The effect of varying the screening interval on false positives and duration of undiagnosed disease in a screening programme for type 2 diabetes

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 toleranc e 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 popu lation for each screening interval. Setting-Ely, Cambridgeshire, UK. Methods-We used the glucose tolerance data from 965 participants of the fly Diabetes Project, who were tested 4.5 years apart, to calculate the popula tion's between and within person variance for 2 hour plasma glucose, and co nstructed a probability matrix of observed v true glucose tolerance categor ies. 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 h alf of those labelled with diabetes would not have the disease. For a 5 yea r interval, the false positive percentage was 27.6%, but the population wou ld have been exposed to undiagnosed diabetes for 144 person years. Conclusions-Screening can be associated with both benefit and harm; the bal ance is dependent on characteristics of the disease and the screening progr amme. This study has quantified the trade off between exposure to undiagnos ed diabetes and false positive results to inform the debate about screening for type 2 diabetes.