2.3. Study population
As shown in Supplemental Figure 1 , 738,254 patients 18 years of age or older were identified from January 1, 2005 to December 31, 2006. After excluding 16,995 subjects with malignancy during the washout period, 697,281 patients were selected. Among these, 80,356 patients had at least one claim for ICD-10 codes for asthma (J56–46).
Among the 80,356 patients with asthma, we excluded patients who had asthma during the three-year washout period (n = 15,069), patients who had only one claim under ICD-10 codes for asthma (n = 28,333), patients who did not receive asthma-related medication (n = 16,592), and patients who died during the baseline period (n = 55). Finally, a total of 19,319 patients with incident asthma was identified (asthma cohort). Next, we performed 1:4 age and sex matching for each asthma patient from cohort without asthma (control cohort). Patients were followed-up until the date of death or December 31, 2015.
This study was approved by the Ethics and Review Board of Hanyang University Guri Hospital (IRB No. 2019-11-018). The requirement of informed consent from the participants was waived because the NHIS database was constructed after anonymization.