Diabetes is one of the most rapidly growing diseases in the world. The International Diabetes Federation has predicted that every 10 seconds, three patients are diagnosed with diabetes [1], and its prevalence rate around the world was 6.6% in 2010 [2]. It is estimated that 415 million people, aged 20 to 79, would be diagnosed with diabetes in 2015, and approximately 5 million people died from it. Moreover, the number of people with diabetes would be expected to be 642 million by 2040 [3]. According to the American Diabetes Association, the direct expenditures on diabetes by the United States government amounted to 91.8 billion United States dollar (USD) in 2002, and 156 billion USD in 2010. It has been estimated that by 2020, 192 billion USD will be spent on diabetes. In 2002, the United States government's total expenditures on health and medication were 865 billion USD; among others, 160 billion USD, equivalent to 18.5% of its total health expenditures, were used for diabetes. Health expenditures per capita in the United States were 2,560 USD in 2002, and this number will increase by 5-fold to 13,243 USD if the individual is a diabetes patient. Moreover, if age, sex, and ethnicity are adjusted, the heath expenditures per capita will increase by 2.4 times [4].

In addition, a constant increase in medical expenditures due to complications of diabetes, proven in several studies [5,6,7], is also a public concern. Cost Of Diabetes in Europe-Type 2 (CODE-2) diabetes studies carried out in seven countries in Europe, including over 7,000 participants, have suggested that medical expenditures on complications of diabetes would increase 1.7 times for patients who develop microangiopathy, two times for patients with macroangiopathy, and 3.5 times for those who develop both types of complications [7].

Through continuous care, however, it is possible to prevent patients with diabetes from further development of the disease and reduce their medical expenses. Regardless of the type of care or treatment, continuous care of diabetes is important because absence of treatment will eventually lead to hospitalization or a number of medical complications [8]. According to the Centers for Disease Control and Prevention, patients with diabetes are 1.5 to 1.8 times more likely to be hospitalized from heart disease or stroke than patients who do not have diabetes. Cases of lower-extremity amputation due to diabetes have increased from 73,000 in 2010 to 108,000 in 2014 [9,10].

In the case of chronic diseases such as diabetes or hypertension, because the duration of the illness is so long, there are many cases in which the patient frequently changes medical institutions, which distorts usage behavior of medical services by casting about for a better medical practitioner [11,12,13]. If a patient visits different medical institutions for the same disease, it reduces continuity of care because patients need to be re-examined, requiring new physical examinations and laboratory and radiological tests for the same symptoms.

The objective of this study, therefore, was to analyze the effect of Continuity of Care (COC) on health outcomes and health care costs for patients with type 2 diabetes mellitus (T2DM), using the Korean National Health Insurance database.

This study uses data from the Korean National Health Insurance database gathered between 2002 and 2013. Information about one million people (2% of the whole population) was extracted from the nation's health insurance claims data, including socioeconomic status, disability, death, and use of medical care institutions. Data about patients aged 20 years old or over who were newly diagnosed with T2DM in 2004 were collected for the study.

The observation period of the study is 10 years, consisting of two periods, the first one observing whether managing diabetes continuously for 2 years after initial diagnosis (continuity observation), and the following one observing the health outcome for 8 years thereafter (outcome evaluation). We excluded patients from the analysis who had fewer than four physician visits [14], had a record of emergency visits or hospital admission during the continuous observation period, had diabetic complications, or died within the first 2 years of initial diagnosis. In addition, patients who had no record of physician visits or who died for conditions not related to T2DM during the 8-year period were also excluded. In the end, the data from 2,259 patients in total were eligible for analysis (Fig. 1).

The effects of continuity of treatment for T2DM on diabetic complications and health care costs were assessed using the National Health Insurance cohort database (2002 to 2013). Continuity of care was evaluated based on managing diabetes continuously for 2 years after initial diagnosis during the continuity observation period and the diabetes-related complications for the 8 years of follow-up thereafter during the outcome evaluation period. The COC index was used to evaluate continuous care. The total costs of diabetes treatment were measured based on the outcome evaluation period (8 years). The results of this study demonstrated that high COC significantly reduced the risk of diabetes-related complications (HR, 0.69) and medical costs (3,496 USD vs. 3,973 USD) compared with low-COC patients with T2DM.

