2.1. Data Source

This retrospective, observational cohort study examined data from the Registry of Catastrophic Illness Database (RCID), a subset of the National Health Insurance Research Database (NHIRD). The Taiwan Bureau of National Health Insurance established the universal National Health Insurance (NHI) program in 1995, which currently provides comprehensive medical coverage to 99% of the population of Taiwan (23.72 million residents) (https://www.nhi.gov.tw/english). The National Health Research Institutes (NHRI) maintain the NHIRD for research purposes. The RCID includes data for all patients who fulfill the guidelines of the National Health Insurance Administration for a catastrophic illness certificate. Catastrophic illnesses are defined as severe conditions requiring advanced health care, such as malignancies, post transplantation status, and ESRD. Patients with a catastrophic illness certificate are exempted from making any copayment for medical care, and this certificate is formally reviewed by physicians at the time of application. The NHIRD identifies diseases based on the International Classification of Diseases, Ninth Revision, Clinical Modification (International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM)). The accuracy and validity of NHIRD diagnostic codes have been established [18,19].