Legesse et al. BMC Infectious Diseases 2011, 11:89
http://www.biomedcentral.com/1471-2334/11/89
RESEARCH ARTICLE
Open Access
Community-based cross-sectional survey of latent
tuberculosis infection in Afar pastoralists,
Ethiopia, using QuantiFERON-TB Gold In-Tube
and tuberculin skin test
Mengistu Legesse1,3*, Gobena Ameni1, Gezahegne Mamo2,3, Girmay Medhin1, Gunnar Bjune3 and Fekadu Abebe3
Abstract
Background: There is little information concerning community-based prevalence of latent tuberculosis infection
(LTBI) using T-cell based interferon-g (IFN-g) release assays (IGRAs), particularly in TB endemic settings. In this study,
the prevalence of LTBI in the Afar pastoral community was assessed using QuantiFERON-TB Gold In-Tube (QFTGIT)
and tuberculin skin tests (TST).
Methods: A community-based cross-sectional survey of LTBI involving 652 apparently healthy adult pastoralists
was undertaken in the pastoral community of Amibara District of the Afar Region between April and June 2010.
Results: The prevalence of LTBI was estimated as 63.7% (363/570) using QFTGIT at the cut-off point recommended
by the manufacturer (≥ 0.35 IU/ml IFN-g), while it was 74.9% (427/570) using a cut-off point ≥ 0.1 IU/ml IFN-g. The
QFTGIT-based prevalence of LTBI was not significantly associated with the gender or age of the study participants.
However, the prevalence of LTBI was 31.2% (183/587) using TST at a cut-off point ≥ 10 mm of skin indurations, and
it was higher in males than females (36.8% vs. 23.5%, X2 = 11.76; p < 0.001). There was poor agreement between
the results of the tests (k = 0.098, 95% CI, 0.08 - 0.13). However, there was a positive trend between QFTGIT and
TST positivity (X2 = 96.76, P < 0.001). Furthermore, individuals with skin indurations ≥ 10 mm were 13.6 times more
likely to have positive results using QFTGIT than individuals with skin indurations of 0 mm (adjusted OR = 13.6;
95%CI, 7.5 to 24.7, p < 0.001).
Conclusions: There is currently no agreed gold standard for diagnosis of LTBI. However, the higher prevalence of
LTBI detected using QFTGIT rather than TST suggests that QFTGIT could be used for epidemiological studies
concerning LTBI at the community level, even in a population unreactive to TST. Further studies of adults and
children will be required to assess the effects of factors such as malnutrition, non-tuberculosis mycobacterial
infections, HIV and parasitic infections on the performance of QFTGIT.
Background
Tuberculosis (TB) is one of the major public health problems in sub-Saharan Africa and Asia [1]. Globally, it is
responsible for approximately two million deaths, and
eight million new cases are reported each year; approximately 80% of all new cases occur in the 22 countries
with a high burden of TB [2]. Furthermore, it is estimated
* Correspondence: dlegessem@yahoo.com
1
Aklilu Lemma Institute of Pathobiology, Addis Ababa University, Addis
Ababa, Ethiopia
Full list of author information is available at the end of the article
that one third of the world’s population has LTBI [3].
This global prevalence of LTBI was estimated predominantly on the basis of data obtained from the tuberculin
skin test (TST) survey. However, TST has several limitations including a high rate of false-positive results among
individuals vaccinated with bacille Calmette-Guérin
(BCG) or exposed to non-tuberculosis mycobacteria
[4,5], and a high rate of false-negative results among
immune-suppressed individuals [6]. Therefore, uncertainty remains concerning the accuracy and reliability of
this previously estimated global prevalence of LTBI [7].
© 2011 Legesse et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
Legesse et al. BMC Infectious Diseases 2011, 11:89
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Recently, T-cell-based IFN-g release assays (IGRAs)
have been developed and approved for the diagnosis of
LTBI [8], and one study demonstrated that IGRAs have
a higher specificity than TST as the results are not
affected by the BCG status of the subject [9]. In addition, these tests are affected less by anergy than TST
[10], although their sensitivities are questioned by some
studies [11,12]. IGRAs have limitations including the
inability to differentiate between active TB and past
infection [13], and concerns regarding the validity of the
present cut-off values recommended by the manufacturers [14]. However, regardless of their limitations, it is
believed that IGRAs could improve existing information
about the global epidemiology of LTBI [15], but the
majority of studies concerning LTBI and IGRAs have
been limited to patients with active TB or health care
workers and refugees [16-20]. Few studies have assessed
community-based prevalence of LTBI using IGRAs
[21,22 ].
