This thesis focuses on glycemic control measured as HbA1c in type 1 diabetes (T1D) patients during childhood and especially during adolescence, both in a Swedish and an international context, and relates the glycemic control to the risk of complications in young adults.  

In studies I and II, the Swedish Pediatric Diabetes Quality Register (SWEDIABKIDS) and the Swedish National Diabetes Register (NDR) were used. More than 4000 young adults with T1D and data on HbA1c in NDR both in 2011 and 2012 as well as data on HbA1c in SWEDIABKIDS were used. The T1D patients with poor glycemic control during their teenage period had a risk for retinopathy several times higher than those with good glycemic control. The risk for micro- and macroalbuminuria was also higher in those with poor glycemic control and was most pronounced in the T1D patients with high HbA1c in both registers. Females had worse glycemic control than males during the teenage period and an increased risk of retinopathy as young adults.  

In studies III and IV, pediatric diabetes quality register data from, respectively, eight and seven Western high-income countries were collected in the year 2013. Data on about 60 000 T1D patients were analyzed according to mean HbA1c levels in the countries and related to actual age and T1D duration to determine if there were differences in glycemic control between the countries. There were large differences in mean HbA1c between the countries, both when related to age and T1D duration. Despite the differences in mean HbA1c, the increase in mean HbA1c with increasing age and T1D duration was very similar in all countries.  

The overall picture of these studies is that good glycemic control is very important to avoid complications of T1D as young adults, and it seems particularly important to maintain a good glycemic control during adolescence. Furthermore large differences in glycemic control in T1D patients in Western high-income countries were found. Despite the differences in glycemic control, the pattern of rising HbA1c with increasing age and duration of T1D was very similar in all countries. Females have worse glycemic control than males during their teenage period, both in Sweden and internationally, and they also have more retinopathy as young adults.   

This thesis shows that it is of the utmost importance to treat T1D patients intensively directly after diagnosis, to treat the young T1D patients intensely and to reduce the rise in HbA1c with increasing age and duration of T1D in order to avoid complications early in life. Diabetes quality registers give the opportunity to compare results and share experiences, both within and between countries, so treatment of T1D can be designed in the best possible way and thereby minimize T1D complications.