Patients with T2DM are at significant risk for a number of complications, such as retinopathy, renal disease, and heart disease [31]. The prevalence of retinopathy is related to the duration of diabetes and is regarded as the most substantial cause of blindness among adults aged 20 to 74 years [32]. The rate of cardiovascular disease is relatively high among patients with T2DM, resulting in an increased mortality rate compared with the general population [33,34,35]. These complications related to diabetes, however, can be reduced by continuity of care, because it may lead to escalating the likelihood of cancer screening [36] and to increasing follow-up appointments [37]. In fact, continuity of care with a primary care provider is associated with better glucose control among patients with T2DM, with a fairly high probability of detecting minor conditions at earlier stages to reduce the incidence of diabetes complications and incur fewer hospital admissions [18,38]. This study also reports the positive effect of continuity of care for people with diabetes, as do previous studies. Patients who received high COC score are significantly less likely to have diabetic complications. Furthermore, patients with high COC scores have higher chances of survival. The COC score in the study was 0.83±0.24. In other studies, it was 0.84±0.17 for 3 years [15] or ranged from 0.64 to 0.66, using only diabetes-related visits between 2001 and 2006 [18].

Another issue is economic burdens on the health care system as well as on patients. Direct medical costs (91.8 billion USD in 2002) in the United States comprise 25.3% for diabetes care, and 26.8% for chronic complications attributable to diabetes [4]. The age-gender weighted average of lifetime medical costs for diabetic patients in the United States is 85,200 USD, of which 53% is due to treating diabetic complications, based on data from 2009 to 2010 from the National Health and Nutrition Examination Survey [8]. Continuity of care, however, leads to decreasing medical costs from patients with diabetes. In other words, there is a general reduction in health care costs as continuity of care improves [15,39,40]. Moreover, patients who do not fulfill continuity of care are more likely to spend more in medical costs than those who carry out continuity of care [41]. In this study, the average medical cost is approximately 3,496 USD per patient for patients with a higher COC score, whereas it is 3,973 USD per patient for patients with a lower COC score. So the study finds results consistent with those of previous studies. Diabetics with higher COC scores had significantly lower medical expenses than those with lower COC scores after adjusting for other factors (β=−0.152).

This study is meaningful in the sense that it uses national claims-based data to verify the relationship between COC and medical costs and health outcomes for diabetes patients. However, the use of secondary data in the analysis created a few limitations. First, this research considered only available variables in the claims database, such as patients' socioeconomic status, characteristics of disease, and medical institution, that could affect diabetes patients' health outcome. It did not consider commonly addressed socio-behavioral characteristics in most studies of chronic conditions. For example, Kim et al. [42] explained that a patient's occupation, education, and family history can affect continuity of care. Ji [43] also stated that factors such as drinking and smoking habits, obesity, stress level, and average sleep hours can affect a patient's ability to control glycemic hemoglobin. However, these factors were not considered in this research. Severity of diabetes was not properly considered; this research instead applied the Charlson comorbidity score to consider the patient's overall health condition. In the study by Cho et al. [30], the prescription of hypoglycemic agents (types and number) and comorbidity index together were used to consider the severity of the disease. In this study, prescriptions of hypoglycemic agents and comorbidity score were also considered to adjust for the severity of diabetes.

Second, out of all direct costs, this research only considered medical cost to calculate the cost of diabetes. It fails to recognize that diabetes is a chronic disease that lasts for a long period of time. To manage this disease, a significant portion of the cost could be taken up by direct nonmedical costs, such as time, and transportation cost, and indirect cost such as decrease of income incurred by productivity loss and death. Even if direct medical costs had been calculated, it only considered the covered benefit costs of medical care services.

This study demonstrated, by using the National Health Insurance cohort database, that continuity of care has significant impacts on complications and medical costs of patients with T2DM. Despite the limitations of the study variables and design due to the nature of the NHI cohort data, there would be no difficulty in generalizing the results. Last, this study will be valuable for clinicians in understanding the concept and utility of care continuity.