In this study we evaluated the use of the TST and QFT-GIT assays in the diagnosis of LTBI in a country with high burden of TB and routine BCG vaccination at birth. We observed strong agreement between TST (at a cut-off of ≥10mm) and the QFT-GIT assay. Our observation is comparable with a study conducted in a cohort of health care workers in India [16
]. However, a recent community based study conducted in the Afar region of Ethiopia by Legesse et al.
] reported weak agreement between the two tests.
In our study, all of the study participants were males, unlike the study by Legesse et al.,
which reported a higher prevalence estimate in LTBI in males than females using TST, but no difference using the QFT assay. Despite this, the report by Legesse et al.
found a lower rate of TST positivity (36.8%) than that we report here (46.7%). In addition, although 48% of the participants in the study by Legesse et al.
had a TST induration of 0 mm, compared with 45% in our study, 42.7% of those were QFT-GIT positive, compared with 0% in our study. It therefore appears that the participants in the Afar region study were more likely to have a weak or absent TST response, regardless of their QFT status. Factors associated with TST false-negative reactions may include, but are not limited to, malnutrition, parasitic infections, HIV/AIDS infection, overwhelming TB disease, or difficulties with the method of TST administration and interpretation of the reaction [13
]. Any of these factors may be an issue, as the two populations studied in the two studies are quite different – relatively affluent, urban and healthy based on medical history, physical examination, chest radiography and blood tests including HIV, in this study, relatively deprived and rural with no in depth evaluation of their health in the Legesse et al.
study. It is therefore possible that the discordance seen in the study of Legesse et al.
may reflect an increase in false negative TST reactions which could contribute to an under-estimate of the incidence of LTBI in this population.
The strength of agreement between TST and QFT-GIT is slightly weaker in BCG vaccinated as compared with the group without BCG vaccination at both TST cut-offs (See Table ). The simplest explanation is the presence of false positive TST results due to sensitization to the TST reagent, Purified Protein Derivative (PPD), which contains mixture of antigens that are common to the MTB complex,
environmental non-tuberculous mycobacterium strain and BCG [9
]. This interpretation is supported by the stronger effect seen in BCG-vaccinated, QFT-negative donors at the lower TST cutoff. Some studies have shown that BCG-vaccinated individuals are more likely to have positive TST results [18
]. Interestingly, however, the effects (if any) in this study are small: we did not find a significant difference in TST positivity rate, at cut-offs of either TST≥10 mm or TST
5 mm, in BCG-vaccinated and BCG-unvaccinated participants. This finding could suggest either BCG vaccine administered in infancy has had a minimal effect on false TST positivity in young adults, or that the high rate of TST positivity due to true LTBI resulting from the high incidence of active pulmonary TB in our study setting has over-ridden the effect of BCG.
The BCG vaccine is not expected to have an effect on the QFT-GIT assay since the antigens that are contained in the assay are not present in any BCG strains [20
]. Consistent with this, we saw no difference in QFT-GIT assay positivity rate between the BCG vaccinated (47.9%) and the non-vaccinated groups (40.7%). This is also consistent with studies elsewhere [21
], and this might imply that BCG vaccination at infancy provides limited protection against MTB infection of adults. In addition to its specificity, studies have shown that QFT-GIT assay is as sensitive as TST by correlating its results with the degree of exposure (duration and proximity) to a source patient and the likelihood of acquiring infection from that source [11
]. Therefore, we used the QFT-GIT assay as the baseline definition for the diagnosis of LTBI. Using this approach we were able to see an apparent effect of BCG vaccination on the TST. Among QFT-GIT negatives (those thought not to have LTBI), the rate of TST positivity in participants with a BCG scar was 28.0% (7/25), significantly higher than those without a scar (1/35 or 2.9%), at cut-off of >5mm. At a cutoff of ≥10mm there was no significant difference. These observations suggest the effect of BCG vaccine administered in infancy on TST reactivity in adults is weak. It is detectable at a cut-off of >5mm, but is unlikely to lead to larger indurations (≥10mm), which are presumably indicative of LTBI. Our hypothesis is in agreement with that indicated by a meta-analysis of several studies on the effect of BCG vaccination on TST, which indicated little effect 15 years after vaccination, especially on TST tests using the higher cutoff [23
]. A more recent meta-analysis has also shown that the effect of BCG received in infancy on TST is minimal 10 years after vaccination [24
]. This study confirms those finding and adds the observation that in countries with a high incidence of TB, where exposure to infection is likely to be substantial, does not appear to lead to significant boosting of BCG-induced PPD sensitivity.
Moreover, we have found a good correlation between the amount of IFN-γ
(IU/ml) in the QFT-GIT assay and the magnitude of TST induration (mm) (spearman correlation coefficient
0.81 and P
<0.0001), which is also consistent with the hypothesis that LTBI leads to larger TST induration (Figure ). This has been suggested by others as well [25
In highly endemic areas, increasing age is usually associated with an increase in the LTBI rate, assessed by immunodiagnosis [27
] which is easily explained by cumulative exposure to MTB with time. However, a new finding is that in the multivariate analysis, both age increment and Khat (Catha edulis
) consumption were both found to be risk factors for LTBI, based on both TST (TST≥10 mm) and QFT-GIT assay. Khat is usually chewed in groups, and this often involves sitting together in small crowded rooms for long periods of time, especially in urban settings. In addition, the chewed leaves are spat out, a process which is likely to efficiently generate aerosols. If there is one TB patient within the group the chance of infecting others is probably relatively high. One recent study conducted in Afar region of Ethiopia where Khat consumption is a common practice, has also described it as risk factor for the development and spread of pulmonary TB [29
]. Although smoking has been associated with LTBI [30
], the number of smokers in our study was very low (2 out of 107 study participants) and hence we do not expect it to confound the effect of Khat consumption even if both habits are often found in the same individuals. While the association found here is strong, the small number of participants who consumed Khat means this finding cannot be regarded as definitive. Further studies to confirm the role for Khat consumption as a risk factor for LTBI should be conducted as Khat chewing is becoming a common practice all over the country, and indeed, throughout the Horn of Africa.