In patients receiving treatment for pulmonary tuberculosis (TB), change in sputum culture from positive to negative is the principal outcome measure of a therapeutic response in both clinical practice1 2 and drug trials.3 Patients will often stop producing sputum early in the course of treatment.4 We have tested the assumption that “no sputum” means that the patient is “culture negative”, as this has never been confirmed experimentally.

We prospectively followed 16 patients with newly diagnosed pulmonary TB. Sputum samples were collected at diagnosis, during weeks 1 and 2, at months 1, 2 and 4, and on completing treatment. Those patients who were not producing sputum spontaneously had specimens collected by induction. This was performed in a purpose-built negative pressure isolation chamber (Elwyn E Roberts Isolators, Shropshire, UK) where …