This study was performed to analyze treatment outcome and factors associated with unsuccessful treatment outcome of the TB cases treated at DOTS clinic of Arsi-Robe Hospital, Ethiopia between January 2013 and December 2017 (Table 1). Evaluation of the treatment outcome and associated factors for TB patients has the greatest importance in assessing the effectiveness of DOTS program in a country .
In this study, the frequency of male TB patients (57.9%) was higher than female (42.1%) in agreements with studies in Arsi zone, central Ethiopia (male: 54.4%; female 45.6%) . Similar studies from Southern Ethiopia (61.3%)  reported higher frequency of TB patients in males than females. Globally, gender inequalities have been reported in TB case notification, and TB is more common in males than females . It was suggested that higher proportion of males being exposed to the infection as a result of more social contact or engagement in professions associated with a higher risk for tuberculosis , or underutilization of the DOTS service by female patients as a result of various socioeconomic and cultural influences  or might be due to the biological differences between male and female in vulnerability to development of active disease .
In our study, more TB patients were noted from rural areas (62.6%) than urban areas (37.4%). A study from southern Ethiopia also revealed higher frequency of TB patients from rural Ethiopia (66%) than that of urban areas (44%) . This might be because of better awareness of tuberculosis and health facility in urban areas than rural areas. The frequency of TB infection was low in earlier ages (≤ 14 years) (Table 1). The low infection frequency in lower ages might be due to vaccination of children with vaccine bacille Calmette-Guérin (BCG) in earlier stages. Ethiopia is one of the countries with BCG coverage in the range of 50–89% in 2018 .
The frequency of TB infection was also higher among new patients as compared other TB treatment categories (Table 1). Similar result was also reported in other study in Debre Tabor . This might indicate that disease was widespread in the society, and the retreatment cases in the society may be the sources for subsequent infection new infections. TB cases were predominantly PTB- (48.3%) followed by PTB+ (32%) and the least was EPTB (19.8%) (Table 1). Similarly, earlier studies from Bale Robe Hospital, south eastern Ethiopia (40.7%)  and from Brazil (65.2%) reported higher frequency of PTB- over PTB+ .
According to WHO , continuous tracing of TB treatment outcome, mainly in TB/HIV co-infected patients, is essential. The overall TB-HIV co-infection rate at Arsi-Robe Hospital was 8.6% (Tables 1 and 2). This was lower than studies in northeastern parts of Ethiopia (24.3%) , and Ethiopia as a whole (29.4%)  and Kenya (41.8 %) . Studies indicated that the unavailability of HIV counseling and testing services or refusal of patients to be tested for HIV  or low prevalence of HIV in the study population  could contribute to lower TB-HIV co-infection. In this study, PTB- cases had higher number of TB/HIV co-infection than PTB + cases (Table 2). This is the fact that TB cases with HIV are less likely to be smear-positive  and high in HIV prevalence countries, the number of patients with SNPT is increasing rapidly .
The percentage of cured patients in our study (31.5%) was higher than the rate in the Bale Robe Hospital (19.7%) . More than half of the study population completed their treatment. It was higher than study in Gonder Teaching Hospital, North Ethiopia (46.4%) . The Arsi zone was the zone that historically commenced the first DOTs program . Deployment of health extension workers all across the zones of Ethiopia that provide preventive and curative interventions against TB has been suggested for increase in the treatment outcome (cure and treatment completion) of TB .
The failure rate in this study (5.1%) was higher than in Asella Teaching Hospital, Ethiopia (0.2%) . Treatment failure is a result of MDR-TB development in previously treated TB cases . Treatment failure steadily decreased across the study period although the trend was not consistent through the years. The defaulting rate in our study (5.1%) was slightly lower than in Bale Robe Hospital (6%) . The overall death rates of TB patients in this study (5.8%) was similar to that in Asella Hospital, central Ethiopia (6%) , but lower than in Arsi Zone for smear positive cases (7.4%) . In our study, higher death rates occurred in TB patients who were female, urban residents, older, HIV- positive, and in PTB- (Table 2). In agreement with our study, previous studies also reported that older ages (44–54 years), smear- negative pulmonary TB  and HIV- positive  were more likely to die than their counter parts. High death rates have been reported for urban residents and in PTB-  in concordance with our study. According to Dangisso et al. , the higher proportion of deaths in PTB- cases than PTB + and EPTB cases might be due to diagnosis and treatment delays, as well as HIV infection among PTB- cases. The recorded higher death in TB patients of older ages might be due to increasing comorbidities as well as the general immunological deterioration with age [35–36].
TB patients who were HIV positive experienced more default (9.1%), death (18.2%) and treatment failure (13.6%) than HIV negative TB patients (Table 3). It was in agreement with study in Arsi Asella . Studies indicated that TB/HIV co-infection has significant impact on treatment outcome [37–38]. HIV affected the performance of TB control programmes by increasing the number of TB cases and by compromising the treatment outcome.
