Please see Additional file 1 for translations of the abstract into the six official working languages of the United Nations.

Tuberculosis (TB) is an infectious disease caused by various strains of mycobacteria, especially Mycobacterium tuberculosis and usually attacks the lung [1]. It remains to be a major cause of morbidity and mortality throughout the world. It is estimated that one-third of the world's population is infected, 8.8 million people develop TB, and 1.45 million people die annually from the disease [2]. In Africa, about 2.8 million incident TB cases and 390 thousands TB deaths occurred in 2009 [3]. Ethiopia ranks seventh among the world’s 22 high-burden TB countries [2]. According to the World Health Organization’s (WHO) Global TB Report 2011, Ethiopia had an estimated incidence rate of 261 cases per 100,000 population and 29 thousands deaths in 2010, with an estimated prevalence rate of 394 cases per 100,000 population [2]. The incidence of TB was reported greatest among people with impaired immunity, HIV infection, or diabetes [4].

Diabetes mellitus (DM) is complex metabolic disorder that is characterized by a high level of blood sugar either because the body does not produce enough insulin or cells do not respond to the insulin [5]. This high level of blood sugar produces the classical symptoms of frequent urination, and increased thirst and hunger. The total number of people with DM worldwide is projected to rise from 285 million in 2010 to 439 million in 2030 [4]. In 2010, 12.1 million people were estimated to be living with diabetes in Africa, and this is projected to increase to 23.9 million by 2030 [6]. The WHO estimated that the number of cases of diabetes in Ethiopia was about 800,000 in 2000, and projected that it would increase to about 1.8 million by the year 2030 [7].

Besides HIV, malnutrition, alcoholism, and smoking, DM has received recent recognition as a risk factor for TB [8]. Epidemiological studies have elucidated an association between DM and the development of TB [9]. According to a study done by Jeon CY et al., people with DM had approximately a three times higher risk of developing TB than people without diabetes, and this has been evidenced by biological support of the causal relationship between diabetes and host immunity to TB [10]. Evidence from an experimental study revealed that diabetes was associated with reduced macrophage functions, such as chemotaxis, phagocytosis, and bactericidal actions, and also impairs the function and proliferation of T-helper 1 cells and their production of cytokines [11].

On the other hand, a study done in Brazzaville, Congo showed that TB patients with diabetes were more likely to have an increased rate of treatment failures, deaths, defaults and relapses, delayed smear, and culture conversion [12]. It was also found that TB treatment reduced the effectiveness and concentration of diabetes medications, which makes it difficult to control diabetes [13]. However, the prevalence of PTB among diabetic patients was not assessed in the study area. Therefore, this study was conducted to determine the magnitude of smear positive PTB and its associated risk factors among TB suspected diabetic patients at the Dessie referral hospital.

Previous studies have identified an important association between diabetes and TB, in that DM is one of the risk factors for the development of active PTB [8, 9]. However, in Ethiopia, particularly in Dessie, little is known about prevalence and associated risk factors of PTB in the population of those with diabetes. Hence, the present study tries to provide insights into the prevalence of PTB in diabetic patients, as well as outline some possible risk factors.

In this study, the prevalence of PTB among TB suspected diabetic patients was 6.2%. It is higher than the 2011 WHO estimated prevalence of TB in the general population of 0.39% [3]. This finding is in line with the reports from Tanzania (5.4%) and India (6%) [16–18]. However, the finding of the current prevalence of PTB among diabetics was higher than the prevalence reported in Korea (2.12%) [19]. On the other hand, the burden of PTB in Dessie was lower than the prevalence reported from Pakistani studies that revealed the prevalence of PTB to be from 9.5% to 14% among diabetic patients [20–22]. This variation could be due to the use of an advanced diagnosis technique in Pakistani studies, and the fact that the study participants were admitted patients and thus were more likely to be positive.

