2.1. Database

All Koreans are obligated by law to register in national health insurance. The National Health Insurance Service (NHIS) stores medical information (age, sex, diagnosis, drug prescription, surgery types, types of medical insurance, hospitalization, and outpatient examinations) of subscribed Koreans (about 51 million). The Health Insurance Review and Assessment Service (HIRA) is an institution that examines the adequacy of medical expenses to prevent disputes over insurance payments between NHIS and hospitals. The HIRA shares most of the medical record information stored by the NHIS. This population-based retrospective cohort study used the medical data of HIRA gathered between 1 January 2007 and 31 December 2020. The data of 154,781 patients who underwent national examinations by the Korean Health Insurance Corporation from 2011 to 2014 were extracted.