Increasing prevalence of type 2 diabetes in a Scottish population: effect of increasing incidence or decreasing mortality?

We examined incidence, prevalence and mortality from type 2 diabetes mellitus in a Scottish population over 12 years, and evaluated the effects on prevalence of increasing incidence and decreasing mortality. We used a diabetes clinical information system in Tayside (population 387,908), Scotland, to identify new cases of type 2 diabetes between 1993 and 2004 and to calculate incidence rates and mid-year prevalence. We defined mortality rates as the number of deaths of diabetic people divided by mid-year prevalence. We used logistic and Poisson regression to analyse trends. We then modelled the increase in prevalence for each year for three scenarios, based on whether mortality or incidence rates remained unchanged from 1993. There was a doubling in incidence and prevalence of type 2 diabetes in Tayside over the 12 years, with statistically significant increasing trends of 6.3 and 6.7% per year respectively. The mortality rate decreased. If incidence and mortality had remained at 1993 levels, there would have been an increase in prevalence of 855 per 100,000 in 2003, accounting for 60.1% of the actual increase of 1,423 per 100,000. If there had been no mortality decrease, prevalence in 2003 would have been very similar to the actual prevalence observed. Decreasing mortality rates in Tayside had less effect on the increase in prevalence than did increasing incidence. Even if incidence and mortality remain unchanged, prevalence will increase by over 20% in the next decade.