A study done in Addis Abeba showed that 4.14% of diabetic patients had PTB, which is lower than the present study finding [7]. The increased prevalence of PTB in our study might be due the involvement of TB suspected diabetic patients as compared to the study in Addis Ababa, which was retrospective and considered all diabetic patients.

The socio-demographic characteristics of PTB suspected diabetic patients and other risk factors for active PTB infection were also investigated. Several studies have shown that socio-economic status is a risk factor for active TB occurring [23–25]. The result of this study showed that the higher the age of the patients, the more likely it is that they will have TB infection if they are also diabetic. The majority of the patients who developed PTB were older than 40 years of age and this was comparable to other age reports from Indian, Korea, and Pakistan [17, 20, 22]. The possible reason for this may be due to a compromised host’s immunity that increased susceptibility to TB due to aging, and most diabetic patients were in the over 40 age group as well.

The association of sex with active PTB infection was not statistically significant. However, among the 14 PTB patients, 11 (78.6%) were male which indicates a higher proportion of PTB infection among male diabetic patients rather than females (21.4%). This result was consistent with a study in Pakistan (78% versus 22%, respectively) [22].

Urban residence was also associated with the development of PTB. The prevalence of smear positive PTB in this study was higher in urban dwellers (52.7%) than rural residents. This finding was consistent with a study done in India, in which the prevalence of smear-positive TB was reported to be 69.2% [26]. The possible reasons for this could be because of crowded living conditions in urban areas, as well as urban dwellers having lower levels of physical activity and mostly consuming a calorie rich diet, which increases fat accumulation in the body. Physically inactive or sedentary lifestyles lead to excess body fat accumulation which increases insulin resistance and ultimately results in higher blood glucose levels which impairs the immune cells against TB infection [23].

The role of smoking in the development of active TB is well established [27], but smoking was not associated with active TB in this current study and this could probably be attributed to a social desirability bias whereby smokers denied their smoking status.

This study also identified the variable that has the most important influence on the occurrence of PTB. It revealed that patients who had a previous history of TB were significantly associated with developing smear positive PTB than those who did not have a previous history of TB. Moreover, living with a TB patient had been one of the most important predictors of smear positive PTB in this study, which is consistent with previous findings in Pakistan and India [17, 20, 28]. This might be due to the fact that frequent contact with TB patients in a household could lead to increased transmission of PTB.

The duration of DM has been also associated with the risk of developing smear positive PTB. Those patients who have had DM for more than ten years had a higher proportion of PTB than those who had shorter durations (<5 years) of DM, which is consistent with a study done by Jabbar et al. that showed a high prevalence of PTB among diabetic patients who had DM for more than ten years [21]. Uncontrolled high levels of blood glucose for long periods of time could be another factor associated with the development of TB. In addition, in low-resource settings, early diagnosis and adequate glycemic control of DM is difficult, and this will probably further increase the proportion of diabetic patients developing TB.

In general, the present study and other studies carried out so far show the high burden of PTB among the diabetic populations. Therefore, active screening and treatment of PTB among patients with DM is especially relevant in TB-endemic countries, such as Ethiopia.

This study had some limitations. Direct smear microscope alone may underestimate the prevalence of PTB in the study population. We believe that comparing the prevalence of PTB with non-diabetic patients to that of diabetic patients might disclose important information about the occurrence of PTB. However, due to financial constraints, we were unable to determine the prevalence of PTB among non-diabetic patients. Selection bias may also arise from convenience sampling.

The overall prevalence of smear positive PBT among TB suspected diabetic patients was high (6.2%) in Dessie. Pulmonary TB was occurring more often among patients with DM (>10 years), and among patients who had high blood glucose levels. Patients with a previous history of TB, history of contact with TB patients in the family, living in urban areas, and prolonged duration of DM were independent risk factors for the occurrence of active PTB. Therefore, we recommend that, at the time of their diagnosis, diabetic patients be screened for PTB and have regular check-ups. Further studies are also recommended to strengthen and explore the problem among diabetic patients in depth with large sample sizes and advanced diagnostic techniques.