Disseminated TB (DTB) is a potentially lethal form of TB that affects other organs, other than the lung parenchyma, through lymphohematogenous spread with typical extrapulmonary disease locations including lymph nodes, pleura, genitourinary tract, bones and joints, meninges, peritoneum and pericardium. DTB has been increasingly observed in immunocompromised hosts, especially in developing countries, where DTB is the principal cause of morbidity and mortality due to higher rates of TB-HIV co-infection . However, little is known about the burden, risk factors and treatment outcome of this condition in Uganda.
In both the DTB and non-DTB cases in our study, the modal age was 15-34 years, which is consistent with the most frequent age group affected by TB . The prevalence of DTB was found to be 9%, which is similar to that found in Oman (10%)  but lower than that in Portugal (20%) , Ghana (33%)  and South Africa (31%) . Further, our prevalence is higher than that found in Tanzania (5.7%) . These differences can be explained by variations in the study populations and definition of DTB used. The study populations consisted of EPTB patients in Ghana, hospitalised HIV patients in South Africa, and febrile inpatients in Tanzania. It is often difficult to establish the diagnosis of DTB as there is no standardised diagnostic criteria and the clinical presentation of DTB is commonly non-specific, with symptoms varying according to the affected organs . In our study, the commonest site of dissemination was the abdomen. In contrast, Leeds et al  in their retrospective review of EPTB cases in the United States found the most common site of dissemination to be lymphatic. Ultrasound scan services are more readily available in Uganda than other advanced diagnostic methods such as lymph node biopsy and histology. This may have influenced the frequency of abdominal TB that we observed due to higher diagnostic capability for abdominal TB than other sites. It is possible that some sites of TB dissemination are not confirmed due to lack of readily available diagnostic resources.
Casual laborers were more likely to have DTB in our study. Casual labor as a source of income is a proxy for low socio-economic status and low education level. Low socio-economic status is associated with increased TB susceptibility, TB infection, TB progression to active TB, severe forms of TB (such as DTB) and poor TB treatment outcomes due to poor living and working conditions, undernutrition and poor health seeking behavior .
There were more Bantu-speaking individuals with DTB than without. While the reason is not apparent from our study results, genetic differences (as is expected in different ethnic backgrounds) are widely recognized to influence immune responses against Mycobacterium tuberculosis(Mtb) and TB dissemination. The Bantu-speaking individuals in our study may have had a low frequency of HLA sub-types that are important in mycobacterial epitope recognition as has been suggested by a study among a South African Colored population . It is noteworthy that ethnic background was unknown for >50% of the study population. There is, therefore, a risk for misclassification bias. The association between Bantu ethnicity and DTB deserves further investigation.
Similar to previous reports by Meira et al,  and Wang et al , we found DTB to be associated with comorbidities of which HIV was the most predominant (100% of patients with DTB had HIV co-infection). This should be expected because HIV globally impairs immune responses against Mtb resulting in TB dissemination .
Typical chest radiographic findings in DTB are not well established. In our study, only 35% of DTB cases had military pattern. In contrast, Khan et al  estimate that the classic miliary pattern occurs in 85% - 90% cases of DTB. It is unclear whether this is among patients with any form of DTB per se or among patients with bacteremia DTB (miliary TB). Interestingly, we found that DTB patients were more likely to have empyema but less likely to have bronchopneumonic opacities. More studies are needed to characterize chest x-ray findings among patients with DTB with or without bacteremic DTB.
Our findings about the treatment outcomes are consistent with other studies that found DTB to be highly fatal . We found that DTB patients had a fourfold mortality rate as compared to those without DTB. In Portugal, Meira et al.  found a mortality rate of 36% among patients with DTB but was not significantly higher than that among patients without DTB (21%). However, many patients in their study had other comorbidities and HIV was prevalent in only 47% (cf. 100% in our study) of patients with DTB. The rate of cure was lower in the DTB than in non-DTB cases in our study. This is expected since patients with predominantly extrapulmonary forms of TB may be unable to produce sputum during treatment follow up to enable confirmation of TB cure . From our results, it is evident that patients with DTB were more likely to be assigned “treatment completion” as opposed to cure.
Our study had limitations. We had a small number of cases with DTB which limited our ability to construct a robust model for predictors of DTB. We also did not evaluate some biomedical differences such as anemia, organ dysfunction, and clinical symptoms. These were not consistently documented in the charts. Our discussion of the relevance of ethnicity in DTB was limited by not knowing the ethnicity of 201 study participants and being a retrospective study, we could not obtain this information . Lastly, our study was at a national referral center and our estimate of the prevalence may be an over-estimate due to referral bias.