Asymptomatic Tuberculosis in the Namibia National Tuberculosis Prevalence Survey: Occurrence Under Various Case Definitions

ABSTRACT Background Asymptomatic tuberculosis (TB) is emerging as a disease state warranting attention because it is presumed infectious, yet affected individuals lack symptoms. The World Health Organization recommends screening for cough, fever, weight loss and night sweats and recently defined asymptomatic TB. However, the place of these symptoms in the definition has not yet been standardised. We determined the proportion of patients with bacteriologically confirmed asymptomatic TB without four symptom definitions: persistent cough (> 2 weeks), cough of any duration, W4SS of any duration or W4SS for > 2 weeks. Methods Secondary analysis of the Namibia TB prevalence survey data was performed; a representative sample had been interviewed for symptoms, underwent chest radiography and was offered bacteriological testing. The proportion of asymptomatic TB was computed for each definition. Logistic regression was used to determine factors associated with asymptomatic TB across four definitions. Results Among 29,495 participants in the nationally representative sample (86.8% of the target sample size), 123 TB cases met the case definition. Mean age was 45 years, and 74 (60%) were male. Of TB cases, 60/123 (48.8%) reported a cough of any duration, 20/123 (16.3%) a fever of any duration, 25/123 (20.3%) weight loss and 26/123 (21.1%) night sweats. Asymptomatic TB ranged from 39.8% to 63.0% by definition, with screening based on prolonged cough missing 63.0% of bacteriologically confirmed TB patients, while screening based on W4SS any duration missed 39.8% of TB cases. Conclusion Even when using an inclusive symptom screening system that includes any duration of the W4SS, asymptomatic TB occurred in 39.8%, highlighting a public health concern for asymptomatic TB. There is a need to standardise the symptoms being used when defining asymptomatic TB and we recommend the W4SS of any duration.