Ethiopia is ranked 7th among the 22 countries with a
high-burden of TB and second in Africa [1]. It is
expected that there are a large number of reservoirs of
LTBI in Ethiopia, although there are no reliable data
and available information is based on the results of TST
surveys conducted several years ago [23,24]. Afar Region
is one of the main pastoral areas of Ethiopia, where TB
is a major public health problem [1,25], although the
extent of LTBI is unknown. In this study, the prevalence
of LTBI in the Afar pastoral community was assessed
using QFTGIT and TST.
provide a three ml blood sample, volunteered to be
injected with purified protein derivative (PPD), and gave
written consent.
Prior to data collection, a list of all the pastoral kebeles
(smallest administrative units) in the district was
obtained from the District Health Office. On the basis of
this list, nine pastoral kebeles were randomly selected out
of a possible 14 kebeles. The selected kebeles were stratified into manageable villages (if there was more than one
village in the kebele) and a list of the head of households
of each kebele/village was noted. Using the number of
households in each kebele, the pre-estimated sample size
(713) was proportionally distributed between these
kebeles. The required number of households from each
kebele/village was selected using systematic random sampling from the lists of the heads of the households of that
kebele/village. From each of the selected households, one
individual (male or female) was systematically selected
and requested to participate in the study. In the event of
refusal, the next household on the list was asked to participate. Participants were requested to attend a respective
health post or central place of each Kebele for clinical
examination and a skin test and to provide a blood sample. Each study participant underwent a clinical and physical examination and was interviewed to ascertain any
previous history of TB or contact with TB patients, and
to investigate the presence of other acute or chronic illness, using a questionnaire. Information concerning
socio-demographic characteristics of the participants was
included in the questionnaire.
Methods
QuantiFERON-TB Gold In-Tube assay
Study area and population
The QFTGIT assay was performed according to the
manufacturer’s instructions (QuantiFERON-TB Gold InTube, Cellestis Ltd., Carnegie, Australia). Briefly, a
venous blood sample (1 ml) was collected from each
individual into three tubes (one containing TB-specific
antigens, one containing mitogen and a nil tube). The
samples were transported to Awash Health Centre
within 4-6 h of collection and incubated for 24 h at
37°C. The samples were centrifuged at 3000 × rcf (relative centrifugal force) for 10 min, and the plasma was
collected and stored at 4°C until the IFN-g assay was
performed using ELISA kits provided with the TB-Gold
tube. The optical density (OD) of each test was read
using a 450 nm filter with a 620 nm reference filter,
using an ELISA plate reader. The results were interpreted as positive, negative or indeterminate on the
basis of the manufacturer’s recommended cut-off value
(IFN-g ≥ 0.35 IU/ml) using QFTGIT analysis software
developed by the company.
Prior to its utilization for the present epidemiological
study, the performance of QFTGIT in the Afar Region
was evaluated in patients clinically suspected of having
A community-based cross-sectional study was conducted in the Amibara District of the Afar Region,
north-east Ethiopia, between April and June 2010. Amibara District is located in the Middle Awash valley
~260 km to the north-east of Addis Ababa, the capital
city of Ethiopia. The study area has been described in
detail elsewhere [26].
Sample size determination and data collection
There was no previous information concerning the general prevalence of LTBI in the Afar Region, or in the
Amibara District specifically. From the assumption that
50% of the study participants would have LTBI, and the
following targets: a 95% confidence level, a 5% degree of
accuracy, a design effect of 1.5, 10% compensation for
non-respondents, 10% compensation for exclusion during screening and 15% compensation for those participants lost to follow up; it was hoped to enrol a total of
713 study participants. Individuals were considered eligible for participation if they were apparently healthy,
aged over 18 years, not pregnant (females), able to
Legesse et al. BMC Infectious Diseases 2011, 11:89
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active pulmonary TB (PTB) and in healthy control subjects using a different study design and different study
participants [27]. The QFTGIT results were interpreted
using the population-specific cut-off point (IFN-g ≥ 0.1
IU/ml), which was estimated by receiver operator characteristic (ROC) curve analysis [27] prior to the implementation of the study.