The overall TSR of all forms of TB in Arsi-Robe Hospital in this study was 84.0% (Table 4). It was nearly similar with studies in Southwest Ethiopia (85.2%) , but lower than the global target (> 90%) , WHO target of Ethiopia for the new cases in the years of 2016 (90%)  and 2017 (96%) , and China (95.01%) . But, it was higher than Asella Teaching hospital (81.7%) , Bale Robe Hospital (78.2%) , and Kenya (82.4%) . The variation in the TSR with other studies could be due to differences in variation in DOTs performance in various study areas, socio-economic of the patients, geographic setting, sample size, study period and the TB clinic management (Table 4).
Higher treatment success was noted in male TB patients than female (Table 4). Similar findings were also noted from Addis Ababa , and South region , but different from studies in Bale Robe Hospital , and Arsi Asella hospital . The treatment success rate was high in younger TB patients, and decrease in older ages but the trend was not consistent. Studies indicated that TB patients at higher age might be under immune-compromised situations as a result of associated diseases, and general physiological deterioration with age thereby resulted in poor-treatment outcome [35, 39].
In our study, PTB + had the highest treatment success rates (95.1%) as compared other TB types, and was higher than Arsi Zone (83.6%) . The TSR in TB/HIV co-infection cases (59%) in our study was lower than Bale Robe Hospital (67.1%) , and Asella Teaching Hospital (79.8%) . The treatment success in Arsi-Robe Hospital showed an increment during the first two years (76.6% in 2013 and 86.8% in 2014) followed by an inconsistent increase and decrease in the following years (Table 4). The national treatment success rate (NTSR) was 91% in 2013 , 89% in 2014 , 84% in 2015  and 90% in 2016 . This shows that the treatment success of Arsi-Robe Hospital was lower than the NTSR except in 2016 (90%) (Table 4).
In multivariable logistic regression, the odd of unsuccessful treatment outcome of TB was higher in TB patients residing urban areas than rural residents (Table 4). It was agreement with recent study in Debre Tabor, Northern Ethiopia  and Sidama Zone, Ethiopia , but against study in south Ethiopia . A model based study from India indicated that average treatment success rate decrease with increase in the proportion of urban population and with increase higher degrees of urbanization . Co-morbidities (HIV, and diabetes mellitus), temporary interruption of treatment and other addiction related factors were suggested for poor treatment outcome of TB among urban residents , and difficulties in ensuring quality of care and prevention of infection and transmission in densely populated areas with large disparities in sanitation and access to health care  have been suggested to explain the observed difference.
TB patients with failure treatment had high odd of significant unsuccessful treatment as compared to new patients (the reference patient category). This was in agreement with the study of Melesse and Zeleke (2018)  where new cases have lower odd poor treatment as compared to retreatment cases. It was found that previously treated TB cases (including failure) had a multidrug resistance TB (MDR) [11, 44] and was associated with unsuccessful treatment outcomes [10, 44].
In our study, TB patients with HIV co-infection had higher odd of unsuccessful treatment of TB as compared HIV negative TB patients. Several studies documented lower TSR of the HIV/TB co-infected cases [9–10, 13]. HIV co-infection also increases the risk of latent TB reactivation 20-fold, is the most known risk factor for progression of M. tuberculosis infection to active disease [18, 33]. The low treatment success in TB/HIV co-infected patients could be due to co-administration of ART along with anti-TB therapy which can lead to drug-drug interactions, overlapping drug toxicities and immune reconstitution syndrome .
Smear positive pulmonary TB patients (PTB+) had lesser odd of unsuccessful treatment of TB as compared to smear negative pulmonary TB and extra pulmonary TB patients. This is in agreement with studies in Ethiopian University Hospital , and Southern Ethiopia . According to Biruk et al. , there is a monitoring of the treatment outcome and testing of the sputum in the 2nd, 5th and 7th months of treatment for PTB + but for PTB- and EPTB, monitoring is only in clinical conditions. HIV co-infection in PTB- and EPTB patients may decrease the treatment success and increase mortality . Global reports also indicated an increase in TB treatment success from 2012 to 2016 [1, 42]. The higher TSR of PTB + in our study could be attributed to improved diagnosis and good adherence to DOTs program in the Arsi-Robe Hospital.
This study was performed on retrospective basis and used already registered socio-economic and clinical data. As compared to WHO international target (85%) [11, 18], our study revealed an appreciable treatment success rate of TB (84%) in the study area. Many variables that might help to relate with treatment outcome were missed because they were absent in data registry or log book of the hospital. Secondly, this study doesn’t find data on other potential factors such as sputum smear test result at second month after initiation treatment, duration, malnutrition status, drug resistance pattern of the TB patients. The data was also collected from a single hospital, though it gives treatment services to more than a half a million of people of the town and the surrounding district. The absence of these critical factors has impact on the conclusion of the results of our study and any interpretation of the results should consider the mentioned limitations, and based on the variables considered.