Screening for type 2 diabetes: literature review and economic modelling.
To reconsider the aims of screening for undiagnosed diabetes, and whether screening should be for other abnormalities of glucose metabolism such as impaired glucose tolerance (IGT), or the 'metabolic syndrome'. Also to update the previous review for the National Screening Committee (NSC) on screening for diabetes, including reviewing choice of screening test; to consider what measures would be taken if IGT and impaired fasting glucose (IFG) were identified by screening, and in particular to examine evidence on treatment to prevent progression to diabetes in these groups; to examine the cost-effectiveness of screening; and to consider groups at higher risk at which screening might be targeted. Electronic databases were searched up to the end of June 2005. Literature searches and review concentrated on evidence published since the last review of screening, both reviews and primary studies. The review of economic studies included only those models that covered screening. The new modelling extended an existing diabetes treatment model by developing a screening module. The NSC has a set of criteria, which it applies to new screening proposals. These criteria cover the condition, the screening test or tests, treatment and the screening programme. Screening for diabetes was considered using these criteria. Detection of lesser degrees of glucose intolerance such as IGT is worthwhile, partly because the risk of cardiovascular disease (CVD) can be reduced by treatment aimed at reducing cholesterol level and blood pressure, and partly because some diabetes can be prevented. Several trials have shown that both lifestyle measures and pharmacological treatment can reduce the proportion of people with IGT who would otherwise develop diabetes. Screening could be two-stage, starting with the selection of people at higher risk. The second-stage choice of test for blood glucose remains a problem, as in the last review for NSC. The best test is the oral glucose tolerance test (OGTT), but it is the most expensive, is inconvenient and has weak reproducibility. Fasting plasma glucose would miss people with IGT. Glycated haemoglobin does not require fasting, and may be the best compromise. It may be that more people would be tested and diagnosed if the more convenient test was used, rather than the OGTT. Five economic studies assessed the costs and short-term outcomes of using different screening tests. None examined the long-term impact of different proportions of false negatives. All considered the costs that would be incurred and the numbers identified by different tests, or different cut-offs. Results differed depending on different assumptions. They did not give a clear guide as to which test would be the best in any UK screening programme, but all recognised that the choice of cut-off would be a compromise between sensitivity and specificity; there is no perfect test. The modelling exercise concluded that screening for diabetes appears to be cost-effective for the 40-70-year age band, more so for the older age bands, but even in the 40-49-year age group, the incremental cost-effectiveness ratio for screening versus no screening is only 10,216 pounds per quality-adjusted life-year. Screening is more cost-effective for people in the hypertensive and obese subgroups and the costs of screening are offset in many groups by lower future treatment costs. The cost-effectiveness of screening is determined as much by, if not more than, assumptions about the degree of control of blood glucose and future treatment protocols than by assumptions relating to the screening programme. The very low cost now of statins is also an important factor. Although the prevalence of diabetes increases with age, the relative risk of CVD falls, reducing the benefits of screening. Screening for diabetes meets most of the NSC criteria, but probably fails on three: criterion 12, on optimisation of existing management of the condition; criterion 13, which requires that there should be evidence from high-quality randomised controlled trials (RCTs) showing that a screening programme would reduce mortality or morbidity; and criterion 18, that there should be adequate staffing and facilities for all aspects of the programme. It is uncertain whether criterion 19, that all other options, including prevention, should have been considered, is met. The issue here is whether all methods of improving lifestyles in order to reduce obesity and increase exercise have been sufficiently tried. The rise in overweight and obesity suggests that health promotion interventions have not so far been effective. The case for screening for undiagnosed diabetes is probably somewhat stronger than it was at the last review, because of the greater options for reduction of CVD, principally through the use of statins, and because of the rising prevalence of obesity and hence type 2 diabetes. However, there is also a good case for screening for IGT, with the aim of preventing some future diabetes and reducing CVD. Further research is needed into the duration of undiagnosed diabetes, and whether the rise in blood glucose levels is linear throughout or whether there may be a slower initial phase followed by an acceleration around the time of clinical diagnosis. This has implications for the interval after which screening would be repeated. Further research is also needed into the natural history of IGT, and in particular what determines progression to diabetes. An RCT of the type required by NSC criterion 13 is under way but will not report for about 7 years.