[Number of sputum cultures by MGIT system needed to diagnose pulmonary tuberculosis]

To study the number of sputum cultures by MGIT system (Becton-Dickinson) needed to diagnose pulmonary tuberculosis.

Prospective study of all patients who visited our hospital, and were strongly suspected of pulmonary tuberculosis during the period from Jan. 2002 to Sept. 2003. In these patients, 3 consecutive sputum cultures were done by both MGIT-system and egg-based Ogawa medium (1 slant).

Altogether 290 cases of sputum-culture positive pulmonary tuberculosis were available for analysis. In 210 first-sputum-smear positive cases, incremental yield of 3rd sputum culture in 3 consecutive MGIT cultures was equal to or less than 1.0% and 98.1% of culture positive cases were detected by 2 consecutive MGIT cultures. In 80 first-sputum-smear negative cases, incremental yield of 3rd sputum culture in 3 consecutive MGIT cultures was equal to or more than 5.0%, and 90.0% of culture positive cases were detected by 2 consecutive MGIT cultures. This detection rate was almost the same as the calculated detection rate (91.4%) by 3 consecutive Ogawa (2 slant) cultures (previous standard method).

It was suggested that 2 consecutive sputum cultures by MGIT were sufficient to detect M. tuberculosis in first-sputum-smear positive cases, but 3 consecutive sputum cultures by MGIT were relatively useful in first-sputum-smear negative cases.