Objectives
The aims of this study were to quantify the proportion of people diagnosed as having type 2 diabetes by standard 75 g oral glucose tolerance test, in a hypothetical screening programme, who would actually be false positives (false positive percentage), and the effect on the false positive percentage of varying the time between repeat screens. We also calculated the duration in person years of exposure to undiagnosed disease in the population for each screening interval.
Setting
Ely, Cambridgeshire, UK.
Methods
We used the glucose tolerance data from 965 participants of the Ely Diabetes Project, who were tested 4.5 years apart, to calculate the population's between and within person variance for 2 hour plasma glucose, and constructed a probability matrix of observed v true glucose tolerance categories. The progression of the population between glucose tolerance categories was modelled assuming exponential times to progression.
Results
After one year, 47.5% of test positives were disease free: almost half of those labelled with diabetes would not have the disease. For a 5 year interval, the false positive percentage was 27.6%, but the population would have been exposed to undiagnosed diabetes for 144 person years.
Conclusions
Screening can be associated with both benefit and harm; the balance is dependent on characteristics of the disease and the screening programme. This study has quantified the trade off between exposure to undiagnosed diabetes and false positive results to inform the debate about screening for type 2 diabetes.
