Study population
This study used data from KNHANES V (2010–2012), VI (2013–2015), and VII (2016–2018). The KNHANES, conducted by the Korea Centers for Disease Control and Prevention (KCDC), is a nationwide, cross-sectional survey to assess the health and nutritional status of the general population [15]. The study population was selected using a stratified multi-stage sampling method. Of 92,592 participants, 72,751 agreed to participate (participation rate: 78.6%). In the KNHANES, spirometry was assessed only in participants over 40 years of age. Therefore, excluding 43,635 participants without spirometry, 29,116 with spirometry were included as a first step. Of these, 3902 with missing baseline data were excluded. Among the 25,214 remaining participants, 1210 who had experienced pulmonary TB and 6050 without a past pulmonary TB history were selected using 1:5 propensity score matching for age, sex, body mass index (BMI), marital status, smoking status, education, regular exercise, functional expiratory volume in 1 s (FEV1), functional vital capacity (FVC), and FEV1/FVC. Finally, 7260 participants were included in the final analysis (Fig. 1).
Past pulmonary tuberculosis history
A standardized interview was conducted by trained staff in the homes of the participants to determine past pulmonary TB history. Past pulmonary TB history was defined as a previous pulmonary TB diagnosis, excluding patients with active pulmonary TB. A previous diagnosis of pulmonary TB was assessed using the question “Have you ever been diagnosed with pulmonary TB by a doctor?” Among the patient group, 1219 participants reported a previous diagnosis of pulmonary TB. Of the 1219 patients, 9 who underwent treatment of active pulmonary TB were excluded. Finally, 1210 participants were defined as those with past pulmonary TB history.
Health-related quality of life
The HRQoL was assessed using EQ-5D, which consists of the five dimensions of mobility, self-care, usual activities, pain/discomfort, and anxiety/depression. Patient responses to each dimension were scored as follows: one for no problem, two for a moderate problem, and three for a severe problem. The EQ-5D utility scores were calculated by converting response values using the Korean formula for quality of life weighting [16]. The EQ-5D utility score was converted into EQ-5D disutility score (1-EQ-5D utility score) due to the non-normal distribution of EQ-5D utility score.
Measurements
Income was defined as low for the lowest quartile of monthly household income, intermediate for the second to third quartiles of monthly household income, and high for the highest quartile of monthly household income. Past smoking was defined as those who have smoked more than five cigarettes in their lifetime but currently do not smoke. BMI was calculated by dividing weight by the square of height (kg/m2). Lung function test was performed by trained technicians with a spirometer (Vyntus Spiro; Care Fusion, San Diego, CA, USA; or dry rolling seal spirometer Model 2130; Sensormedics Corporation, Yorba Linda, CA, USA), and the best scores of three pre-bronchodilator measurements were recorded. Percent predicted FVC and FEV1 were calculated using the predicted values for Korean patients [17]. Blood pressure (BP) was measured using a standard mercury sphygmomanometer (Baumanometer Wall Unit 33 [0850]; Baum Co., Inc., Copiague, NY, USA). Measurements were performed three times, and the average was used. Hypertension (HTN) was defined as systolic BP ≥ 140 mmHg, diastolic BP ≥ 90 mmHg, or treatment with antihypertensive drugs. Blood glucose level was obtained after at least eight hours of fasting and analyzed in a central laboratory (Neodin Medical Institute, Seoul, Korea) using a Hitachi Automatic Analyzer 7600 (Hitachi, Tokyo, Japan). Diabetes mellitus (DM) was defined as fasting glucose ≥ 126 mg/dL and treatment with insulin or oral antidiabetic drugs. Cancer was defined as a previous diagnosis of stomach cancer, liver cancer, lung cancer, or colon cancer. Depressive disorder was defined as a previous diagnosis of major depressive disorder. Regular walking was defined as walking at least five days a week for 30 min or longer. Regular muscle-strengthening exercise was defined as muscle-strengthening exercising at least two days a week. Severe stress was determined based on answer to a self-reported questionnaire item.
Statistical analysis
Sample weights were used for post-survey adjustment with the ‘survey’ package of R, and participants in the single primary survey unit (PSU) stratum were centered at the sampled grand mean. Before and after propensity score matching, clinical characteristics were compared based on past pulmonary TB history. In the analysis, all variables were categorized, and weight percent (Wt %) was calculated for each. Differences between individuals with and without pulmonary TB history were confirmed using Pearson’s Chi-square test. The EQ-5D disutility according to past pulmonary TB history was assessed using density plots and a design-based t-test. Multivariable Poisson regression analysis was used to analyze the association between past TB history and EQ-5D disutility score because the distribution of EQ-5D disutility score followed the Poisson distribution. Thus, the odds ratio (OR) of multivariable Poisson regression analysis indicated a likelihood of low HRQoL. For analysis, all dimensions were categorized into positive for two or three respondents and negative for one respondent. Multivariable logistic regression analysis was performed to investigate the effect of past pulmonary TB history on each dimension. In subgroup analysis, we analyzed all variables used in multivariable Poisson regression analysis and selected those that showed statistically significant results or that were clinically important. Interaction analysis was performed by including interaction terms in the multivariable Poisson regression analysis. Marital status was analyzed as a factor variable, and other variables were analyzed as ordinal. The p-values less than 0.05 were considered statistically significant. All analyses were performed using R version 4.0.3 (R core 172 Team 2020; R Foundation for Statistical Computing, Vienna, Austria).