Tuberculin skin test
Immediately following blood collection from the right
hand, 0.1 ml (2T.U/0.1 ml) tuberculin PPD RT23
(Statens Serum Institute, Copenhagen, Denmark) was
administrated intradermally in the middle of the left
forearm by an experienced nurse. The diameter of the
indurations was measured transversely after 48-72 h
using a ball-point pen and flexible plastic ruler [28].
Diameters of skin indurations ≥ 10 mm were considered positive, while diameters of 0 mm were considered non-responsive [29,30].
Ethical considerations
The study protocol was approved by the Ethical Clearance Committee of the Aklilu Lemma Institute of
Pathobiology (ALIPB), Addis Ababa University, and by
the Regional Committee for Medical Research Ethics of
Southern Norway. The aim of the study was explained
to each of the study participants and written consent
was obtained. Blood sample collection and tuberculin
PPD injections were carried out under aseptic conditions by well experienced technicians and nurses,
respectively. Individuals who presented with signs/symptoms of active TB or other diseases were advised to
attend the nearest health centre. Similarly, individuals
whose TST indurations were >10 mm and/or tested
positive for LTBI using QFTGIT were advised to consult
the nearest health facilities lest they develop signs/symptoms suggestive of active TB.
Data analysis
Data were entered into EpiData Software v.3.1 and analyzed using Stata version 11. Frequencies and percentages were used to summarise socio-demographic
characteristics and the prevalence of LTBI as diagnosed
using QFTGIT and TST. The prevalence of LTBI was
estimated by dividing the number of participants with
positive TST or QFTGIT results by the total number of
study participants who had undergone the TST or
QFTGIT test. The combined prevalence of LTBI was
estimated by dividing the number of participants who
tested positive using TST and QFTGIT by the total
number of study participants who had test results for
both TST and QFTGIT. Pearson’s chi-square was used
to compare the proportions of target outcomes. Univariable logistic regression analysis was performed to assess
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the association between LTBI and background characteristics of study participants including age, sex, and history of BCG or close contact with TB patients.
Multivariable logistic regression analysis was used to
assess the effect of each of the independent variables
(such as gender, age, occupation and marital status) on
the outcome variable by adjusting each independent
variable for all other variables. A p-value less than 0.05
was considered statistically significant. Agreement
between the TST results (positive, negative or anergy)
and QFTGIT results (positive, negative or indeterminate) was assessed using Cohen’s Kappa (k) coefficient.
K values greater than 0.75, between 0.4 and 0.75, and
less than 0.4 were considered excellent, fair and poor
agreement, respectively [22].
Results
Study participants
A total of 723 study participants, comprising 41.1%
(297/723) females and 58.9% (426/723) males, were
invited to participate. Of these subjects, 71(9.8%) were
excluded owing to common signs/symptoms of active
TB or a previous history of TB treatment. Of the
remaining 652, 73 did not volunteer to provide blood
samples, and blood samples collected from nine individuals were not labelled correctly. Sixty-five (10%) subjects were lost to follow up for TST measurement. Five
hundred and eighty-seven (90.0%), 570 (87.4%) and 505
(77.5%) participants had complete data concerning TST,
QFTGIT and both tests, respectively. The ages of the
652 study participants ranged from 18 to 70 years, with
a median of 30 years. Socio-demographic characteristics
and baseline data for the 652 study participants are presented in Table 1.
Prevalence of tuberculosis infection
Table 2 demonstrates the prevalence of LTBI assessed
by TST and QFTGIT. The prevalence of LTBI was
31.2% (183/587) using TST at a cut-off point ≥ 10 mm,
and it was higher in males than females (36.8% vs.
