Defining the target population for a screening program involves describing the settings and the characteristics of the collection of persons who would be eligible for screening (Last 1995). This should be the first step in planning a population-based screening program because screening programs should be tailored to meet the needs of a defined target population. In practice, however, screening programs are developed and implemented in response to the availability of an effective screening test for a particular health condition. Concepts and principles for selecting a target population are defined at the beginning of this chapter to reinforce the advantages of thinking about ends before means. Important concepts related to defining the target population include community, settings, mass versus selective screening, risk stratification, and high-risk versus population strategies.
Community. A community is "a group of individuals organized into a unit, or manifesting some unifying trait or common interest; loosely, the locality or catchment area population for which a service is provided, or more broadly, the state, nation, or body politic" (Last 1995). Green and Kreuter have refined the definition of community to distinguish between the structural and functional dimensions of the concept. They state that, structurally, a community is a neighborhood, township, city, county, district, or metropolitan area demarcated by geographic and often political boundaries; and functionally, a community is a place where "members have a sense of identity and belonging, shared values, norms, communication, and helping patterns." Further, they continue, "unrestrained by place, a functional community of interest or constituency of concerned citizens also may exist, scattered across one or more geopolitical jurisdictions" (Green and Kreuter 1991).
Several important considerations are warranted when defining a target population for screening within the community. First, the size and characteristics of the target population should be tailored to the availability and appropriateness of the resources available to provide advice, referral, and follow-up for participants (Rose 1992; Wilson and Jungner 1968). Second, the characteristics of the target population should be similar to those of the populations in which the screening test has been accurate and early detection and follow-up have been proven effective (USPSTF 1996). And third, governmental public health agencies often must define target populations within geopolitical boundaries and constituent groups prescribed by a legislative mandate. Nongovernmental public health agencies often have more flexibility in defining target populations that cut across the boundaries of governmental mandates. Thus, coalitions between governmental and nongovernmental agencies offer the greatest flexibility in ensuring that a successful screening program reaches the widest possible target population (Grad 1990; Institute of Medicine [IOM] 1988).
Settings. Mullen and Evans reviewed the importance of settings in health promotion and disease prevention (Mullen et al. 1995). They defined settings as major social structures or institutions that provide channels and mechanisms of influence for reaching defined populations. These institutions are characterized by patterns of formal and informal membership and communication and involve frequent and sustained interaction among members. Potential settings for screening programs include community units (e.g., neighborhoods, counties), workplaces, schools, places of worship, and health care settings. Settings often vary in their capacity to reach populations differentiated by age, gender, socioeconomic status, race, ethnicity, or combinations of such characteristics.
Settings are important to screening policy, programs, and research because they obviate the need to create new social structures, are organized for purposes more deeply binding than the single mission of health improvement, and create efficiencies in time, resources, access to selected populations, and potential for social influences (Mullen et al. 1995).
Mass and Selective Screening. In mass screening, the entire target population, regardless of level of risk for the target condition, is eligible to be screened (Last 1995). For example, in a screening program to detect hypertension, all residents of county X, regardless of age, may be deemed eligible for screening. In selective screening, the target population is refined to include only those persons who meet a predetermined set of eligibility criteria. Such eligibility criteria may include age, sex, race, ethnicity, comorbid conditions, or a combination of risk factors. For example, to prevent end-stage renal disease (ESRD) among residents of county X, a highly selective screening program for hypertension may restrict eligibility to black men with non-insulin-dependent diabetes mellitus (NIDDM) because risk is very high for this population group. Sackett has used the term "case finding" to character ize the strategy of selective screening when the target population is restricted to persons who visit a physician for intercurrent illnesses or other purposes unrelated to the condition being screened for. For example, the physicians in a managed care plan may decide to screen for hyperlipidemia or hypertension among all enrollees 40 years or older who are seen for any reason during a 2-year period (Sackett et al. 1991).
Selective screening and care usually are more cost effective than mass screening (Rose 1992). Opportunities for the application of cost-effective mass screening are likely to be confined to special conditions (e.g., neonatal screening) or in selected settings. For example, screening by chest radiography to rule out or detect active tuberculosis (TB) in jails with high turnover, some homeless shelters, and among immigrants and refugees recently arrived from countries with a high incidence or prevalence of TB might be considered an application of mass screening in selected settings (CDC 1995b). This latter example illustrates that the distinction between mass and selective screening is sometimes blurred.
Risk Stratification. The risk for occurrence of a health event (condition or disease) in an individual (or population) is the probability that the event will occur within a stated time period (Last 1995). Risk stratification is a process of classifying the members of a population into levels of risk (i.e., high, medium, or low) for the occurrence of a target health condition on the basis of risk factors (e.g., genotype, demographic characteristics, environmental exposure, or personal behavior) that are epidemiologically associated with that event (Kelsey et al. 1996). Often, risk stratification is essential to defining a target population for selective screening. For example, subgroups at higher risk for death from breast cancer include black women of all ages (20% higher mortality rate than white women) and black women >65 years of age (73% higher mortality rate than black women ages 45-64 years) (Brownson et al. 1993; Beckles et al. 1994).
A useful distinction exists between the high-risk and population strategies of preventive medicine (Rose 1992). The high-risk strategy of screening for hypertension to prevent stroke would entail the early detection and treatment of black or elderly residents of state X with diastolic blood pressure of >90 mmHg or systolic pressure >140 mmHg. Black or elderly residents would be targeted for screening because hypertension and stroke are more common in these groups than in their counterparts (USPSTF 1996).
The population-wide strategy for preventing stroke by blood pressure reduction would seek to shift the entire continuous population distribution of diastolic (or systolic) blood pressures toward lower values. Thus, some researchers have estimated that a nationwide moderation of salt intake would lower the blood pressure distribution by about 5% and prevent as many as
25% of all strokes (Stamler et al. 1989; Law et al. 1991). Population-wide strategies (e.g., nationwide reduction of salt intake to reduce prevalence of hypertension) are potentially more effective than high-risk screening and treatment strategies (e.g., early detection of black or elderly persons with hypertension) for the prevention of some conditions (e.g., stroke). However, the two types of strategies are complementary (i.e., they require different types of resources), potentially synergistic, and often can be implemented together (Rose 1992).
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