The diagnostic gap: Characterising the profile of undiagnosed infectious tuberculosis patients in the community

Abstract

Background. In tuberculosis (TB)-endemic countries, about half the total TB caseload remains undiagnosed within the community. The proportion of such patients that can potentially transmit the disease has been poorly characterised, and there is insufficient data to inform on strategies to target potentially infectious TB cases in the community. Objective. To characterise the nature and profile of smear-positive patients diagnosed with TB in the community. Methods. We analysed data from culture-positive TB cases in the community during the course of an intensified case finding (ICF) study. The parent study was a randomised controlled trial comparing conventional and novel diagnostics for ICF in communities in Cape Town, South Africa and Harare, Zimbabwe, where trained healthcare workers screened patients at transmission hotspots. The results of the parent study are reported elsewhere. Results. A total of 2 261 persons were screened and 875 (39%) met the criteria for diagnostic testing. A total of 53/630 (8.4%) had confirmed tuberculosis. Smear microscopy detected 22/53 (42%) of the culture-positive patients. The specificity, positive predictive value (PPV) and negative predictive value (NPV) for smear microscopy were 98.4%, 70.0% and 95.3%, respectively. No clinical or demographic variable predicted smear positivity. Only decreased culture time-to-positivity was associated with smear grade (odds ratio 0.93, 95% confidence interval 0.91 - 0.96; p<0.001). Conclusion. A considerable proportion of patients with undiagnosed TB in the community (almost half) were smear positive and hence potentially infectious. Interestingly, neither HIV status nor symptoms identified those patients who were potentially infectious, despite them having a higher mycobacterial burden.