Diabetes mellitus (DM) is one of the most common and rapidly increasing chronic metabolic disorders in the world. In 2013, more than 382 million people had diabetes worldwide [1], corresponding to 8.3% of all adults. This number is expected to increase to 592 million about 10% individuals by 2035 [1]. Diabetes can cause serious complications and mortality, resulting in a large burden to the public health care system as well as to patients.

The management of patients with diabetes is a major challenge for health care systems. From the public health perspective, epidemiological data is useful to estimate the necessary manpower and resources needed for disease control and prevention of common chronic diseases. In the absence of reliable population-based data on diabetes, nationwide claims data can provide credible evidence for investigating the actual epidemiologic status and the medical cost of diabetes [2]. Claims data include a large population-based longitudinal registry and are widely used in epidemiology and health services research [3]. In Korea, a universal coverage was achieved by the government in 1989 and provides coverage for nearly 100% of the total population [2]. In 2013, the National Health Insurance system covered 97.2% (n=49,989,620) of the whole population in Korea, and the medical aid system covered the remaining 2.8% (n=1,458,871).

Because of the significant impact of diabetes on public health, the incidence of diabetes and associated annual medical claim cases in the general population of Korea has been investigated. Studies in initial stage have been carried out to examine the epidemiology of diabetes in Korea based on urban or rural community cohorts [4,5,6,7,8]. Recently, the Task Force Team for Basic Statistical Study of Korean Diabetes Mellitus from the Korean Diabetes Association has reported a 12.4% prevalence of diabetes in adults aged older than 30 years based on the nationwide Korea National Health and Nutrition Examination Survey (KNHANES) [9]. Although the KNHANES is a well-designed survey that reflects representative citizens in Korea, it does not assess trends in diabetes incidence or the status of medical care utilization in diabetic patients in Korea. Korean diabetes fact sheets 2015 showed that the prevalence of diabetes steadily increased from 5.6% in 2006 to 8.0% in 2013 from the health insurance claim forms made by National Health Insurance Service (NHIS) [10]. However, we could not find the incidence of diabetes. The latest study showed that the incidence decreased from 9.8/1,000 person-year in 2009 to 9.5/1,000 person-year in 2011 and prevalence of type 2 diabetes mellitus (T2DM) increased from 6.1% in 2008 to 6.9% in 2010 using the Health Insurance Review and Assessment database [11].

Therefore, we aimed to evaluate trends in the annual medical claims in diabetic patients and the incidence of diabetes in the general population of Korea using nationwide claims data from the NHIS database over the last decade.

In this nationwide study using claims data, we investigated the trends in the annual incidence of diabetes and the number of medical claims and patients with diabetes in the general population of Korea. During the 10 years from 2003 to 2012, the annual number of diabetes claim patients continuously increased by 186.7%, resulting in 2,571,067 individuals in 2012. Furthermore, the annual number of total claim cases for diabetes dramatically increased by 272.7% over this period, which can be attributed to the increase in the average number of claim cases per patient. However, the incidence of new diabetes claim patients gradually decreased from 2005 to 2009 and has remained steady since 2009, resulting in an overall 30.9% decrease from 2005. The average age of new claims subjects with diabetes codes decreased from 64 and 68 years in men and women, respectively, in 2004 to 57 and 61 years in 2012.

Although the prevalence of diabetes is influenced by diagnostic method and criteria, it has dramatically increased by 6.6-fold from 1.5% to 9.9% in the past 40 years in Korea [4]. A recent paper based on data from the KNHANES 2011 reported that the prevalence of diabetes was 12.4% (estimated number of subjects with diabetes in 2011 [4,006,686]) in Korean adults older than 30 years [12]. In agreement with these statistics, we observed a steady increase in the annual number of total claim patients with diabetes, though this increase is lagging behind the estimated number of subjects with diabetes in Korea. In 2011, a total of 2,459,952 people visited a hospital for diabetes, accounting for only 61% (2,459,952/4,006,686) of subjects with diabetes. This low proportion may have several causes: (1) subjects with undiagnosed diabetes [13]; (2) subjects with known diabetes who do not receive treatment [14]; (3) inaccurate coding for diabetes by medical professionals; and (4) differences in research design and methods.

Our report found that the increase in annual number of total claim cases for diabetes outnumbered that of total diabetes claim patients. This can be explained by the increase in the average number of claim cases per patient-year from 6.1 to 8.9 over the 10 studied years. Economic development, increased public interest in health-care, and changes in health policies that expanded health insurance coverage may have all influenced the increase in total number of diabetes claim cases [15]. Moreover, subjects with a longer duration of diabetes are likely to have more comorbidities that may require frequent hospital visits for medical care [14]. Further detailed investigation should be implemented to examine the overall characteristics of patients with diabetes in Korea. In terms of gender differences, a relatively rapid increase in the annual number of diabetes claim patients from 2003 to 2012 was seen in men (196.1%) compared to women (177.6%). Similarly, the annual number of total diabetes claim cases also increased more in men (286.1%) than in women (260.6%) during the 10-year interval.

