Effect of population screening for type 2 diabetes and cardiovascular risk factors on mortality rate and cardiovascular events: a controlled trial among 1,912,392 Danish adults

As governments seek to apply the principles of prevention to chronic disease, health check programmes have been proposed or introduced in a number of countries, including the UK and the USA [1, 2]. These usually include assessment and management of risk factors for chronic disease, most of which are related to cardiovascular disease (CVD). Modelling studies suggest that screening for diabetes and cardiovascular risk assessment might be both effective and cost-effective; however, these studies rely on a number of assumptions [3‐7]. There are relatively few trials assessing the benefits and harms of screening at the population level [8, 9]. A Cochrane review of randomised trials comparing health checks with no health checks in adult populations found that they did not reduce morbidity, all-cause mortality or cardiovascular-related mortality rates, although the number of new diagnoses increased [10]. The review included data from a number of historical cohorts that were initiated before the widespread introduction of effective treatments such as statins. More recent studies examining the impact of systematic population-wide screening have shown mixed results. INTER-99 reported no effect and the Västerbotten Intervention Programme, which combined screening with a wider public health promotion programme, reported mortality rate reductions [9, 11].

Given the limited evidence of the impact of population-based screening programmes, it is important to explore whether health checks might have different impacts in contemporary populations. It is also critical to evaluate the impact of screening on overall population mortality rates rather than simply disease-specific mortality or disease event rates to quantify overall benefits and harms at the population level [12]. Between 2001 and 2006, a population-based cardiovascular risk assessment and diabetes screening programme was introduced in five Danish counties as part of the Anglo–Danish–Dutch Study of Intensive Treatment in People with Screen-Detected Diabetes in Primary Care (ADDITION) study [13]. The Danish national registration system enables a post hoc analysis of the rates of mortality and cardiovascular events in individuals who were invited to take part in the ADDITION-Denmark screening programme compared with individuals who were not invited during the same time period.

ADDITION-Denmark consists of two phases: a pragmatic screening programme; and a cluster-randomised trial comparing the effects of intensive multifactorial therapy with routine care among individuals with screen-detected type 2 diabetes [13, 14]. We report results from a post hoc analysis of data from the screening phase of the study in conjunction with outcome data from Danish national registers. Ethical approval for the ADDITION-Denmark study was granted by a local scientific committee (no: 20000183). As this was a registry-based study using anonymised data, participants did not give informed consent. This approach was approved by the Danish Data Protection Agency and the Danish Health and Medicine Authority.

The screening and control groups were well balanced for age, sex and citizenship at baseline (Table 1). Compared with the control group, a larger proportion of the screening group (330,096 [19.3%] vs 34,648 [23.1%]) had received >15 years of education. Slightly higher proportions of the control group had experienced IHD, stroke or cancer compared with the screening group.

Of 153,107 eligible people in the screening group, 27,177 (18%) attended their GP for a diabetes test and a cardiovascular risk assessment. Of these, 1533 participants (1% of those eligible for screening) were diagnosed with diabetes and 2526 (1.6%) had IFG/IGT. Of all individuals who attended screening for ADDITION-Denmark with complete data for calculating CVD risk, 9693/20,223 (48%) had high CVD risk (European Heart SCORE ≥ 5 points). In total, 6855 (28%) of attenders with available data reported being a current smoker. There were 1,759,285 individuals in the no-screening control group.

In this population-based sample of nearly two million middle-aged Danish adults, a single round of screening for type 2 diabetes and cardiovascular risk assessment was not associated with a reduction in the rates of all-cause mortality or cardiovascular events between 2001 and 2012. Similarly, rates of cardiovascular-, cancer- and diabetes-related mortality were not reduced by invitation to screening.

Modelling studies have previously suggested a benefit of population screening for diabetes and related risk factors for vascular disease [3‐7]. This finding was supported by examination of the Ely cohort [22], in which individuals aged 40–65 years who were invited to diabetes screening every 5 years between 1990 and 1999 had a non-significant 21% lower all-cause mortality rate than individuals who were not invited to screening; however, this finding was not replicated in between 2000 and 2008.

