Prevalence of smear negative pulmonary tuberculosis among patients visiting St. Peter's Tuberculosis Specialized Hospital, Addis Ababa, Ethiopia.

BACKGROUND: Smear negative pulmonary tuberculosis is an increasing clinical and epidemiological problem, particularly in areas that are affected by the dual tuberculosis/Human immunodeficiency Virus infections. OBJECTIVE: This study was initiated to investigate the value of clinical parameters, chest x-ray findings and culture in the diagnosis of smear negative pulmonary tuberculosis. DESIGN: A cross sectional study was conducted among suspected pulmonary tuberculosis patients visiting St. Peter Tuberculosis Specialized Hospital, Addis Ababa, Ethiopia between November 15, 2004 and October 30, 2005. METHODS: A total of 297 informed and consented patients with suspected pulmonary tuberculosis were screened for acid fact bacilli by direct smear microscopy. All smear negative pooled sputum samples were further processed for culture using conventional Lowenstein-Jensen solid medium and automated BACTEC MGIT 960 system liquid medium at the Ethiopian Health and Nutrition Research Institute. RESULTS: 247/297 (83.2%) patents with suspected pulmonary tuberculosis have had a negative smear results for acid fast bacilli. Abnormal chest x-ray findings were observed in 196 (79.4%) patients. 43/247 (17.4%) patients whose smears were negative for acid fast bacilli found to be positive for mycobacterial culture. The Mycobacterium species identified were M. tuberculosis (n = 40) (93%) and non-tuberculous mycobacteria (n = 3) (7%). Significant difference was not demonstrated statistically between BACTEC MGIT 960 and Lowenstein-Jensen medium in terms of mycobacterial recovery rate (p > 0.05). CONCLUSIONS: The present study showed 82.6% smear negative pulmonary tuberculosis cases were still etiologically unexplained by culture. Therefore, there is a need to develop a scheme to determine the most cost-effective approaches for the diagnosis of smear negative pulmonary tuberculosis in the Ethiopian setting, such as improving the screening method patients with tuberculosis and other chronic pulmonary diseases, chest-x-ray readings and interpretation, specimen collection and processing, smear microscopy, culture and applying laboratory quality control schemes in parallel.