23.5%, X2 = 11.76; P = 0.001). The prevalence of LTBI
was not significantly different across age categories (X2
= 3.13; P = 0.372), or between BCG vaccinated and
non-vaccinated individuals (37.3% vs. 30.0%; X2 = 2.09;
P = 0.148). The prevalence was comparable in individuals who reported a history of close contact with TB
patients and those who did not (40.5% vs. 30.6%; X2 =
1.61; P = 0.204). The prevalence of anergy was 48.0%
overall (282/587), but the proportion was higher in
females than in males (54.3% vs. 43.5%; X 2 = 6.59;
P = 0.010).
The prevalence of LTBI was 63.7% (363/570) using
QFTGIT at the cut-off point recommended by the manufacturer, while it was 74.9% (427/570) at the cut-off
Legesse et al. BMC Infectious Diseases 2011, 11:89
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Table 1 Socio-demographic characteristics of study
participants
Characteristic
Number (%)
Gender:
Female
Male
Age (years):
262 (40.2)
390 (59.8)
18-29
269 (41.3)
30-44
241(37.0)
45-60
133 (20.4)
61+
9 (1.4)
Marital status:
Married:
527 (80.8)
Unmarried:
Other:
100 (15.3)
25 (3.83)
Ethnicity
Afar
648 (99.4)
Other
4 (0.6)
Religion :
Muslim:
650 (99.7)
Other:
2 (0.3)
Occupation :
Pastoralist
522 (80.0)
Agro-pastoralist
104 (16.0)
Other
26 (4.0)
Educational status
Illiterate:
574 (88.0)
Literate:
78 (12.0)
BCG scar present :
Yes
No
113 (17.4)
538 (82.6)
History of contact with TB patient:
Yes
44 (6.8)
No
608 (93.3)
Table 2 Prevalence of LTBI by TST and QFTGIT test
Test and Cut-off point
Number (%)
TST (n = 587)
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point ≥ 0.1 IU/ml of IFN-g. Six (1.1%) subjects had indeterminate results. The proportion of infection did not
significantly differ between genders (male 64.3% vs.
female 62.7%; p = 0.908), across age categories (P =
0.363), or between those subjects who reported a history
of close contact with TB patients and those who did not
(78.1% vs. 62.8%; X2 = 3.19; P = 0.203). The proportion
of infection was higher in BCG vaccinated subjects than
in non-vaccinated individuals (76.0% vs. 61.2%, X 2 =
9.83, P = 0.007). Among the 505 subjects who had TST
and QFTGIT test results, 168 (33.3%) tested positive
using TST and 326 (64.6%) were positive using QFTGIT
(X 2 = 70.8; P < 0.001) (Table 2). When subjects with
anergy and indeterminate results were excluded from
the analysis, the prevalence of LTBI was 61.6% using
TST and 83.8% using QFTGIT.
Agreement between TST and QFTGIT for the diagnosis of
LTBI
At the cut-off points ≥ 0.35 IU/ml of IFN-g for QFTGIT
and ≥ 10 mm for TST, 151 (29.9%) subjects tested positive using both assays, whereas 28 (5.5%) subjects were
negative in both tests (Table 3). Out of 232 subjects
who had skin indurations of 0 mm, 99 (42.7%) tested
positive using QFTGIT. The percentage agreement
between the overall results (positive, negative and indeterminate/anergy) of the tests (36.04%) was poor (k =
0.098; 95% CI; 0.08 - 0.13)). There was also poor agreement (66.05%) between the results of the tests when
only positive and negative results (excluding indeterminate and anergy) were compared (k = 0.195; 95% CI;
0.09 - 0.30). When the cut-off point for positive TST
was reduced to ≥ 5 mm of skin indurations, 207 (41.0%)
subjects were positive in both tests and six (1.2%) were
negative (Table 4). The percentages of agreement
between the results (positive, negative and indeterminate/anergy) of the tests were 42.77% (k = 0.136; 95%
CI; 0.11- 0.15) and 78.60% (k = 0.058, 95% CI, -0.07 0.19) when positive and negative results were considered, respectively.
There was a positive trend between the positivity of
QFTGIT and TST (X2 for trend = 96.76; P < 0.001). Figure 1 presents the trend of the proportion of positivity
by QFTGIT against the skin test indurations.
