Active case finding for pulmonary tuberculosis using mobile digital chest radiography: an observational study

BACKGROUND: Mobile digital chest radiography (CXR) is used routinely to screen for pulmonary tuberculosis (PTB) in London among homeless populations, persons accessing drug treatment services and prisoners. OBJECTIVE: 1) To establish the sensitivity and specificity of mobile digital CXR, and 2) to test the hypothesis that actively identified cases have reduced odds of sputum smear positivity vs. those presenting passively to health care services from the same populations. METHODS: Sensitivity and specificity were calculated using a gold standard comparator of culture-confirmed cases of PTB reported to the national surveillance system within 90 days of screening. Logistic regression was used to determine whether actively detected cases had reduced odds of smear positivity compared to passively detected cases after adjustment for confounding. RESULTS: The intervention had a sensitivity of 81.8% (95%CI 64.5-93.0) and a specificity of 99.2% (95%CI 99.1-99.3). After adjusting for confounding, there was evidence that cases identified through screening were less likely to be smear-positive than passively identified cases (OR 0.34, 95%CI 0.14-0.85; likelihood ratio test P = 0.022). CONCLUSION: Digital CXR achieves a high level of sensitivity and specificity in an operational setting; targeted mobile radiographic screening can reduce the risk of onward transmission by identifying cases before they become infectious.