in 2014 , the national health insurance service ( nhis ) signed a memorandum of understanding with the korean diabetes association ( kda ) to provide limited open access to its databases for investigating the past and current status of diabetes and its management . a previous review by song et al .   described in detail regarding the history , structure , contents , and way to use data procurement in the korean national health insurance ( nhi ) system . briefly , the nhis in korea is a single - payer organization that is mandatory for all residents in korea . because it has adopted a fee - for - service system to pay health care providers who treat or examine korean patients , nhis obtains information on patient demographics , medical use / transaction information , insurers ' payment coverage , and patients ' deduction and claim database ( diagnosis / prescriptions / consultation statements ) . the nhis database represents the entire korean population ; therefore , it can be used as a population - based , nationwide study for various diseases . recently , several epidemiologic studies with a large population using the nhis database have been reported . a single insurer , nhi system consists of two major health care programs for universal coverage of all residents of korea : nhi and medical aid ( ma ) . approximately 97% of the population is covered by nhi , and the remaining 3% of the population is covered by ma . since 2006 , information of ma beneficiaries has been incorporated into a single nhis database . therefore , the nhis database during 2002 to 2005 included only information of nhi beneficiaries , but not ma beneficiaries , which should be considered in caution when interpreting the findings in this period . retrospective data of individuals aged more than 30 years were extracted using the korean nhis database from january 2002 through december 2013 . among sub - datasets of the nhis database , we used qualification db , claim db , health check - up db , and death information . qualification db includes sex , age , income , region , and types of qualification . using this database , we showed the distribution of study participants aged more than 30 years from the nhis database from 2002 to 2013 by gender and age ( table 1 ) . claim db includes general information on specification ( 20 t ) , consultation statement ( 30 t ) , diagnosis statements defined by the international classification of diseases 10th revision ( icd-10 ; 40 t ) , and detailed statements about prescriptions ( 60 t ) . health check - up db generally consists of four areas : general health check - up , lifetime transition period health check - up , cancer check - up , and baby / infant health check - up . among them , we used the database from the general health check - up , which includes ( 1 ) employee subscribers and regional insurance subscribers who are a regional householder , ( 2 ) employee subscribers ' dependent and household members ( 40 years or older ) , and ( 3 ) ma beneficiaries who are a householder of 19 to 64 years of age and household members of 41 to 64 years . all examinees were requested to have biannual health check - ups , except non - office workers who are employee subscribers ( annual ) . the proportion of complete health check - ups was approximately 40% in 2002 , whereas it increased up to 68% in 2013 . considering the characteristics of the nhis database , an operational definition of diabetes was applied for further analysis . for claim db , individuals having diabetes were defined if anti - diabetic drugs were prescribed with the presence of icd-10 codes e11 , e12 , e13 , or e14 , as either principal diagnosis or 1st to 4th additional diagnosis at least once a year . for the health check - up db , patients with fasting glucose levels this operational definition of diabetes was concluded by the following data analysis using study participants in the 2013 nhis database . table 2 shows numbers of nhi beneficiaries aged 30 years and general health check - up examinees in 2013 by age . if the prevalence of diabetes is calculated among nhi beneficiaries aged 30 years based on either prescription of anti - diabetic drugs ( insulins , sulfonylureas , metformin , meglitinides , thiazolidinediones , dipeptidyl peptidase-4 inhibitors , and -glucosidase inhibitors ) , icd-10 codes , or both , there are substantial discrepancies by the different categories . the proportion of patients with diabetes was 8.50% ( 3.95%+4.55% ) defined by combination of diagnosis and prescription data , whereas it was 13.15% ( 4.14%+0.51%+3.95%+4.55% ) by icd-10 codes alone and 8.69% ( 0.12%+0.07%+3.95%+4.55% ) based on prescription data alone ( table 3 ) . the taskforce team concluded the operational definition of diabetes as either ( 1 ) patients who had both data of diagnosis and prescription of anti - diabetic drugs or ( 2 ) patients whose fasting glucose levels from health check - up db are more than 126 mg / dl . according to this definition , since 2014 when the kda and nhis signed an agreement for open access to the nhis database , 16 subjects of collaboration projects are currently ongoing . depending on the project objectives , overall projects can be divided into two categories : analyses of disease status and analyses of casual relationships among diseases , management , and drugs . analyses of disease status demonstrated the annual prevalence and age - standardized prevalence of specific diseases , such as diabetic nephropathy . analyses of casual relationships ( e.g. , effects of anti - diabetic drugs on cancer , association between diabetes , and percutaneous coronary intervention ) applied the study design with washout periods and cox - hazard regression models . the nhis database represents the entire korean population ; therefore , it can be used as a population - based nationwide study for various diseases . because it contains detailed information regarding statement of prescriptions and medical examination or treatments , such as medical care and in - hospital administration of medicine , procedures , and surgery , investigation of the trends or status of specific diseases is feasible . furthermore , long - term follow - up of a single individual can allow us to perform longitudinal studies of casual relationships . by combining laboratory and standard questionnaire information from the health check - up db , limitation of claim db ( without having any laboratory or personal history data ) can be overcome . despite its strengths , one of the most critical drawbacks is the discrepancy between diagnosis of individuals in real practice and that recorded in claim db . generally , proportion of discrepancy in diagnosis might be more prominent in claim data from outpatient clinics , less - severe illnesses , and primary care clinics , compared with inpatient hospitalization , severe illnesses , and tertiary or general hospitals , respectively . therefore , appropriate operational definitions should be required to minimize the inconsistent and inaccurate results . moreover , because the nhis covers only insured benefits , uninsured payments could not be estimated from this database . because information of ma beneficiaries was incorporated into a single nhis database from 2006 , nhis database during 2002 to 2005 included only information of nhi beneficiaries but not ma beneficiaries , which should be considered with caution when interpreting the findings in this period . in terms of health check - up db , the proportion of complete health check - up examinees was only 40% in 2002 , and different intervals of health check - up among beneficiaries should be considered in the study design .
in 2014 , the national health insurance service ( nhis ) signed a memorandum of understanding with the korean diabetes association to provide limited open access to its databases for investigating the past and current status of diabetes and its management . 
 nhis databases include the entire korean population ; therefore , it can be used as a population - based nationwide study for various diseases , including diabetes and its complications . 
 this report presents how we established the analytic system of nation - wide population - based studies using the nhis database as follows : the selection of database study population and its distribution and operational definition of diabetes and patients of currently ongoing collaboration projects .