Improved diagnosis of Ziehl-Neelsen smear negative tuberculosis using sodium hypochlorite sedimentation method

Abstract

Background: Bacteriological diagnosis of tuberculosis (TB) is largely dependent on Ziehl-Neelsen (ZN) microscopy. This method has a low sensitivity. Although concentration of sputum with sodium hypochlorite (NaOCl) followed by sedimentation increases the sensitivity of direct smear microscopy, no study has focused on the effect of NaOCl on smear negative sputum specimens. Objective: To establish whether 3.5% NaOCl sedimentation method specifically improves the diagnosis of Ziehl-Neelsen smear negative tuberculosis. Design: A prospective study. Setting: Mbagathi District Hospital and Center for Respiratory Diseases Research, Kenya Medical Research Institute. Subjects: Two hundred and thirty confirmed direct ZN smear negative sputum specimens from new TB suspects were analysed. Results: Seventy (30.4%) specimens were culture positive. Of these, 19 were ZN smear positive. The ZN sensitivity, specificity, positive and negative predictive values were 27.1%, 99%, 95% and 76%, respectively, after sedimentation with 3.5% NaOCl. Conclusion: Overnight sedimentation using 3.5% NaOCl significantly improves diagnosis of ZN smear negative TB. This technique has potential to improve diagnosis in TB diagnostic services especially in settings with high burden of dual TB/HIV infection.