QuantiFERON-TB Gold and the TST are both useful for latent tuberculosis infection screening in autoimmune diseases

One-third of the world's population is believed to harbour a latent tuberculosis infection (LTBI) 1, defined as the presence of quiescent mycobacteria. Approximately, 10% of immunocompetent persons with LTBI will ever develop tuberculosis (TB). In countries with a low incidence of TB, such as Italy, the tracing and targeted treatment of LTBI subjects constitutes a major pillar of TB control 2.

Ageing of the population and the increased use of immunosuppressive treatments represent risk factors for progression to active TB, and highlight the need for additional strategies to maintain and improve TB control 3.

Tumour necrosis factor (TNF)-α inhibitors are approved for the treatment of immunomediated inflammatory diseases (IMID) and provide clinical benefit. However, treatment with TNF-α inhibitors showed an increased risk of serious life-threatening infections, including reactivation of TB infection in patients previously exposed to mycobacteria 4. Thus, screening for active TB and LTBI has become mandatory prior to the initiation of TNF-α inhibitor therapy 5, 6. However, to date, no agreement exists on the best strategy for detecting LTBI in this population, and the tuberculin skin test (TST) remains largely used 7–10.

The TST attempts to measure cell-mediated immunity in the form of a delayed-type hypersensitivity response to purified protein derivative (PPD). However, the TST lacks specificity in populations with high bacillus Calmette–Guérin (BCG) coverage and nontuberculous mycobacteria (NTM) exposure. Moreover, the TST may have a higher probability to have false-negative results in IMID patients than in the general population because of the immune disregulation linked to the disease itself or due to immunosuppressive drugs use 11.

Recently, two new assays have been developed to identify LTBI, QuantiFERON®-TB Gold in-tube (QFT-GIT; Cellestis Limited, Carnegie, Australia) and T-SPOT TB (Oxford Immunotec, Abingdon, UK). They measure antigen-specific interferon (IFN)-γ secretion by peripheral blood CD4+ T-lymphocytes in response to in vitro stimulation with early secreted antigenic target-6 and culture filtrate protein 10 12, 13, which are more specific for the detection of a Mycobacterium tuberculosis infection than PPD, as they are not shared with BCG substrains or with most environmental mycobacteria 13, 14.

Several unresolved issues remain on the potential clinical use of IFN-γ release assays (IGRAs). One area of controversy is whether they can be used in immunosuppressed patients such as IMID subjects, in addition or as an alternative to the TST. The lack of a gold standard for LTBI diagnosis has hampered the assessment of diagnostic accuracy of IGRAs, and the comparison of these new tests with the TST. Therefore, it has been suggested that tests for LTBI diagnosis can be validated by analysing the association of positive results with risk factors for TB, and different tests can be compared by evaluating differences in their association with risk factors 15, 16. The current authors used this approach to compare the performance of QFT-GIT and the TST for the detection of LTBI among IMID subjects coming from a low endemic TB country. Moreover, the impact of different classes of drugs on the response to these tests was analysed.

METHODS

RESULTS

Studied population

A total of 398 consecutive subjects with IMID were enrolled (fig. 1⇓). Indeterminate results to QFT-GIT were found in five (1.2%) out of 398 subjects. No association was found between risk factors for TB and immune suppression (data not shown) and these samples were excluded for further analysis.

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Fig. 1—

Flow diagram of the study population. QFT: QuantiFERON®-TB Gold in-tube (Cellestis Limited, Carnegie, Australia); TST: tuberculin skin testing; +: positive; -: negative.

Demographic and clinical features of the 393 remaining subjects are reported in table 1⇓. The present population was characterised by a high median age (54 yrs), a low prevalence (4.1%) of BCG vaccination and a small number (4.6%) of subjects coming from high endemic TB countries. None of the enrolled subjects were found to have active TB.

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Table 1—

Associations of demographic, epidemiological and clinical characteristics and therapy with QuantiFERON®-TB Gold in-tube (QFT-GIT)- and tuberculin skin test (TST)-positive results in subjects with rheumatic diseases: univariate analysis

DISCUSSION

Immunosuppressed subjects are one of the most important targets for the screening of LTBI because of the high risk of progression to active TB. Despite its widely recognised limitations, the TST remains the standard method for identifying TB infection before TNF-α inhibitor therapy, due to the lack of sufficient data on IGRA use in this population 7–10. To date, there are limited data describing performance of IGRA for detecting LTBI in IMID subjects awaiting TNF-α inhibitor therapy. Of them only seven studies reported direct comparison with the TST (table 5⇓) 21–30. Moreover, the published results were not easily comparable, due to the small number of enrolled subjects, a high rate of BCG vaccination in the studied population, the nonhomogeneous type of underlying diseases considered and the kind of IGRA used (table 5⇓).

To the present authors' knowledge, the current study is the largest that has compared the results of an IGRA to the TST for TB screening in IMID subjects coming from a low endemic TB country (only 4.6% subjects coming from a high TB prevalence country) and with a low (4.1%) BCG vaccination rate.

The first relevant finding of the current study was a high positive rate of LTBI for both TST and QFT-GIT testing (18.8 and 13%, respectively). This result was probably due to the high median age of the IMID population (55.7% >50 yrs), who were most likely exposed to the M. tuberculosis infection at a young age when Italy was a high TB prevalence country. Therefore, the need for TB screening was confirmed in IMID subjects even in a low TB prevalence country such as Italy.