0 mm (anergy)
282 (48.0)
>0 mm & <5 mm
34 (5.8%)
≥ 5 mm & < 10 mm
88 (15.0)
≥ 10 mm
183 (31.2)
QFTGIT (n = 570)
≥ 0.35 IU/ml
Table 3 Discordant between TST at a cut-off point ≥ 10
mm of skin induration and QFTGIT at a cut-off point
≥ 0.35 IU/ml level of IFN-g
363 (63.7)
≥ 0.1 IU/ml
QFTGIT
427 (74.9)
TST
Positive
(≥ 10 mm)
QFTGIT &TST (n = 505)
Negative
(< 10 mm)
Anergy (0 mm)
≥ 0.35 IU/ml & ≥ 10 mm
151(29.9)
Positive ( ≥ 0.35 IU/ml)
151
76
99
≥ 10 mm
168 (33.3)
Negative (< 0.35 IU/ml)
16
28
130
≥ 0.35 IU/ml
326 (64.6)
Indeterminate
1
1
3
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Table 4 Discordant between TST at the cut-off points ≥ 5
mm of skin indurations and QFTGIT at a cut-off point
≥ 0.35 IU/ml level of IFN-g
QFTGIT
TST
Positive
(≥ 5 mm)
Negative
(<5 mm)
Anergy (0 mm)
Positive ( ≥ 0.35 IU/ml)
207
20
99
Negative (< 0.35 IU/ml)
38
6
130
Indeterminate
2
0
3
Factors associated with positivity using TST and QFTGIT
Results from logistic regression analysis demonstrated
that males were more likely than females to have positivity for LTBI using TST (adjusted OR = 1.8; 95%CI;
1.2 to 2.7; P = 0.004; Table 5). High positivity for
QFTGIT was associated with being vaccinated for BCG
(adjusted OR = 2.2; 95%CI; 1.2 to 4.1; P = 0.014). Individuals who had skin indurations ≥ 10 mm were 13.6
times more likely to have positive results using QFTGIT
than individuals with skin indurations of 0 mm
(adjusted OR = 13.6; 95%CI; 7.5 to 24.7; P < 0.001).
100
90
Positivity by QFTGIT
80
70
60
50
40
30
20
10
0
0mm
> 0<10mm
10mm
Skin induration
Figure 1 The trend of proportion of positivity by QFTGIT
against the skin test indurations. Whole blood samples were
collected from study participants directly into tubes containing TBspecific antigens, mitogen and nil. The samples were incubated for
24 hours at 37°C, plasma was collected and IFNg assay was
performed using ELISA. The results were interpreted as positive,
negative or indeterminate on the basis of the manufacturer’s
recommended cut-off value (IFN-g ≥ 0.35 IU/ml) using QFTGIT
analysis software developed by the company. Tuberculin skin test
(TST) was also performed by administrating 0.1 ml (2T.U/0.1 ml)
tuberculin PPD RT23 (Statens Serum Institute, Copenhagen,
Denmark) intradermally in the middle of the left forearm. The
diameter of the indurations was measured transversely after 48-72 h
using a ball-point pen and flexible plastic ruler. The subjects were
categorized in to three groups based on the size of skin test
induration (0 mm, >0 < 10 mm and ≥ 10 mm) and the positivity by
QFTGIT was compared for the three groups.
Discussion
In this study, the prevalence of LTBI in the pastoral
community of Afar, one of the regions in Ethiopia with
a high TB notification rate [1], was assessed. The TST
results revealed a prevalence of 31.2% for LTBI among
study participants who had complete TST results. This
is comparable with the estimation that one-third of the
world’s population is infected with Mycobacterium
tuberculosis (Mtb) [3], and with the prevalence of LTBI
reported in India using TST [21]. However, it is lower
than the prevalence of LTBI reported in HIV-negative
adult factory workers from Ethiopia [31] and from adult
study participants in South Africa [32]. The discrepancy
between the results of the present study and those of
previous studies carried out in Ethiopia could be
explained by several factors including the study setting,
the BCG status or immune profile of the study participants, and other socio-demographic factors [23,31].
