In high-income countries like Canada, patients with subclinical pulmonary tuberculosis (TB) are a heterogeneous group, accounting for one-fifth of all pulmonary TB culture-positive cases, according to findings published in Chest.
Researchers sought to discover the prevalence of subclinical pulmonary TB in Canada, the diagnostic chest radiographic (CXR) features, the relationship of those features to time-to-culture-positivity, and the association between DNA fingerprint clustering and radiographic features in the foreign-born. For the analysis, researchers used a retrospective cohort study design, which included using the Provincial TB Registry in Alberta, Canada, to identify 1656 adults aged over 14 years who were diagnosed with culture-positive pulmonary TB between 2005 and 2020.
Patients were described according to age, sex, whether they were Canadian-born vs foreign-born, new active disease vs relapse/retreatment, HIV status, and mycobacteriologic features. Subclinical patients were also described according to reason-for-assessment. In subclinical patients, diagnostic CXR features were read. Researchers also used linear regression to compute time-to-culture-positivity relative to change in CXR from normal/minimally abnormal to moderately/far-advanced, adjusted for age and sex, stratified by reason-for-assessment. Associations between DNA fingerprint clustering of mycobacterium tuberculosis isolates, and, age, sex, CXR features, and time since arrival were determined by multivariable logistic regression in foreign-born subclinical patients.
Of the total study population, 78.9% were symptomatic (clinical), 21.0% were asymptomatic (subclinical), and 0.1% had unknown symptoms.
Subclinical vs clinical patients were more likely to be foreign-born (90.2% vs 79.6%) and smear-negative (88.2% vs 43.5%). Time-to-culture-positivity had a median of 18 (IQR, 14-25) days vs 12 (IQR, 7-17) days. CXR showing more advanced disease associated with shorter time-to-culture-positivity. Male sex and a longer time between arrival and diagnosis was associated with DNA fingerprint clustering.
Study limitations included the retrospective design and the limited discriminatory power of the DNA fingerprint clustering.
“Subclinical PTB patients comprise a substantial and heterogeneous minority in high-income countries,” the study authors noted.
Lau A, Lin C, Barrie J, et al. The radiographic and mycobacteriologic correlates of subclinical pulmonary tuberculosis in Canada: a retrospective cohort study. Chest. Published online February 1, 2022. doi:10.1016/j.chest.2022.01.047