A high overall prevalence of psychological distress (32.9 % and 81 % according to the K-10 score ≥28 and K-10 score ≥16, respectively) was found in this large sample of tuberculosis public primary care patients in South Africa. Caseness of psychological distress or common mental disorder was assessed using two different cut-offs (K-10 score ≥28 and K-10 score ≥16), as found in two different previous validation studies in South Africa [26, 27]. The uncertainty regarding the correct K-10 cut-off for this study group is a major limitation of this study.
Several studies showed that the K-10 had good psychometric properties [25, 26] and can discriminate between cases and non-cases reported in the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) [15, 34]. The finding of 32.9 % to 81 % (using cut off scores of 28 and 16 respectively) the prevalence of psychological distress in this study is in line with the prevalence rates of depression or common mental disorders in most other studies with tuberculosis patients 46 %-80 % in LMICs [10–12, 15–18]. The differences in prevalence of common mental disorders in the studies cited in the literature could be attributable to several factors including the population being studied, the study periods during the TB treatment course and the diagnostic tools used . It is possible that increased rates of psychological distress were found in this study because the assessment followed within a short time (within one month) of the TB diagnosis which might not have persisted at a later stage of the disease course or upon completion of TB treatment. Andersen et al.  have suggested that the Kessler scales had significantly lower discriminating ability in regard to depression and anxiety disorders among the Black African than among the combined non-Black African population group in South Africa, and they attribute this to differential item biased measurement. The Black African population group is usually highly represented among the lowest socioeconomic status groups in South Africa and often lack basic necessities as compared to the other population groups in South Africa. Consequently, they may more likely endorse K-10 items such as “How often do you feel that everything is an effort?” In this study, however, there were no significant differences in psychological distress rates between Black African, Indian/Asian or Whites population groups. Spiess et al.  also did not find a significant difference in validity of the K-10 in detecting depression and anxiety disorders among HIV infected South Africans across gender, age, education, or ethnicity categories.
Given the relatively large difference between the levels of psychological distress using a cut-off score of 28 versus 16 on the K-10 in this study, it is important to consider which cut-off score is more appropriate for use in a clinical setting within the public health sector. The authors in this study recommend the use of a cut-off score of 16 for use in South Africa, particularly within the public sector health clinics in order for cost-efficient treatment programmes to be implemented on a large scale.
In multivariable analysis the study found that lower formal education and poverty were associated with psychological distress. Low socioeconomic status has also been found in other studies to be associated with common mental disorders in TB patients [13–15]. Most studies in developing countries showed an association between indicators of poverty and the risk of mental disorders, the most consistent association being with low levels of education [3, 35, 36]. Many patients in low and middle income countries suffer from common mental disorders because of the stress caused by poverty [3, 15, 37] and associated factors such as the experience of insecurity and hopelessness, rapid social change and the risks of violence and physical ill-health may explain the greater vulnerability of the poor to common mental disorders . Financial empowerment of patients may reduce their levels of depression, and improve the compliance rate to anti-TB medication which could ultimately result in an improved quality of life .
Older age in this study was associated with psychological stress. This finding is consistent with the findings of other studies among TB patients [13, 14] but was not consistent with findings in general population studies . . This increased prevalence of distress in older participants may be due to increased responsibilities such as child care, care of other family members, employment and economic responsibilities, having to cope with chronic illness conditions, including HIV in the older age group . Marital status (being married or cohabitating) has been found in this study to serve a protective factor from psychological stress (K-10 ≥28) and this finding is consistent with the findings of other studies [39, 40]. Being married or cohabitating provides social support thereby reducing the levels of psychological distress . In contrast to some studies [11, 13] but in agreement with other studies , we did not find a significant association between gender and psychological distress.
Being TB/HIV co-infected was found in this study to be associated with a higher rate of common mental disorders (using the K-10 cut off of ≥16) than among non-coinfected patients, a finding consistent with other studies . Being diagnosed with HIV which is a terminal life-long disease associated with high levels of stigma may also lead to high rates of mental disorder . Substance use (hazardous or harmful alcohol use and current tobacco use) in this study was associated with psychological distress. This finding is similar to the findings in other studies which found substance use to be associated with depression and psychological distress in general patient population groups [43–46]. Daily or almost daily tobacco use was also found in this study to be associated with psychological distress a finding similar to the findings of other studies . In this study there was no association found between TB and HIV treatment non-adherence and common mental disorders as found in other studies [11, 15, 20, 47]. It is possible that the impact of psychological stress on treatment adherence may take longer to take effect than at the beginning of TB treatment.
Caution should be taken when interpreting the results of this study because of certain limitations. Generalisability of our findings is limited to TB and HIV patients on treatment in public primary care centres in South Africa. Furthermore, measures of anxiety and depression at a general population level in South Africa may be needed so that the diagnostic accuracy of common mental disorders among patients with anxiety and depression can be compared to those without these disorders. This study was a cross-sectional one which implies that no statement of causality between the variables can be made. A further limitation was that most variables were assessed by self-report and socially desirable responses may have been given. The Kessler 10 scale is not 100 % sensitive and specific which may have resulted in misdiagnosis or missed cases of common mental disorder. Some areas of assessment such as number/severity of symptoms reported, illness perceptions [10, 13, 47] and perceived stigma  were not included in the study. These factors are known to be related to common mental disorder in TB patients.