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Epidemiological
A history of being born or residing in a TB-endemic area or being a close contact of a patient with active pulmonary TB places the patient at risk of LTBI. A list of countries where TB is endemic can be accessed on the WHO website. 1 It is important to note that TB was endemic in Australia before the 1940s, so anyone alive during that period may have
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CC / Sanjay Mukhopadhyay: bit. ly / 3SHd0ep |
A comparison of TST and IGRA appears in table 1.
The author’ s approach to making a diagnosis of LTBI using a combination of epidemiological, radiological and immunological factors is shown in figure 7.
EXCLUSION OF ACTIVE TB
EXCLUSION of active TB is impor-
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been exposed. 14 |
tant before embarking upon a man- |
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These epidemiological risk fac- |
agement plan for LTBI as the features |
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tors can be, and often are, tem- |
and treatment are different( see fig- |
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porally distant. For example, an |
ure 2). |
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elderly person who spent most of |
History and examination are |
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their life in Australia but spent their |
important in screening for active TB. |
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childhood years in India is at risk |
Specifically ask the patient if they |
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of LTBI. |
have experienced fever, night sweats, |
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Within Australia, there are dis- |
weight loss, cough, haemoptysis or |
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proportionately high rates of TB in |
the development of new masses. |
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the NT: a 2021 report showed that |
Extrapulmonary TB is more frequent |
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44 % of TB in the NT occurred in |
in immunocompromised patients, so |
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Indigenous Australians. 15 |
it is important to have a high index of |
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Radiological
Chest imaging may show evidence
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suspicion that any new symptom or sign might be TB. 25
Findings on chest imaging can
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of previous TB infection. In immunocompromised patients, the author orders a chest X-ray in addition to a tuberculin skin test( TST) or an |
Figure 1. Non-necrotising granuloma in a lymph node in the neck. |
help distinguish LTBI from active TB. If there are symptoms suggestive of TB and abnormalities are present on chest X-ray, order a CT scan |
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interferon-gamma release assay |
to further investigate. If features on |
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( IGRA) for three reasons: first, the |
Gold( Qiagen, Germantown, US) |
lymphocytes, reduced lymphocyte |
example, in patients with HIV, 1.5- |
the CT scan are suspicious for TB, |
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X-ray may reveal signs of LTBI in |
is currently the only commer- |
activity from improper specimen |
16 % have an indeterminate IGRA |
obtain specimens for mycobacterial |
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patients with a false-negative TST |
cially available assay in Australia. |
handling, incorrect filling or mix- |
result. Furthermore, the risk of an |
staining, culture and PCR. If sputum |
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or IGRA; second, to stratify the risk |
In immunocompromised patients, |
ing of the mitogen tube or an inabil- |
indeterminate result is increased in |
cannot be obtained, referral for fur- |
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of reactivation of LTBI; and third, to |
the key limitations of IGRAs are |
ity of the patient’ s lymphocytes to |
patients with lower CD4 + lympho- |
ther tests, such as bronchoscopy, is |
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exclude active pulmonary TB. 16
About 15 % of patients with LTBI have abnormalities on chest X-ray. 16 These changes may include calcified
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false-negative and indeterminate results. 12, 22 In an IGRA assay, a mitogen is used as the positive control. This mitogen is a powerful |
generate interferon gamma. If the mitogen does not produce adequate amounts of interferon gamma, this represents failure of the positive |
12, 23, 24
cyte counts.
There is no gold-standard immunological test for the diagnosis of LTBI; TST and IGRA each have
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recommended.
The author recommends against initiating treatment for LTBI until active TB has been investigated and
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nodules, fibrotic lesions and pleu- |
non-specific T-cell stimulant and |
control. The most frequent reason |
their own advantages and disad- |
excluded. |
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ral thickening; however, none of these are pathognomonic of TB. 16 A fibrotic scar 2cm2 or greater and / or a calcified nodule 1.5mm or greater have been shown to be significantly associated with LTBI. 17 Patients with LTBI who have abnormali- |
should produce interferon gamma when combined with the patient’ s serum. If the mitogen response is low, the test is considered indeterminate. An indeterminate response can occur because of insufficient |
for mitogen failure is deficiency of T-cell function.
The mitogen itself may be a barometer of the net state of immunocompromise and, in itself, may have prognostic significance. 23 For
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vantages. Recommendations vary between guidelines, with older guidelines recommending the use of TST, while more recent guidelines endorse the use of IGRA. 7 The author uses IGRA to screen for LTBI. |
ASSESSING THE RISK OF LTBI REACTIVATION
IMMUNOCOMPETENT patients
with LTBI have approximately a
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ties on chest X-ray are more likely to reactivate than those without abnormalities. 18
The author does not routinely perform CT scans when screening patients for LTBI or in patients with positive TST / IGRA but uses this modality to differentiate between active TB and LTBI.
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Figure created in BioRENDER. com |
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In immunocompromised |
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patients, it can be clinically difficult |
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to exclude active TB because |
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of the multiple comorbidities and |
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the possible associated lung disease |
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. Features on a chest CT that |
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are more likely to represent active |
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TB compared with LTBI include the |
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presence of tree-in-bud, non-calcified |
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nodules, lymph node enlargement |
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and splenic calcified nodules. 19 Figure 6 demonstrates CT chest |
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findings of cavitating lesions of pulmonary |
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TB. |