Diagnostic value of repeated sputum examinations in pulmonary tuberculosis: how many sputum specimens are adequate?

The aim of this study was to assess the value of examining multiple sputum specimens in the diagnosis of pulmonary tuberculosis (PTB). We analyzed sputum smear and culture results of patients diagnosed with culture-proven PTB during 2002. In 1027 patients, the diagnosis was established by detection of Mycobacterium tuberculosis bacilli in sputum samples. The number of sputum specimens submitted to laboratory was one in 634 cases, two in 167 cases, three in 186 cases and more than three in 48 cases. 760 (74%) cases had positive smear examination result. The first sputum smear examination was positive in 82.3% of smear positive cases. Either the first or the second sputum was diagnostic in 94.9% of these cases. Smear examination of third sputum revealed 4.2% additional diagnostic yield. In 863 (84%) cases, culture examination of the first sputum was positive. The second and the third sputum culture examinations revealed additional diagnostic yield of 11% and 4.5%, respectively. Percent 95 of culture-proven cases were diagnosed with the first two sputum cultures. In conclusion the majority of PTB cases can be diagnosed with the examination of two sputum specimens. Three or more sputum specimens submitted obtain a small additional diagnostic yield.