A more recent screening and health promotion CVD intervention undertaken in Västerbotten County, Sweden, also showed a significant long-term reduction in all-cause and CVD mortality rates when compared with the general Swedish population [11]. This complex intervention was different from the ADDITION-Denmark study. The screening programme and health counselling offered to individuals was nested within a public health prevention programme targeting the wider collective determinants of diabetes and CVD. However, our findings mirror those of a pragmatic, parallel-group, cluster-randomised trial in 33 general practices in eastern England (ADDITION-Cambridge) [8], in which screening for type 2 diabetes in those at high risk did not result in a reduction in all-cause or cardiovascular- or diabetes-related mortality rates over 10 years. Our results also reaffirm a recent Cochrane review [10], which found no long-term impact of general health checks on mortality and morbidity rates following population screening.

There are many potential explanations for the lack of difference in mortality rates and cardiovascular events observed in this trial. The study was undertaken against a background of national interest about screening and early treatment for diabetes in Denmark. The effect of screening may therefore have been diluted by opportunistic screening in primary care and by continuing improvement in the detection and management of CVD risk factors. Only 27,177 (18%) of individuals in the screening group attended their general practice for testing which combined with the less than 100% sensitivity at each step of the screening programme might have contributed to the low yield of individuals found to have detectable disease (1% of the invited population) [14]. Although earlier detection may have benefitted those diagnosed with diabetes [18], the proportion was probably too small to affect population mortality rates. Furthermore, screening attendees are frequently those at lowest risk [23].

For cancers such as prostate, any effect of screening on population mortality rates is achieved largely through the effect of treatment in the lead time between detection by screening and clinical diagnosis [24]. By contrast, CVD risk factor screening identifies a significant proportion of the population that might benefit from health promotion and lifestyle change, which could also impact other health outcomes such as cancer. Unlike the ADDITION-Cambridge trial, the Danish screening programme included assessment of CVD risk factors alongside screening for undiagnosed prevalent diabetes. This is similar to recommendations in the UK NHS Health Checks Programme [1]. Danish GPs were encouraged to intervene in individuals with IFG/IGT and those with high CVD risk but without diagnosed diabetes. While small numbers of individuals were diagnosed with diabetes following screening in Denmark (1%), 1.6% were found with IFG/IGT and 48% with elevated cardiovascular risk (SCORE > 5 points), providing a larger at-risk group with the potential to benefit from interventions to prevent both diabetes and CVD [25, 26]. Indeed, larger proportions of people in the screening practices redeemed cardioprotective medication compared with the control practices during the screening phase of the study (2001–2006). However, these differences were small and were not sustained. Furthermore, there is evidence of sub-optimal treatment among individuals identified at high risk in health check programmes [27] and among those with screen-detected diabetes [18], which might have contributed to the lack of difference between groups. Indeed, follow-up of ADDITION-Denmark participants who did not have diabetes but had high CVD risk and dyslipidaemia showed that only 20% were started on lipid-lowering treatment following screening [28]. Of these, only 53% reached the treatment goal of total cholesterol <5.0 mmol/l and LDL-cholesterol <3 mmol/l within 1 year of starting treatment [29]. Among those with screen-detected diabetes, there was wide variation between general practices in prescription of lipid-lowering treatment [30]. As such, there is evidence of both under-treatment and considerable delay in starting treating among this high-risk population. Benefits to the population might have been increased by identification of non-attenders, targeting of screening to those at greatest risk, strategies to maximise uptake of screening (particularly among those at highest risk), repeated rounds of screening and optimal treatment of detected disease. Benefits might also be increased by introducing complex interventions that target collective determinants of chronic disease alongside individualised screening and treatment programmes. For example, the Västerbotten Intervention Programme aimed to raise public awareness of CVD risk factors and lifestyle behaviours by tailoring activities to the local community, including the introduction of ‘the green keyhole’ food labelling system denoting low fat and high fibre foods, development of healthy school lunches, production and distribution of health educational materials, and health information meetings [11]. Participation rates in Västerbotten were 48–67%. Our results underline the continuing uncertainty about the overall benefits of population screening for diabetes and CVD.

As with all screening programmes, positive health outcomes are associated with uptake. In common with findings from other population-based screening programmes, we observed the lowest mortality rates in attenders, probably due, at least in part, to healthy volunteer bias.