Although the prevalence of diabetes and annual number of claim patients with diabetes in Korea steadily increased during the last 10 years, the incidence of new claim patients has remained steady since 2009. The maximal incidence rate was 1.28% and 1.36% in men and women, respectively, in 2005 compared to 0.85% and 0.82% in 2012. Similar findings were observed in a nationwide population-based pharmacoepidemiological study in Sweden [16]. A previous study reported that the estimated incidence of diabetes among Korean adults aged 20 to 79 years was 0.76% in 2003 [14]. A community cohort study by Cho [6] showed that the annual incidence range of T2DM was 1.33% to 2.35% in those in their 40s, 1.48% to 3.18% in those in their 50s, 1.85% to 4.55% in those in their 60s, and 3.08% to 5% in those in their 70s. Consistent with our findings, Koo et al. [11] reported a decrease in the incidence of T2DM from 0.98% in 2009 to 0.95% in 2011. However, the prevalence of obesity has gradually increased in Korean adults aged ≥19 years from 26.9% in 1998 to 32.0% in 2011 [17]. This discrepancy may be due to earlier detection of diabetes with improved accessibility to primary medical care and the establishment of the National Health Screening Exam (NHSE). Since 2000, the health insurance systems in Korea have been incorporated into a unified system operated by the NHIS [2]. As part of the NHIS health-care system, all insurance subscribers are allowed a free biannual NHSE [18]. According to the report of Ministry of Health and Welfare, the rate of undergoing NHSE was 35% in 1999, 48% in 2003, 55% in 2006, and 65% in 2008, there has been increased up to 71% in 2011 and 2012. However, increasing health check-up rate can be hardly expected to increase in number of new diabetes patients, the assumed reasons as follows: First, the increment of screening rate was rapidly increasing 20% from 1999 to 2008, and then it showed only 5% after 2008. This screening rate almost reached a plateau [19]. Second, the increasing proportion of examination subjects is occupied by previous examiners and the people who already have the disease including diabetes and hypertension [19]. Therefore, additional diagnosis of new diabetes may be reduced. In addition, the health checkup rate of younger ages increased because of increasing interests of health through various media; however, the proportion of young diabetes patients in young population is significantly lower than in the old age. As well as, the changing demographic structure of our country seems to be associated. As progressing aging country, the elderly populations are increasing; in contrast, young populations are actually shrinking.

While the number of new diabetes claim patients did not significantly change over the last 4 years, the average age of new diabetes claim patients has decreased by 6 years from 2003 to 2012, regardless of gender. Furthermore, the age range of the majority of new diabetes claim patients has decreased from the 70s in 2003 to 2007 to the 50s in 2008 to 2012. These findings imply two possibilities: (1) the age at initial diagnosis of diabetes decreased due to diabetes awareness campaigns, positive attitudes for individual health-care, and enhanced accessibility of primary medical care and NHSE [15] or (2) diabetes developed at an earlier age due to increased obesity in younger adults [17].

The present study demonstrates a steady increase in the annual number of total diabetes claim patients and cases during the last 10 years. Although the fit of the models was not ideal, this data suggests that a further 1.4- to 1.6-fold increase in the number of total diabetes claim patients and cases can be expected in 2020. This translates into a future increase in medical expenses for the management of patients with diabetes in Korea, which should be considered a crucial issue for health-care providers.

The current study has several limitations that should be addressed by further investigation. We defined subjects as having diabetes based on the diagnosis information from the claims data without considering medication prescription data. Therefore, we were unable to calculate the prevalence of diabetes from the latter dataset. Furthermore, the typical limitations of research based on a medical claims database also apply to our study. Only subjects who actively utilized health care and were able to access medical personnel were represented in the NHIS data and analyzed in this study. Thus, the actual incidence rate of diabetes may be underreported due to individuals with undiagnosed diabetes. Lastly, the accuracy of diagnosis coding for diabetes in the claims data has not been intensively validated by independent research such as confirmation with laboratory data or medical records. Despite these limitations, the present study demonstrates for the first time trends in the annual number of total diabetes claim patients and cases in Korea from 2003 to 2012. The use of the claims database illustrates actual changes in claims patterns in clinical practice in Korea. Furthermore, we analyzed trends by gender and age groups and predicted the future number of claim patients and cases with diabetes.

In summary, there have been increasing numbers of diabetes claim subjects and cases in Korea in the last 10 years, although the annual number of new diabetes claim patients did not increase during the last 4 years. Considering the fact that new claim subjects with diabetes are getting younger, the number of patients with a longer duration of diabetes who are at higher risk for related comorbidities will gradually increase. Thus, it is necessary for health-care policy makers to establish a long-term health-care management plan for preventing chronic diabetic complications.