refers to its
ability to accurately describe: a) the occurrence of STD over time, and b)
its distribution in the population by place and person. This may be examined
through special studies or surveys that seek to identify a probability sample
of all cases. Although this information is not generally available in specific
detail, some judgment of the representativeness of surveillance data is possible
on the basis of knowledge of the following:
characteristics of the population (e.g., age, geographic location, etc.)
natural history of the STD (e.g., latency period, outcome, etc.)
prevailing medical practices (e.g., site performing diagnostic tests, and physician-referral patterns, etc.)
multiple sources of data (e.g., laboratory reports for comparison with physician reports, etc.)
An important benefit to determining the representativeness of a STD surveillance system is the opportunity to identify population subgroups (e.g., migrant workers or prison inmates) that may be systematically excluded from the reporting system. Errors and bias can make their way into a STD surveillance system at any stage. Because STD surveillance data are used to identify high-risk groups, to target interventions, and to evaluate interventions, it is important to be aware of the strengths and limitations of the information in the system.
Timeliness of a surveillance system reflects its ability to identify the need to take appropriate action based on the urgency of the problem and the nature of the public health response. Timeliness is usually measured in days or weeks; for diseases that do not necessitate an immediate response, it might be measured in months or even years. Several dates are critical to properly evaluating timeliness of reporting. They are (a) date of symptom onset (date the patient first noticed symptoms); (b) date of examination or specimen collection; (c) date of laboratory tests; (d) date of diagnosis; and (e) date reported to the responsible public health agency. Other dates that can be used to determine program effectiveness include date treated, date assigned, and date interviewed. It is also affected by the time that 1) the clinician takes before sending orders to the laboratory and 2) the laboratory takes to report results to the health department and clinician.
Centers for Disease Control and Prevention
1600 Clifton Rd, Atlanta, GA
30333, USA
800-CDC-INFO (800-232-4636) TTY: (888) 232-6348, 24 Hours/Every Day cdcinfo@cdc.gov
Program Operations Guidelines for STD Prevention
