Mortality rates and the causes of death related to diabetes mellitus in Shanghai Songjiang District: an 11-year retrospective analysis of death certificates

Trends in mortalities tend to reflect the level of economic development and the changing pattern of health services of a country or region [24, 25]. In the present study, it showed an increased trends in diabetes related mortality with growing proportions of CVD prevalence and mortality in a Chinese population. In the first decade of the 21^st century, China has been one of the most rapidly developing countries in the world with an increase in life expectancy and changes in lifestyle [4]. The crude death rates for China, including for Shanghai [26] and Songjiang have varied only slightly during this decade, but the diabetes mortality (diabetes related death) increased about 1.78 fold (see Fig. 2) in Songjiang District. Similarly, the prevalence of diabetes has increased dramatically during a similar period in China, from 5.5 % in 2001 [27] to 11.6 % in 2010[28]. This reveals that the increased diabetics should be owed to the growth of incidence, which should be faster than the growth of mortality. Certainly, the development of medical techniques should help to detect more and more individuals with undiagnosed diabetes since last decades, which could led to a pseudo increase of incidence. In spite of the striking increase of diabetes related death (by 178 %), the mortality of diabetes as the underlying cause increased only 30 %. In addition, the proportion of diabetes as the underlying cause reduced significantly, of which especially the proportion of subject without complications (Fig. 3). This may reflect improvement of health service like effective treatment of hyperglycaemia and efforts to prevent diabetic complications (not counting CVD) by local health care systems. On the other hand, it suggests that the central and local governments should pay more attention to primary preventions aimed at lowering the incidence and ultimately to reduce the burden of diabetes. Moreover, the descending proportion of unspecified type may be attributable to improvements in proper diagnosis by clinicians. That is, the types of diabetes for a large amount of decedents had been already clarified and/or recorded before death. This is in conformity with the progress of local health service (Data from “Statistic Yearbook of Songjiang” 2004 – 2013,published by local government).

People with type 2 diabetes have an increased risk of developing CVD. In the present study, 73.6 % of deceased diabetics had suffered from CVD, and about 30 % died directly of CVD, which is higher than India (13.7 %) [29] and Taiwan (19.8 %) [30], and is lower than the western countries (49.4 % for the U.S. and 49.1 % for U.K.) [31]. But this comparison is not totally appropriate because it can be biased by many factors including the difference of coding system among countries, the habits of recording physicians, the subjective judgement of them, and so on [20, 21]. We used continuous data to describe the variation of mortality of CVD, which is more informative than a static observation and helpful to assess the CVD burden. The changing pattern during the 11-year period manifests that the increasing deaths associated with diabetes should chiefly be owed to cardiovascular diseases but have nothing to do with diabetes as the underlying cause, that is, other diabetic complications and diabetes without complications. As shown in Fig. 2, the curve of death rate for people with diabetes and CVD comorbidity shows nearly parallel to the curve of diabetes related mortality. In fact, the mortality for people with comorbidity increases faster than the mortality for all diabetics, that is, the proportion of deceased diabetics with CVD comorbidity increased from 62–82 % during the 11 years. However, the mortality rate for people with other complications varied very slightly; even the proportion of deaths without complications decreased more than three fold in the same period.

Mortality statistics are an important source of health information for analysing the burden of diseases and are often used to assign priorities in health policy [2]. In China, there are two sets of mortality registration systems, the MOH-VR and the DSP. In general, the DSP provides more representative and more accurate data than the MOH-VR, especially in rural areas [22]. Songjiang District is one of the national DSP sites since the last decades, where the non-agricultural population accounted for 46 % in 2003 and 83 % in 2012 (Data from “Statistic Yearbook of Songjiang” 2004 – 2013). Considering that Songjiang locates in the most developed area in China, the number of hospital beds per thousand people was 6.5 in 2003 and 8.0 in 2012, that is, the local health service are quantitatively and qualitatively in the forefront of the country [26], thus the accuracy of the data should be in a higher level comparing with other areas of the country.

In general, estimation of diabetes mortality is challenging due to various reasons. For example, these challenges stem from people with diabetes most frequently dying of cardiovascular diseases or renal failure, and the physicians who record and determine the causes of death may miss and underestimate the contributions of diabetes on death [11‐19, 32]. In the present study, the death certification in Songjiang listed up to seven causes contributing to death, of which the underlying causes were determined by certified doctors. We collected all records mentioning diabetes in the certificates to assess contributions of diabetes on death; thus the estimate we calculated could be very similar to a mortality statistic for the people with diabetes.

Although we are confident of the higher quality of data, data from death certificates have potential to distort mortality rates because of the ineluctability of incorrect reporting. One possible explanation for the steep increase in diabetes related deaths could be increased awareness of the disorder. Since the Center for Non-communicable Disease of the Chinese Center for Disease Control and Prevention (China CDC) was established in 2002, surveillance on chronic diseases, such as hypertension and diabetes etc., was gradually highlighted by local CDCs. Therefore, an underestimation of mortality should be taken into account, especially in the earlier stage of the investigation. Furthermore, the International Statistical Classification of Diseases and Related Health Problems (known as ICD) facilitates mortality statistics by standardizing causes of death [33], and also request the capacities of certifying doctors in the correct ICD procedures [34]. But even in developed countries there was often substantial use of ICD codes for unknown and ill-defined causes [33]. Also ill-defined causes (ICD-10 codes R00-R99) existed in our data (230 cases for the contributing causes, but none was in the underlying cause), that would also affect the validity of our estimates. Death certificate is one of the most important sources of data regarding people health, but the data quality is often concerned. There were few studies concerning the accuracy of recorded cause of death of diabetic decedents in China. Therefore, assessment and further improvement of death certificate should become a key emphasis in work of the health care system in the future, which would help us to correctly understand the burden of diabetes on survival [35].

Another limitation relates to the representativeness of local population. Due to China’s household registration system, only permanent residents were recorded by local CDCs. People who lived in Songjiang but were registered in other regions were not included in the present study. Because the majority of these people were young adults who worked in Songjiang, the diabetes related mortalities for the total population would be smaller than the data in the present study. Moreover, though the data of the study cannot represent the characteristics of entire Chinese population, it should be also meaningful and suggestive for policymakers. Since Shanghai is one of the most developed area of China and has a very higher proportion (>20 %) of old people (60 years or older, permanent residents) [36]. This is and will be a similar situation in other areas of China.

The present survey provides only information restricted in deceased diabetics, therefore the results should be explained carefully. For example, although the proportions of those who died from CVD rose rapidly from 2002 to 2012, we cannot attribute the increase of CVD mortality only to the prevalence of diabetes. Because we did not collect mortality data of people who did not have diabetes, we do not know the situation of CVD mortality for them [37]. In contrast, from the changing proportions it could be clearly determined that CVD contributed more and more on diabetes deaths from 2002 to 2012.