[Positive sputum smear results after two consecutive negative smears during treatment of pulmonary tuberculosis]

During treatment of pulmonary tuberculosis, sputum smear may turn positive after 2 consecutive negative results. In the present study, we analyzed the infectivity in such cases.

The study involved 201 patients with sputum smear positive pulmonary tuberculosis who were admitted to our hospital between January 2004 and June 2009. Chart review was performed on the bacterial culture results and clinical course of patients in whom the sputum smear test turned to positive after 2 consecutive negative smears.

There were 37 such cases (42 events). The event occurred after the treatment of 2 weeks or longer and less than 1 month in 6 cases (7 events). The culture examination of the smear-positive sputum was negative in only 1 of these cases. There were 9 cases who turned smear positive after the treatment period of 1 month or longer and less than 2 months, and 6 of these cases showed negative results in the bacterial culture of the smear positive sputum. In these cases, the grade of smear positivity was generally low and subsequent tests yielded negative results. There were 22 cases (26 events) in which this phenomenon was observed after treatment for 2 months or longer. Subsequent bacterial culture yielded negative results in all but 1 of these cases.

In patients whose sputum smears gave positive results after 2 consecutive negative smear tests, the bacterial load of the sputum decreased or disappeared after treatment for 1 month or longer, and bacterial discharge was almost completely absent after treatment for 2 months or longer. If the clinical condition is favorable in such cases, we may judge that they are no longer infectious.