Impact of sputum gross appearance and volume on smear positivity of pulmonary tuberculosis: a prospective cohort study

BACKGROUND: Although checking specimen quality upon sputum collection for acid-fast smear of suspected tuberculosis (TB) cases is recommended, this procedure is based on expert opinion. The purpose of this study was to elucidate the impact of sputum gross appearance and volume on smear positivity among patients with suspected pulmonary TB, according to sex. METHODS: From November 2010 through June 2011, we enrolled consecutive patients suspected to have active pulmonary TB. The association of sputum gross appearance and volume with smear positivity, along with other variables possibly affecting smear positivity such as symptoms, disease extent, and cavity on chest radiograph, were investigated. RESULTS: Among 2,439 patients undergoing TB examination, 170 (113 men, 57 women) with active pulmonary TB were enrolled. They submitted 492 sputa. There were 73 smear-positive patients (42.9%) and 164 smear-positive sputa (33.3%). While gross appearance was associated with smear positivity in both sexes (purulent or blood-tinged sputum (rather than mucoid sputum or saliva); odds ratio (OR), 2.05, 95% confidence interval (CI), 1.21–3.47 in men; OR, 2.78, 95% CI, 1.23–6.26 in women), the amount of sputum specimens was associated with smear positivity in only female patients (≥4 ml versus <4 ml; OR, 4.96, 95% CI, 1.98–12.37). CONCLUSIONS: Sputum gross appearance and volume were associated with smear positivity. A volume of 4 ml seems to be the the minimum sputum volume acceptable for smear microscopy in females suspected of TB. Those suspected of TB should be encouraged to expectorate grossly qualified sputum specimens.