The gold standards used for defining patient groups in this study are notoriously imperfect. For example, while active TB was defined by positive microbiology, it is possible that some patients designated symptomatic controls may actually have had smear and culture negative TB. This might have resulted in an underestimate of the specificity of our diagnostic pattern for active TB, although our 6 months follow-up and appropriate re-labelling should have identified most of these. The lack of an adequate gold standard for defining LTBI must also be considered. While IGRAs show greater specificity than TSTs, sensitivity may be compromised especially in early active TB [22] . Thus some patients with unrecognized smear and culture negative TB may have been mislabeled as symptomatic controls without LTBI.