Introduction: tuberculosis causes illness among millions of people each year and ranks as the second leading cause of death from infectious disease worldwide. The aim of this study was to investigate the detection rate of microscopic examination and estimate risk of transmission of TB by smear negative pulmonary TB patients.

Methods: a cross-sectional study and retrospective data analysis on TB were undertaken in Northwest Shewa, Ethiopia. Microscopic examination, bacterial culture and PCR were performed. The statistical analysis was made by using STATA software version 10.

Results: a total of 92 suspected TB cases was included in the study. Of these, 27.17% (25/92) were positive for microscopic examination and 51% (47/92) for culture. The sensitivity and specificity of microscopic examination with 95% CI were 48.94% (34.08% to 63.93%) and 95.56% (84.82 to 99.33%), respectively. The positive and negative predictive values were 92% (73.93% to 98.78%) and 64.18% (51.53% to 75.53%), respectively. Of 8150 pulmonary TB cases in the retrospective study, 58.9% was smear negative. The proportion of TB-HIV co-infection was 28.66% (96/335).

Conclusion: the sensitivity of microscopic examination was 48.94% which was very low. The poor sensitivity of this test together with the advent of HIV/AIDS elevated the prevalence of smear negative pulmonary TB. This in turn increased the risk of TB transmission.