Study design and subjects
We enrolled adult patients with pulmonary TB from the cohort study of pulmonary tuberculosis (COSMOTB) between November 2016 and September 2018 to compare the clinical characteristics of active and subclinical TB. Briefly, COSMOTB is a prospective observational cohort study to assess the prevalence of discordant results of phenotypic and molecular drug susceptibility tests.[6] COSMOTB was conducted at three university-affiliated tertiary hospitals in South Korea that participate in the public-private mix project for TB control in South Korea. TB specialist nurses under this project educate TB patients and monitor them for medication adherence and adverse drug reactions.
Definition of subclinical and active diseases
Patients were categorized as having active TB or subclinical TB. Active TB was defined as the presence of clinical TB-related symptoms with radiographic abnormalities or microbiologic evidence of M. tuberculosis. Subclinical TB was defined as the presence of radiographic or microbiologic test results consistent with TB without clinical symptoms. We implemented two-stage symptom assessment using a predefined checklist, which listed TB-related symptoms, such as cough, sputum, fever, general weakness, dyspnea, chest pain, body weight loss, and hemoptysis. First, TB patients met a TB specialist nurse at the hospital, who interviewed and identified patients’ TB-related symptoms. Subsequently, patients met with a physician at the clinic, who reconfirmed their symptoms and their duration. As patients were identified as asymptomatic after two-stage assessment, they were categorized as subclinical TB disease.
Data collection
Participants were evaluated at each hospital on study entry. Demographic, clinical, and laboratory data were prospectively collected from enrolled patients using a case report form upon study entry. Acid-fast bacilli (AFB) smears using light and fluorescent microscopy and nucleic acid amplification test (NAAT) were conducted at each hospital. Mycobacterium culture testing using both solid (3% Ogawa media) and liquid (BACTEC MGIT 960 system, BD, NJ, USA) cultures were performed at the reference laboratory. Culture-based phenotypic drug susceptibility tests were performed using the absolute concentration method on Löwenstein-Jensen medium.
Statistical analyses
Continuous variables are presented as means and standard deviations or medians and interquartile ranges, whereas discrete variables are presented as frequencies or percentages. To compare the baseline characteristics of patients with active or subclinical TB, univariate analysis was performed using the chi-squared test. Subsequently, we selected age, sex, and other clinical variables with p-values < 0.20 based on the univariate analysis and further performed multivariate binary logistic regression to evaluate the possible association between variables and subclinical TB.
For regression, unknown data were regarded as missing values. A p-value < 0.05 was considered statistically significant. All statistical analyses were performed using SPSS software (Statistical Product and Service Solutions, Chicago, IL, USA).
Sample size
We selected nine variables a priori for inclusion into our model, such as age at diagnosis, sex, body mass index, chronic respiratory disease, prior TB history, results of AFB smear and culture and NAAT. Assuming 20% of subclinical disease, we required a sample size of 400 to ensure a minimum of 10 events per variable, which are needed to minimize bias in logistic regression models[7].
Treatment outcomes
Participants were evaluated at 2 weeks, 4 weeks, 2 months, 4 months, 6 months, 9 months, 12 months, and 24 months after initiating anti-TB treatment in order to collect their treatment outcome. Those with successful outcomes were also followed for at least 1 year to identify recurrence. Treatment outcomes were defined in accordance with the Korean TB guidelines that were adopted from the WHO’s definition. Treatment success was the sum of cured and treatment-completed patients within 1 year of anti-TB treatment. Favorable outcome was defined as patients who had achieved treatment success without recurrence within the post-treatment 1-year follow-up period. We evaluated association between subclinical disease and treatment outcome in the drug-susceptible cohort comprising patients with positive culture results susceptible to both isoniazid and rifampin and patients clinically diagnosed with TB without microbiological evidence using binary logistic regression and adjusting for age and sex.