Data sources and study population

The Scottish Health Survey is undertaken periodically to monitor the health and health-related risk factors of the Scottish population, and has cardiovascular disease as its principal focus. The survey uses multi-stage, stratified sampling to provide a representative sample of the Scottish population, and includes face-to-face interviews and physical measurements. Different participants are recruited to each survey. We combined data from the last two surveys, undertaken in 1998 and 2003, and included participants aged between 40 and 74 years inclusive. Participants were excluded if they had already been diagnosed with cardiovascular disease or had missing data required to compute their risk score.

Simulation models

The Scottish Health Survey data were used to simulate the impact of five screening strategies to identify those at high risk of cardiovascular events.

1. Mass screening of the whole population

2. Screening individuals living in deprived communities

3. Screening individuals with a family history of premature cardiovascular disease

4. Screening individuals who either lived in deprived communities or had a family history of premature cardiovascular disease

5. Screening individuals who both lived in deprived communities and had a family history of premature cardiovascular disease.

The simulation models were run separately for everyone at risk of cardiovascular disease (ages 40–74 years) and for those at risk of premature cardiovascular disease (men aged 40–54 years and women aged 40–64 years).

Definitions

High risk was defined as an ASSIGN risk score of ≥20. This corresponds to a ≥20% risk of a cardiovascular event over the subsequent 10 years and is the recommended cut-off for primary prevention in Scotland.2 The ASSIGN score is derived from age, sex, systolic blood pressure, cigarette consumption, family history and socio-economic status.3 Deprived communities were defined as those in the bottom quintile of the Scottish Index of Multiple Deprivation (SIMD) (http://www.scotland.gov.uk/Topics/Statistics/SIMD/Overview) for the population as a whole (all ages). The SIMD score is an aggregated measure of material deprivation derived from 37 indicators in seven domains (income, employment, health, education, access to services, housing and crime) and is determined at the data zone level (geographical areas with a median population of 769). In the Scottish Health Survey, a family history of premature cardiovascular disease was defined as death of either natural parent as a result of cardiovascular disease before the age of 65 years.

Cost and cost-effectiveness

We assumed that the screening process would be identical in all of the models studied and therefore that the unit costs would be common to all. We included the costs of contacting people and arranging appointments, a screening appointment undertaken by a practice-based nurse, the laboratory costs of assaying cholesterol and glucose concentrations and the cost of a follow-up appointment at which the results would be fed back. Screening costs were based on the estimated 2008 prices published by the Department of Health for England and Wales and incorporated a sensitivity analysis.3 We included only the costs associated with identifying people at high risk. Subsequent investigation and treatment costs were not included in the models, as this was not the focus of our research question.

We undertook three sets of analyses. First, we determined the absolute costs and effects associated with implementing each of the five screening strategies in isolation referent to no screening. The effectiveness of each strategy was defined as the number needed to screen (NNS) to detect one person at high risk of cardiovascular disease. The cost of detecting one person was calculated as the unit cost per patient screened multiplied by the NNS for that strategy. Second, we determined the additional costs and effects of each strategy referent to the other strategies, by assuming that more effective strategies were substituted in an incremental fashion, from the lowest population coverage (lowest overall cost) up to mass screening of the whole population (highest overall cost). We calculated the cost and NNS of detecting one additional person at high risk and thereby derived the incremental cost effectiveness ratios (ICER) of each strategy. Consistent with standard practice in health economics, any strategies that were both more costly and less effective (higher NNS) than the next incremental strategy were then excluded from the calculations as they were “dominated” by the more effective strategy. In addition, any strategies that were associated with a higher ICER than more effective (lower NNS) strategies were excluded from the calculations as they were “extended dominated” Where this occurred, the ICERs were then recalculated following the exclusion of the dominated or “extended dominated” strategy.

Sensitivity analyses

We tested the robustness of our results by applying sensitivity analyses to the costs, using an analysis of extremes, and to the differential uptake of screened populations, using a threshold analysis. For the sensitivity analysis of costs, the ICERs were recalculated using the lower and upper bounds for screening costs estimated by the Department of Health (table 1).4 For the sensitivity analysis of uptake, we determined the lowest uptake level among the most deprived quintile at which the relative rankings based on ICER still held true.

Screening everyone at risk of cardiovascular disease

Targeting deprived communities would result in 15% of the total population being screened but would identify 25% of the high-risk population (table 2). To identify one high-risk individual would require 3.0 people to be screened at a cost of £69. Targeting the offspring of people who die prematurely from cardiovascular disease would result in 28% of the total population being screened but would identify 43% of the high-risk population (table 2). To identify one high-risk individual would require 3.2 people to be screened at a cost of £75. Combining both strategies would enable 57% of the high-risk population to be identified by screening 39% of the general population. Moving directly from no screening to mass screening would identify all high-risk individuals and would require 4.9 people to be screened to identify one high-risk individual at a cost of £113.

In the incremental cost-effectiveness analysis, targeting deprived communities was dominated by targeting family members which was a more effective strategy, requiring fewer additional people to be screened to detect one additional high-risk person. Compared with the most effective targeted strategy (combining family members and deprived communities) the additional cost of expanding coverage to mass screening was £199 for every additional high-risk person identified because an additional 8.6 people needed to be screened.

Screening those at risk of premature cardiovascular disease

Targeting deprived communities would result in 17% of the total population being screened but would identify 45% of the high-risk population (table 3). To identify one high-risk individual would require 6.1 people to be screened at a cost of £141. Targeting the offspring of people who die prematurely from cardiovascular disease would result in 28% of the total population being screened but would identify 61% of the high-risk population (table 3). To identify one high-risk individual would require 7.4 people to be screened at a cost of £170. Combining both strategies would enable 84% of the high-risk population to be identified by screening only 41% of the general population. Compared with no screening, mass screening would identify all high-risk individuals and would require 16.0 people to be screened to identify one high-risk individual at a cost of £370.