The QFTGIT test results demonstrated a high prevalence of LTBI compared with TST, similar to the findings of a study concerning TB prevalence among
healthy adults in a high endemic area of India [21].
A study conducted in a cohort of health care workers in
India demonstrated a similar prevalence of LTBI using
QFTGIT (40.1%) and TST (41.4%) [17]. However, a previous study that compared the diagnostic efficacy of the
old QuantiFERON-TB to that of TST in study participants from Ethiopia and Baltimore indicated that TST
would be more effective for diagnosing LTBI in Ethiopian individuals [33]. The results of a study conducted
in South Africa also suggested that TST was more effective in identifying LTBI than three generations of a
whole blood IFN-g assay in countries with a high TB
burden [22]. The discrepancy between the present study
and previous studies could be explained by several
factors including differences in the study setting, sociodemographic factors of the study participants, BCG status of individuals and the type of IGRAs test used. For
instance, in the study involving Ethiopian and American
individuals [33], the authors used PPD tuberculin
(Tubersol; Pasteur Merieux Connaught Laboratories)
and defined skin indurations of ≥ 5 mm as a positive
reaction in Ethiopian subjects, while the current study
defined skin indurations of ≥ 10 mm as positive for
LTBI [29].
There is evidence that IGRAs are more sensitive than
TST for detecting LTBI in various groups of immunecompromised individuals [10,34,35]. In the present
study, among the 232 study subjects who had no measurable skin indurations (0 mm), 99 (42.7%) tested positive using QFTGIT. Similarly, among 105 subjects who
had skin indurations < 10 mm, 72.4% tested positive
using QFTGIT. This indicates that the use of QFTGIT
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Table 5 Association of socio-demographic characteristics with TST and QFTGIT positivity
TST
Characteristic
COR (95%, CI)
QFTGIT
AOR (95%, CI)
COR (95%, CI)
AOR (95%, CI)
Gender:
Female
Reference
Reference
Reference
Reference
Male
1.9 (1.3 - 2.7)
1.8 (1.2- 2.7)
1.1(0.8 - 1.5)
1.2 (0.8 - 1.7)
Age (years):
18-29
Reference
Reference
Reference
Reference
30-44
1.1 (0.7- 1.6)
1.1 (0.7 - 1.7)
1.4 (0.9 - 2.0)
0.8 (0.5 - 1.4)
45-60
61+
0.7 (0.4 - 1.2)
1.7(0.5 - 6.5)
0.8 (0.4 - 1.3)
1.8 (0.4 - 7.1)
1.7 (1.1- 2.7)
2.4 (0.5 - 11.7)
1.3 (0.7 - 2.4)
1.4 (0.2 -8.2)
Married:
Reference
Reference
Reference
Reference
Unmarried:
1.3 (0.8 - 2.0)
0.9 (0.5 - 1.6)
0.5 (0.3 - 0.8)
0.5 (0.2 - 0.9)
Other:
0.5 (0.2- 1.4)
0.5 (0.2 - 1.6)
1.0 (0.4- 2.8)
1.6 (0.5 - 4.9)
Marital status:
Occupation :
Pastoralist
Reference
Reference
Reference
Reference
Agro-pastoralist
Other
1.2 (0.7 - 1.9)
2.1 (0.8 - 5.3)
0.9 (0.6 - 1.5)
1.4 (0.5 - 3.8)
1.4 (0.9 - 2.3)
1.3 (0.5- 3.3)
1.5 (0.9 - 2.6)
1.7 (0.6 - 4.7)
Educational status
Illiterate:
Reference
Reference
Reference
Reference
Literate:
2.2 (1.3 - 3.6)
1.7 (0.9 - 3.0)
1.0 (0.6 -1.7
1.1 (0.6 - 1.9)
No
Reference
Reference
Reference
Reference
Yes
1.4 (0.9 - 2.2)
1.4 (0.9 - 2.2)
2.2 (1.3 - 3.6)
2.2 (1.2 -4.1)
BCG scar present
Contact with TB pts
No
Reference
Reference
Reference
Reference
1.6 (0.8 - 3.1)
1.6 (0.8 - 3.3)
2.0 (0.9 - 4.7)
2.0 (0.8 - 4.9)
0 mm
Reference
Reference
> 0 < 10 mm
3.5 (2.1- 5.8)
3.5 (2.1- 5.8)
≥ 10 mm
12.1(6.8 - 21.6)
13.6 (7.5- 24.7)
Yes
TST
for the epidemiological study of LTBI is feasible, even in
populations unreactive to TST owing to diminishing
immune responses [10,34]. However, 16 subjects who
had skin indurations ≥ 10 mm (of whom only three had
a BCG scar) were negative using QFTGIT. This finding
is similar to a study carried out in South Africa, where a
