Survey Overview
The study data were obtained from the Korea National Health and Nutrition Examination Survey (KNHANES), a nationally representative population-based cross-sectional survey that evaluates the health and nutritional status of the Korean population conducted annually by the Centers for Disease Control and Prevention in Korea. The survey collected detailed information on demographic, socioeconomic, and clinical characteristics, including age, educational attainment, economic activity, household income, marital status, alcohol consumption, smoking habits, and previous and current diseases.
Survey participants were selected from 192 regions in Korea based on a stratified multistage sampling method. The survey was composed of several components, including a health behavior questionnaire, health interview, health examination, and nutritional survey. Participants aged over 40 years also performed a spirometry test using a spirometer as part of the health examination. The KNHANES 2014 and 2016 surveys included the Patient Health Questionnaire-9 (PHQ-9), which is a self-reported depression screening scale. Therefore, we used survey data from these two years. The KNHANES provides secondary data that is publicly available and a more detailed description of the survey profile can be found elsewhere [12].
Study Population
This study initially assessed data from 6,329 eligible participants with valid lung function measurements. After excluding participants who did not meet the COPD criteria, and those with any missing variables, including PHQ-9 score, a total of 877 participants were included in the final analysis (Figure 1).
Assessment of Lung Function
Lung function was measured by trained medical technicians according to the manual of the American Thoracic Society/European Respiratory Society Task Force, using dry rolling seal spirometers (Model 2130; Sensor Medics, Yorba Linda, CA, USA) [13]. The forced expiratory volume in 1 second (FEV1), forced vital capacity (FVC), and FEV1/FVC ratio were obtained. COPD was defined as an FEV1/FVC ratio below 0.7 according to 2018 Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines [14]. The severity of COPD (GOLD Stage) was classified based on the percent-predicted FEV1: Stage I (mild; FEV1 ≥80%), Stage II (moderate; FEV1 50 to 79%), Stage III (severe; FEV1 30 to 49%), and Stage IV (very severe; FEV1 <30%).
Assessment of Body Mass Index and Depressive Symptoms
Body mass index (BMI) was calculated as weight/height squared (kg/m2) and classified into: underweight (BMI <18.5), normal (18.5 ≤ BMI < 23), overweight (23 ≤ BMI < 25), and obese (BMI ≥25) according to the revised Asia-Pacific BMI criteria by the World Health Organization Western Pacific Region [15].
Depressive symptoms were assessed using the PHQ-9, a nine-item self-reported questionnaire [16]. The questionnaire is based on the criteria of the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, which is used to diagnose several specific types of depressive disorders [17]. Each item on the PHQ-9 is scored on a scale from 0 to 3. The scores are then summed as a total score ranging between 0 and 27. PHQ-9 total scores of 5, 10, 15, and 20 represent valid thresholds of mild, moderate, moderately severe, and severe depression, respectively. PHQ-9 is in the public domain and the scale can be used free of charge [18]. Following previous studies on depressive symptoms using KNHANES data [19-21], the presence of depressive symptoms was defined as a PHQ-9 ≥5 in this study [22]. All participants were categorized into two groups depending on whether they had depressive symptoms according to this cut-off score.
Assessment of Covariates
We included socio-demographic variables: gender, age, educational attainment, equalized household income, living status, economic activity, and residential area. Age groups were divided into five categories as 40–49, 50–59, 60–69, 70–79, and 80 or older. Educational attainment was categorized as ‘elementary school or below,’ ‘middle school graduate,’ ‘high school graduate,’ or ‘college or above.’ Equalized household income was categorized into quartiles from ‘quartile 1 (low income)’ to ‘quartile 4 (high income).’ Living status was categorized as ‘living alone’ or ‘living together.’ Economic activity was categorized as ‘employed’ or ‘unemployed.’ Residential area was categorized as ‘urban’ or ‘rural.’
We also included health-related variables as additional potential confounders: alcohol consumption status, smoking status, chronic medical diseases, and GOLD Stage. Alcohol consumption and smoking status were categorized as ‘never use,’ ‘former use,’ or ‘current use.’ Chronic medical diseases, including hypertension, diabetes mellitus, angina, myocardial infarction, and stroke, were collected via self-reported doctor diagnosis. The total number of diseases was summed and classified into three groups as ‘zero,’ ‘one,’ and ‘two or more.’ For analyses, GOLD Stages III and IV were combined into one, resulting in three categories, ‘Stage I,’ ‘Stage II,’ and ‘Stage III/IV.’
Statistical Analysis
All data are presented as numbers and percentages. Categorical comparisons were performed using the chi-square test. Multivariable analysis was performed using multivariable logistic regression analysis with prespecified covariates. Odds ratios (ORs) and 95% confidence intervals (CIs) were also calculated. A p-value <0.05 was considered significant for all analyses. Data analyses were performed using SAS software, version 9.4 (SAS Institute, Cary, NC, USA).
Ethical Approval
The research protocol was approved by the Institutional Review Board (IRB) of Severance Hospital (IRB number: 4-2019-0854)