The present study showed a lower rate (1.2%) of indeterminate results at QFT-GIT testing compared with other previously published studies utilising the same test (QFT-GIT) 21, 23, 24, 27, where the rate of indeterminate results ranged between 1.5 and 10.3% (table 5⇑). The current finding confirms the feasibility of QFT-GIT in the IMID population that have undergone LTBI screening and have been pre-treated with immunomodulating drugs (steroids, DMARDs and TNF-α inhibitors). In total, 79% of subjects in the present population had been pre-treated.

A fair concordance between QFT-GIT and the TST (87.5% with κ 0.55; p <0.0001; 95% CI 0.43–0.67) was observed, which is higher than reported in previous studies where concordance ranged from 51.7–82.8% (table 5⇑). This is probably due to the lower rate of BCG vaccinated subjects that led to a reduction in the rate of TST-positive/QFT-GIT-negative discordant results compared with the literature, with the exception of the study by Ponce de Leon et al. 27. The present interpretation is supported by multivariate analyses showing a significant association between TST-positive/IGRA-negative results and BCG vaccination status 23, 28. The current multivariate analysis confirmed this association, although the present population accounted for only 16 BCG vaccinated subjects. Therefore, these discordant results most likely represent TST associated to a TB infection. The alternative possibility of a past sensitisation to NTM antigens present in the TST and not in the QFT-GIT assay is difficult to ascertain.

Regarding the 13 (3.3%) TST-negative/QTF-GIT-positive discordant subjects it may be speculated that TST missed these LTBI diagnoses because of the high specificity of QFT-GIT for the M. tuberculosis infection. The two main reasons to explain this finding are as follows. 1) A two-step TST was not performed which would have maximised the TST sensitivity, but this strategy would have required four visits instead of two for every subject. 2) All TST-negative/QTF-GIT-positive discordant subjects were undergoing treatment with immunosuppresive drugs and six out of 13 received TNF-α inhibitors, which may explain the negative results of TST, considering the negative impact of steroids and TNF-α inhibitors on M. tuberculosis-specific responses 31 and on the TST performance (table 2⇑). Moreover no association between any LTBI test and kind of IMID were found (table 1⇑). The present results confirm those reported by Sellam et al. 22 and are consistent with the lack of a relationship with TB infection risk.

Regarding the association of TST and QFT-GIT with risk factors for TB infection, the current data showed a good association for both tests with known risk factors for TB in Italy, such as older age and a history of close contacts of sputum-positive TB patients. However, the TST seems to be more strictly associated with TB infection, considering that only this test was significantly associated with other known risk factors, such as origin from countries with a high endemic of TB and radiological lesions suggestive of TB history.

Establishing sensitivity and specificity for the M. tuberculosis infection is difficult due to the absence of a gold standard for the diagnosis of LTBI. Thus, an approach to index test validation that goes beyond the diagnostic accuracy paradigm is needed for an alternative evaluation process in the absence of a gold standard. In some studies, the association of positive results with risk factors for TB has been analysed, and different tests have been compared by evaluating differences in their association with risk factors 15, 16, 20. The present authors used this approach to assess the accuracy of the TST and QFT-GIT in detecting LTBI, comparing current results to a gradient of risk factors for LTBI selected a priori according to their prognostic relevance. The current findings showed a clear-cut trend for an increase in odds with a growing number of risk factors for LTBI, confirming that both, the TST and QFT-GIT are predictive of LTBI without a difference in the estimation ability between the two tests (table 3⇑).

Based on previous studies that showed a reduced performance of the TST in immunosuppressed rheumatoid arthritis subjects 12, 27, the current authors analysed the impact of different classes of drugs on the response to TST and QFT-GIT tests. By multivariate analysis, steroids were found to significantly affect both QFT-GIT- and TST-positive outcome (OR 0.4 and 0.3, respectively), while treatment including TNF-α inhibitors significantly decreased only the positive outcome of the TST (OR 0.3). This result disagrees with that reported by Matulis et al. 23. However, Matulis et al. 23 did not evaluate the association with the TST results (TST was not performed simultaneously with QFT-GIT). Matulis et al. 23 explained their result with the TNF-α inhibitors that decrease the activation of CD4+ T-lymphocytes by mycobacterial antigens. However, the present results seem to be more consistent with the crucial role, which TNF-α plays in recruiting macrophages in tuberculin-induced delayed-type hypersensitivity that should justify the reduced response to TST in the current population of whom 24.2% were already exposed to TNF-α inhibitors at the time of screening 31.

In conclusion, the present study confirms that QFT-GIT as a screening tool for LTBI among IMID subjects awaiting TNF-α inhibitor therapy is feasible, due to the low incidence of indeterminate results and the strict association with risk factors for TB for this test. However, the current findings do not support the recently published UK national guidelines on TB suggesting the use of IGRA as a confirmatory test in TST-positive subjects 32. Based on these data in a country with a low prevalence of BCG vaccination, such as Italy, this approach would have had the risk of missing subjects with LTBI. Conversely, the present data suggest the potential use of QFT-GIT as a primary test that may offer the advantage of being less impaired by the treatment with TNF-α inhibitors compared with the TST.

In this context, large-scale cohort studies are needed to estimate risk for progression to active disease in persons who have had positive results to the tuberculin skin test and/or interferon-γ release assay, with a particular interest to the risk of disease in persons with discordant reactions. Until further data are available on the implication of discordant tuberculin skin test/QuantiFERON®-TB in-tube results, a strategy of simultaneous tuberculin skin test and QuantiFERON®-TB in-tube testing in populations with a low prevalence of bacille Calmette–Guérin vaccination should maximise the sensitivity for latent tuberculosis diagnosis.

Statement of interest

Statements of interest for A. Bartoloni, S. Vicidomini, C. Fiorelli, A. Mantella and D. Goletti can be found at www.erj.ersjournals.com/misc/statements.shtml