Abstract
BackgroundInterferon gamma release assays (IGRA) are replacing the tuberculin skin test (TST) as a diagnostic tool for Mycobacterium tuberculosis infection. However research into the test's performance in the high HIV-TB burden setting is scarce. This study aimed to define the sensitivity of an IGRA, QuantiFERON-TB® Gold In-Tube (QGIT), in adult Zambian patients with active smear-positive tuberculosis. Secondary outcomes focussed on the effect of HIV on the test's performance.Principal FindingsPatients attending government health clinics were recruited within 1 month of starting treatment for TB. Subjects were tested with QGIT and TST. T lymphocyte counts were estimated (CD3+, CD4+, CD8+). QGIT was performed for 112 subjects. 83/112 were QGIT positive giving an overall sensitivity of 74% [95%CI: 66,82]. A marked decrease in sensitivity was observed in HIV positive patients with 37/59 (63%) being QGIT positive compared to 31/37 (84%) HIV negative patients [chi2 p = 0.033]. Low CD4+ count was associated with increases in both indeterminate and false-negative results. Low CD4+ count in combination with high/normal CD8+ count was associated with false-negative results. TST was recorded for 92 patients, 62/92 were positive, giving a sensitivity of 67% [95%CI: 58,77]. Although there was little difference in the overall sensitivities, agreement between TST and QGIT was poor.ConclusionsQGIT was technically feasible with results in HIV negative subjects comparable to those achieved elsewhere. However, where under-treated HIV is prevalent, an increased proportion of both indeterminate and false-negative QGIT results can be expected in patients with active TB. The implications of this for the diagnosis of LTBI by QGIT is unclear. The diagnostic and prognostic relevance of IGRAs in high burden settings needs to be better characterised.
Highlights
Infection with Mycobacterium tuberculosis (MTB) results in nine million new cases of tuberculosis disease (TB) and just under two million deaths a year [1] The vast majority of TB is found in South-East Asia, Africa and the western Pacific regions
Where under-treated HIV is prevalent, an increased proportion of both indeterminate and false-negative QuantiFERON-TBH Gold In-Tube (QGIT) results can be expected in patients with active TB
In subjects recorded as being HIV negative, 31/37 were QGIT positive giving a sensitivity of 84% (95%CI: 71,96)
Summary
Infection with Mycobacterium tuberculosis (MTB) results in nine million new cases of tuberculosis disease (TB) and just under two million deaths a year [1] The vast majority of TB is found in South-East Asia, Africa and the western Pacific regions It is only in the African region and in eastern and southern Africa that the incidence continues to rise. The natural history of TB is altered in the presence of HIV infection The risk of both primary progressive disease and reactivation of latent tuberculosis infection (LTBI) is increased, resulting in a high incidence of active disease in this population [4,5]. Secondary outcomes focussed on the effect of HIV on the test’s performance
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