considerable proportion of individuals who had skin
indurations ≥ 15 mm tested negative using QuantiFERON tests [22]. This was likely to be due to false
positive results using TST, as infection with nontuberculosis mycobacteria would cause an individual to
test positive [5]. It is reasonable to suppose that false
negative results using QFTGIT could occur as the present cut-off value recommended by the manufacturer is
thought to be too high, as suggested by this study and
others [14,36]. The possibility of false positive results
due to non-tuberculosis mycobacterial infections is
greater using TST than IGRAs, but it is possible that
the high positive results observed using QFTGIT in the
present study could be due, in part, to cross-reactivity of
specific antigens with non-tuberculosis mycobacterial
infections [37,38]. Therefore, further studies are required
to investigate if there are factors that could contribute
to the high positive results obtained in the present study
area using QFTGIT, other than infection with Mtb
complex.
There was poor agreement between the TST and
QFTGIT results. In the present study setting, where TB
is endemic [1,25], repeated exposure to Mtb and the
presence of more persistent memory T cells than effector cells would be expected, and therefore an increased
response to PPD [28,39,40]. However, the dynamics of
the immune response to Mtb infection are still unclear
[15]. Nevertheless, in the present study, the main cause
of disagreement was due to a high proportion of positive
results using QFTGIT and a high proportion of negative
results using TST, similar to the findings of a previous
study [21], but different to those reported by Kang et al.
[41]. The high prevalence of anergy using the TST and
the low prevalence of indeterminate results using
Legesse et al. BMC Infectious Diseases 2011, 11:89
http://www.biomedcentral.com/1471-2334/11/89
QFTGIT could contribute to the disagreement between
the two tests. Anergy to TST could occur because of
technical problems, malnutrition, other infectious agents
such as HIV/AIDS and parasitic infections, or the
absence of tuberculosis infection. However, the higher
proportion of anergy to TST in female study participants and the lower proportion of positive results using
QFTGIT in anergic individuals imply that anergy in the
present study is likely to have arisen from factors other
than technical problems related to TST. However, the
lower proportion of positivity observed in anergic individuals using QFTGIT indicates that skin indurations of 0
mm in most individuals are probably due to the absence
of infection with Mtb complex rather than anergy.
Therefore, further studies are required to elucidate the
role(s) of possible factors such as malnutrition, HIV/
AIDS and other infectious agents associated with anergy
or false negative results using TST in the present study
area.
Despite the poor agreement between the two tests and
the greater effectiveness of QFTGIT than TST in detecting LTBI, there was a trend towards an increased proportion of positivity using QFTGIT with increasing TST
reactivity (Figure 1); this corroborates the findings of a
study carried out by Adetifa et al. [42]. This trend
between the two tests may suggest that both the specific
antigens and PPD stimulate the same T cell population
and measure the same response, contradicting evidence
that IGRAs measure the response of effector T cells and
that the TST measures memory T-cell responses
[28,39,40]. Furthermore, like TST, IGRAs are susceptible
to factors affecting host immune responses, although the
degree of susceptibility differs between the two tests.
