Objective: To determine the optimum number of sputum specimens for smear and culture in the diagnosis of pulmonary tuberculosis.
Material and Method: A retrospective study was conducted in culture-positive pulmonary tuberculosis patients at Siriraj Hospital during April 2009 to October 2010. Number of sputum specimens and microbiological results were retrieved from the microbiologic laboratory. Positive yield and incremental yield of each sputum specimen were calculated.
Results: There were 401 patients during the study period, 153 (38.2%) had positive smear for acid-fast bacilli. Overall diagnostic yields of solid culture media and liquid culture media, were 72.1% and 95.3% respectively. Incremental of overall diagnostic yield from 1 to 2 and 2 to 3 sputum specimens were 8% and 6% respectively.
Conclusion: In place where a routinely combined smear and culture for every sputum sample submitted to the microbiologic laboratory, two specimens are sufficient for the diagnosis in nearly all pulmonary tuberculosis patients.

Keywords: Pulmonary tuberculosis, Sputum smear, Sputum culture