We compared the QOL of persons with HIV infection with and without active TB. The Amharic version of the WHOQOL-HIV instrument had a good internal consistency to assess the QOL of our TB/HIV co-infected patients. The instrument had strong inter domain and negative correlation with the Kessler scale and the stigma instrument. Strong correlation between the Kessler scale and the psychological domain of the QOL instrument indicated that the two instruments had measured the same concept. Although detail validity study was not done, the above information could indicate that the Amharic Version of the WHOQOL-HIV had good construct validity. The WHOQOL-HIV instrument was previously reported to have a good reliability and validity in different cultures worldwide [26–28]. In this study, co-infected patients had a lower QOL in all of the domains of the WHOQOL-HIV as compared to people living with HIV without TB. The occurrence of two stigmatizing diseases can decrease the QOL by affecting the physical, social and mental wellbeing of the person. In other studies, it was reported that HIV patients had a lower QOL as compared to the general population  and that TB patients had a lower QOL as compared to their neighbors [17, 18].
Different studies identified several factors which affect the QOL of patients. In a multi-country study among patients with HIV, it was found that women, older age groups, and the less educated had a lower QOL . A study conducted among African American HIV positive participants showed that stigma and presence of symptoms of HIV were associated with poor QOL . In our study, depression and lower income were associated with the physical, social and environmental domains of QOL. Depression can decrease QOL  but can also be the result of a poor QOL. Because of the cross sectional nature of our study, we couldn't establish a cause effect relationships between QOL and depression. Perceived stigma was also associated with the psychological domain of QOL. The effect of perceived stigma on QOL was also reported by Yen et al in Taiwan .
Lack of social support, lower level of education and income had been reported to be associated with poor QOL of TB patients [15, 18]. In our study, income, depression and lack of family support were predictors of poor QOL among TB/HIV co-infected. Participants without adequate income and family support might have a poor nutritional and immune status which in turn could affect the QOL.
In contrast with other studies, we couldn't find an association between CD4 count, WHO staging and other socio-demographic characteristics with QOL [8, 15, 18].
The results of our study have to be interpreted with caution. Indeed, although the Amharic version of the WHOQOL instrument was used previously, the content and criterion validity of the instrument was not assessed.