To control for potential confounding factors in the analyses, covariates were identified based on previous studies, expert opinion, and covariate availability within the data. The covariates used in this study included age, sex, household income, region of residence, disability, Charlson comorbidity index (CCI) score, smoking status, body mass index (BMI), co-medications, and baseline year.
The age groups were classified using 10-year intervals. A total of eight groups aged ≥20 years old were included. The household income groups initially provided with 11 classes (class 0, lowest income; class 10, highest income) from the NHIS database were recategorized into three groups (low, class 0–2; medium, class 3–7; high, class 8–10). The region of residence was recategorized into urban (Seoul, Busan, Daegu, Incheon, Gwangju, Daejeon, and Ulsan) and rural (Gyeonggi, Gangwon, Chungcheongbuk, Chungcheongnam, Jeollabuk, Jeollanam, Gyeongsangbuk, Gyeongsangnam, and Jeju). The patients’ CCI scores were estimated from their disease records using previously validated algorithms [25 (link)]. Smoking status was categorized as current smoker, ex-smoker, and never smoked. BMI was calculated as an individual’s body weight in kilograms divided by their height in meters squared (kg/m2). In this study, BMI was categorized into underweight (BMI less than 18 kg/m2), normal (BMI between 18 to 25 kg/m2), and overweight (BMI more than 25 kg/m2). Co-medications, such as calcium channel blockers (amlodipine, felodipine, flunarizine, isradipine, levamlodipine, nicardipine, nifedipine, nimodipine, nisoldipine, diltiazem, and verapamil), antidepressants (amitriptyline, amoxapine, clomipramine, desipramine, doxepin, imipramine, maprotiline, nortriptyline, protriptyline, and trimipramine), theophylline, and anticholinergic drugs (benztropine, dicyclomine, hyoscyamine, isopropamide, and scopolamine), which are known to contribute to the development of GERD, were also considered as covariates if the medication was used for more than 60 days between the index date and the last observation date.
The age groups were classified using 10-year intervals. A total of eight groups aged ≥20 years old were included. The household income groups initially provided with 11 classes (class 0, lowest income; class 10, highest income) from the NHIS database were recategorized into three groups (low, class 0–2; medium, class 3–7; high, class 8–10). The region of residence was recategorized into urban (Seoul, Busan, Daegu, Incheon, Gwangju, Daejeon, and Ulsan) and rural (Gyeonggi, Gangwon, Chungcheongbuk, Chungcheongnam, Jeollabuk, Jeollanam, Gyeongsangbuk, Gyeongsangnam, and Jeju). The patients’ CCI scores were estimated from their disease records using previously validated algorithms [25 (link)]. Smoking status was categorized as current smoker, ex-smoker, and never smoked. BMI was calculated as an individual’s body weight in kilograms divided by their height in meters squared (kg/m2). In this study, BMI was categorized into underweight (BMI less than 18 kg/m2), normal (BMI between 18 to 25 kg/m2), and overweight (BMI more than 25 kg/m2). Co-medications, such as calcium channel blockers (amlodipine, felodipine, flunarizine, isradipine, levamlodipine, nicardipine, nifedipine, nimodipine, nisoldipine, diltiazem, and verapamil), antidepressants (amitriptyline, amoxapine, clomipramine, desipramine, doxepin, imipramine, maprotiline, nortriptyline, protriptyline, and trimipramine), theophylline, and anticholinergic drugs (benztropine, dicyclomine, hyoscyamine, isopropamide, and scopolamine), which are known to contribute to the development of GERD, were also considered as covariates if the medication was used for more than 60 days between the index date and the last observation date.
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