In this study, the prevalence of LTBI was higher in
men than women, as estimated using TST. The finding
corroborates the result of a study carried out in South
Africa [32], but the low prevalence of LTBI in women
could be due to the high prevalence of anergy observed
in female study participants. However, the prevalence
was not altered by gender or by age groups when estimated using the QFTGIT test. This could be additional
evidence that anergy contributes to the low prevalence
of LTBI in women. Studies have demonstrated a high
risk of Mtb infection among individuals in close contact
with infectious TB cases [43,44]. However, in the present study, the positivity of TST or QFTGIT was not
affected by a previous history of close contact with TB
patients, and this is in agreement with the findings of a
study carried out in South Africa [22]. This could be
explained by the high endemic nature of TB, hampering
the association between the prevalence of LTBI and TB
case contact. In other words, in areas highly endemic
for TB, individuals can acquire infection not only
through close contact with TB patients, but also from
Page 7 of 9
the community. Previous BCG vaccination was not associated with positive TST. This finding is consistent with
the results from study carried out in other highly endemic country [22], and differs from the results of studies
carried out in countries with low TB prevalence [45,46].
The lack of an effect of BCG on the results obtained
using TST implies that TST can be of use for the
screening of LTBI in highly endemic areas, particularly
in adult individuals, when the effect of the BCG has
waned [47]. However, the high positivity using QFTGIT
was associated with the presence of a BCG scar. This
finding is consistent with the findings of Tsiouris et al.
[48], who evaluated the use of IGRAs for the diagnosis
of TB using QFTGIT in an area highly endemic for TB.
These findings warrant further investigation as IGRAs
are not supposed to be confounded by prior BCG
vaccination.
Conclusions
QFTGIT demonstrated a higher prevalence of LTBI
than TST in the present study area. Unlike TST, the
QFTGIT test results were not affected by gender or the
age of study participants. This suggests that QFTGIT
could be used for the epidemiological study of LTBI at a
community level, even in a population unreactive to
TST, although there is currently no agreed gold standard for the diagnosis of LTBI. Nevertheless, further
studies in adults and children are required to assess the
effect of factors such as malnutrition, non-tuberculosis
mycobacterial infections, HIV and parasitic infections
on the performance of the test. The high prevalence of
LTBI observed in the present study area could have
implications for the rising prevalence of active TB in the
area, particularly when favourable conditions including
malnutrition and HIV/AIDS coexist. Therefore, any possible intervention that minimizes the risk of progression
of LTBI to active TB is important and its uses should be
thoroughly investigated.
Acknowledgements
We are grateful to study participants, Afar Regional and Amibara District
Health Bureau, Awash and Meleka Werer Health Centres. We would like to
thank nurse Gezahegn Getachew staff of Melka Werer Health Centre for his
technical assistance in TST, physical and clinical examinations of the study
participants. We would like to thank Mr. Sisay Dessie, Mr. Girma Kebede and
Ms Kokobe Gebre-Michael for their technical assistance. We would also like
to thank staff of Armauer Hansen Research Institute for their cooperation
during laboratory work.
Author details
1
Aklilu Lemma Institute of Pathobiology, Addis Ababa University, Addis
Ababa, Ethiopia. 2Faculty of Veterinary Medicine, Addis Ababa University,
Beshofitu, Ethiopia. 3Department of General Practice and Community
Medicine, Institute for Health and Society, University of Oslo, Oslo, Norway.
Authors’ contributions
ML designed the study, participated in data collection, analysis and drafted
the manuscript. GA, participated in study design, data collection, analysis
Legesse et al. BMC Infectious Diseases 2011, 11:89
http://www.biomedcentral.com/1471-2334/11/89
and write-up. GM participated in study design, data collection and write-up.
GMD, participated in study design, data analysis/interpretation and write-up.
GB involved in study design and write-up of manuscript. FA involved in
study design, data analysis and write-up of the manuscript and critically
revised the manuscript. All authors read and approved the final manuscript.
ML is the guarantor of the paper.
Competing interests
The authors declare that they have no competing interests.
The study was financially supported by Norwegian Programme for
Development, Research and Education, NUFU (NUFU PRO-2007/10198).
Received: 19 November 2010 Accepted: 9 April 2011
Published: 9 April 2011
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Pre-publication history
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Cite this article as: Legesse et al.: Community-based cross-sectional
survey of latent tuberculosis infection in Afar pastoralists, Ethiopia,
using QuantiFERON-TB Gold In-Tube and tuberculin skin test. BMC
Infectious Diseases 2011 11:89.
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