In the incremental cost-effectiveness analysis, a combined strategy of targeting both family members and deprived communities “extended dominated” either strategy in isolation, because this more effective strategy could be secured for a lower cost per additional high-risk person identified. Compared with combined screening of family members and deprived communities, expanding coverage to mass screening would require an additional 58.8 people to be screened to identify each additional high-risk person at a cost of £1358.

Sensitivity analyses

The cost sensitivity analysis changed the absolute values of the ICERs but the relative rankings of the screening strategies, in terms of cost-effectiveness, remained unchanged. When we re-ran the models applying lower uptake levels in the deprived quintile, we found that targeting only family members in the most deprived quintile, the narrowest screening strategy, remained a more cost-effective screening strategy than screening family members as long as uptake among the deprived remained at or above 11% for preventing any cardiovascular disease (or 7% for preventing premature cardiovascular disease). Furthermore, the combined strategy of screening both family members and the most deprived quintile remained more cost-effective than mass screening as long as uptake in the deprived quintile remained at or above 7% for preventing any cardiovascular disease (or 5% for preventing premature cardiovascular disease).

Principal findings

Targeted screening is more cost-effective than mass screening as a method of identifying asymptomatic people at high risk of cardiovascular disease in the general population. If the aim is to identify people of all ages who are at high risk, targeting screening at individuals with a family history of premature cardiovascular disease is more cost-effective than targeting deprived communities. If the aim is to identify those at high risk of premature cardiovascular disease, then a combined strategy that targets both family members and deprived communities is more cost-effective than either strategy in isolation. The cost-effectiveness of targeted screening is not achieved at the expense of coverage since this combined strategy identifies the vast majority of high-risk people in the general population.

Other studies

In the United Kingdom, the Department of Health has asked primary care trusts to commission an expansion of primary prevention services.5 The existing evidence on the effectiveness of primary prevention delivered through primary care practitioners is largely based on a mass screening strategy.6 7 From a population perspective, primary prevention is most effective if targeted at individuals with a high global risk of cardiovascular disease.1 However, determining which asymptomatic members of the general population have a high global risk presents obvious logistical problems. Mass screening is the best method to ensure complete coverage but the absolute cost is prohibitive. In our model, the cost of screening all 1.4 million people aged 40–74 in Scotland would be £33 million. An alternative approach is to target screening at a subgroup of the population in which cardiovascular risk is over-represented. Deprived communities have a higher prevalence of cardiovascular risk factors and a higher incidence of cardiovascular events.8 Similarly, people with a family history of premature cardiovascular disease have a twofold risk of developing the condition, owing to a combination of shared genetic predisposition and shared lifestyle.9–11

Strengths and weaknesses

Twenty-nine per cent of the Scottish Health Survey participants were excluded from our analysis because of missing data required to calculate their risk score. However, comparison of included and excluded participants demonstrated no significant differences in their breakdown by deprivation, smoking, family history or sex. Our findings were also robust in relation to the possibility of lower uptake of screening in deprived individuals. The uptake in the most deprived quintile had to be one-tenth of that in other groups before the relative rankings of screening strategies were altered.

The 1998 and 2003 surveys were combined to increase the statistical power of the models. We tested the robustness of our results by re-running the models using only the 2003 survey data. The results for 40–74 year olds were largely unchanged. Among those at risk of premature cardiovascular disease there was even stronger evidence against mass screening, with combined targeting of deprived communities and family members achieving 92% coverage of high-risk individuals. Therefore, extension of targeted screening to mass screening produced even less incremental benefit.

The Scottish Health Survey only provided information on parental death from cardiovascular disease. It did not provide information on parental premature, non-fatal disease or premature disease in siblings and second degree relatives, all of which are included in the ASSIGN definition of family history. Therefore, our model is likely to have underestimated the potential coverage of a strategy targeting family members. In the Utah family health tree study, families with a positive family history accounted for 48% of all cardiovascular events and 72% of premature events.12 In our study the figures were only 43% and 61%, respectively. Use of more complete information on family history would have reduced still further the additional benefits of mass screening.

Our models assumed identical unit costs for screening. Therefore, the cost and cost-effectiveness rankings are not specific to the United Kingdom. However, in practice, the costs may be higher in deprived than affluent communities because of poorer uptake requiring more stringent efforts to attract participants into screening appointments. Our models were compared on the basis that the aim of primary prevention was to reduce cardiovascular events in the population as a whole. We did not consider their impact in relation to health inequalities. We demonstrated that a strategy targeted at family members would cover only 41% of those living in deprived communities (50% of those at risk of premature disease). Therefore, although such a strategy was the most cost-effective method of producing overall health gain, it is likely to be less effective at reducing health inequalities than a strategy targeting deprived communities.

Meaning of study

Screening to identify individuals in need of primary prevention should focus on family members and deprived communities. Mass screening of the whole population may be difficult to justify as the incremental cost is much higher and incremental effectiveness lower.

Unanswered questions and future research

Engagement in health promotion is more difficult to achieve in deprived communities. Further studies are required to determine how best to tailor interventions to improve cardiovascular risk in this group. Guidelines already exist recommending screening of people with a family history,2 13 but surveys suggest that only a minority are, in fact, screened.14 Identifying people with a family history from the general population is difficult. Using hospitalisation of a patient for premature cardiovascular disease as the trigger to contact family members may be a more feasible mechanism and may improve the motivation of asymptomatic relatives. To date, there has been only one published study of this type of intervention.15 Further studies are required to devise effective methods of identifying family members and reducing their cardiovascular risk.