Sputum conversion among patients with pulmonary tuberculosis: are there implications for removal of respiratory isolation?

Abstract

Limited data are available to predict the length of time required for a patient to achieve sputum culture conversion after starting therapy for pulmonary tuberculosis. Rates of sputum smear and culture conversion were determined at weeks 2, 4, 8 and 16 after initiating therapy in patients admitted to our Respiratory Isolation Unit from January 1997 to December 2003. For the 184 patients included in the analysis, the mean time from the initiation of appropriate therapy to sputum culture and smear conversion were 34 +/- 26 and 38 +/- 32 days (mean +/- SD) respectively. Only 53% of patients obtained negative sputum cultures within the first 4 weeks of therapy. Multivariate analysis showed that the persistence of positive cultures during the first 4 weeks of therapy was associated with high bacillary counts in sputum smears at diagnosis [OR: 2.86; 95% confidence interval (95% CI): 1.20-6.66], lung cavitations (OR: 4.0; 95% CI: 1.63-9.09) and a prolonged period of symptoms (OR: 3.57; 95% CI: 1.43-3.57). The only factor associated with the persistence of positive cultures after more than 16 weeks of therapy was infection with a multidrug-resistant strain. High initial sputum bacillary counts and drug resistance result in delayed culture conversion. This should be taken into account when decisions regarding the potential discontinuation of isolation are made. The early identification of drug resistance is important for effective infection control in hospitals.