Mortality trends among people with type 1 and type 2 diabetes in Australia: 1997-2010

OBJECTIVE: With improvements in cardiovascular disease (CVD) rates among people with diabetes, mortality rates may also be changing. However, these trends may be influenced by coding practices of CVD-related deaths on death certificates. We analyzed trends of mortality over 13 years in people with diabetes and quantified the potential misclassification of CVD mortality according to current coding methods. RESEARCH DESIGN AND METHODS: A total of 1,136,617 Australians with diabetes registered on the National Diabetes Services Scheme between 1997 and 2010 were linked to the National Death Index. Excess mortality relative to the Australian population was reported as standardized mortality ratios (SMRs). Potential misclassification of CVD mortality was determined by coding CVD according to underlying cause of death (COD) and then after consideration of both the underlying and other causes listed in part I of the death certificate. RESULTS: For type 1 diabetes, the SMR decreased in males from 4.20 in 1997 to 3.08 in 2010 (Ptrend < 0.001) and from 3.92 to 3.46 in females (Ptrend < 0.01). For type 2 diabetes, the SMR decreased in males from 1.40 to 1.21 (Ptrend < 0.001) and from 1.56 to 1.22 in females (Ptrend < 0.001). CVD deaths decreased from 35.6 to 31.2% and from 31.5 to 27.2% in males and females with type 1 diabetes, respectively (Ptrend < 0.001 for both sexes). For type 2 diabetes, CVD decreased from 44.5 to 29.2% in males and from 45.5 to 31.6% in females (Ptrend < 0.001 for both sexes). Using traditional coding methods, ∼38 and 26% of CVD deaths are underestimated in type 1 diabetes and type 2 diabetes, respectively. CONCLUSIONS: All-cause and CVD mortality has decreased in diabetes. However, the total CVD mortality burden is underestimated when only underlying COD is considered. This has important ramifications for understanding mortality patterns in diabetes.