5 курс / Пульмонология и фтизиатрия / Clinical_Tuberculosis_Friedman_Lloyd_N_,_Dedicoat

Figure 8.52  Multiple thin-walled cystic spaces that are the residua of a previous Pneumocystis carinii infection. These may rupture to produce pneumothoraces. The thin walls are fairly smooth and should not be confused with the cavities of mycobacterial disease.

Figure 8.53  Atypical distribution of P. carinii pneumonia occurs in patients who are on inhaled pentamidine. This appearance may mimic tuberculosis due to its upper lobe distribution, but the history of prophylactic pentamidine should raise the possibility that this is P. carinii pneumonia.

Introduction

Currently available immunodiagnostics Latent tuberculosis

The predictive power of IGRAs for progression to active TB The unmet clinical need in LTBI diagnostics

References

INTRODUCTION

A quarter of the world’s population is estimated to be infected with Mycobacterium tuberculosis (M.tb),1 approximately 1.7 billion people.

This provides a very large reservoir for future active tuberculosis. With the implementation of the World Health Organization (WHO) End TB strategy, aiming to reduce tuberculosis (TB) incidence by 90% by 2035, diagnosing and treating latent tuberculosis infection (LTBI) successfully is of paramount importance.

Whether someone develops active TB, LTBI, or clears the infection following exposure is a complex interaction of bacterial, environmental, exposure, and host factors.

Approximately 10% of those with LTBI will progress to active TB. The risk of reactivation and subsequent disease is significantly increased in those with recently acquired LTBI (the majority within the first 2 years post infection),2 the prevalence of which is estimated to be 0.8% of the global population, amounting to 55.5 million individuals currently at high risk of TB disease.1 Risk factors for progression from LTBI to active TB are all mediated via immunodeficiency and are outlined in Table 9.1.

Current immunodiagnostic tests for TB encompass humoral and cell-mediated immune-based tests, the widely available tuberculin skin test (TST) and interferon-gamma (IFN-γ) release

Table 9.1  Risk factors for progression of LTBI to active TB

153

153

154

157

161

166

Table 9.2  Classes of immunodiagnostic tools available for the diagnosis of TB

assays (IGRAs) as well as other measures of host response (transcriptomics and proteomics) (see Table 9.2).

In the case of LTBI, there are no localizing symptoms, signs, or tissue pathology to target, therefore diagnosis can be challenging. It is in this clinical setting the host response can be used as a marker of infection.

CURRENTLY AVAILABLE

IMMUNODIAGNOSTICS

Cell-mediated immunity-based tests of TB infection

Cell-mediated immunity (CMI) in the host response to M.tb is of central importance in the diagnosis of M.tb. All the commercially

available platforms exploit the fact that M.tb infection, even at very low bacterial burdens as in LTBI, evokes a strong CMI response to M.tb antigens. TSTs are the oldest CMI-based tests in medicine, with IGRAs being the first antigen-specific T-cell-based diagnostic tests. More recently, a low cost, highly specific C-TB skin test has shown promise.

TST

The TST induces a type IV hypersensitivity reaction when the purified protein derivative (PPD) of several M.tb strains is injected into the sub-dermis of a previously exposed individual (exposure to Bacillus Calmette–Guérin [BCG] vaccine, M.tb, or environmental mycobacteria). Along with the chest x-ray, TST was the mainstay of LTBI diagnosis during the twentieth century and is still widely employed today, especially in high-burden regions. The TST has several disadvantages, which will be discussed later on in this chapter. However, despite its limitations, the TST remains widely used, especially in low-income countries with a high-TB burden. Its relative cheapness and simplicity, combined with years of experience in its use continue to make it an attractive option. Its continued clinical utility has recently been clearly exemplified in a large study from Africa showing the clinical benefit of targeting chemoprophylaxis based on TST results in a population with high burdens of both TB and human immunodeficiency virus (HIV) infection.3

IGRA

IGRAs are blood tests, the immunological basis of which involves the ex vivo release of the key anti-M.tb cytokine IFN-γ. Purified white blood cells from the test participant are incubated overnight in a laboratory with antigens from M.tb that are not found in the BCG vaccine. IGRAs yield a quantitative readout of IFN-γ secretion, which has a valid basis in our understanding of the immune response to M.tb. Much research over the last decade has shown that M.tb antigen-specific T-cell-derived IFN-γ is a valid and clinically useful biomarker of M.tb infection. Put simply, if the individual has been infected with M.tb then T-cells specific for M.tb antigens will recognize those antigens upon re-encounter ex vivo and secrete IFN-γ. Commercially, this is measured using one of two platforms: enzyme-linked immunosorbent assay (ELISA), which measures IFN-γ concentration (QuantiFERON®-TB-Gold In-Tube [QFT-GIT] and more recently QuantiFERON-TB Gold Plus [QFT-Plus], Cellestis, Qiagen, NL), or ELISpot, which measures IFN-γ spot-forming cells (T-Spot.TB, Oxford Immunotec, Abingdon, UK) (see Table 9.3 for summary).

Over the last 3–4 years there have been updates to the commercially available immunodiagnostic tests; improvement has been made upon the traditional ELISA and ELISpot methods by allowing the measurement of multiple analytes in one patient sample. The QFT-GIT has been replaced by QuantiFERON Gold Plus (QFT-Plus). This test has two tubes (TB1 and TB2). TB1 includes longer peptides from ESAT-6 and CFP-10 with TB2 including peptides to induce IFN-γ production by both CD4 and CD8 T-cells (TB7.7, present in QFT-GIT, has been removed). The rationale for the inclusion of these new CD8-specific peptides is derived from growing evidence that M.tb-specific CD8+ T-cells are more frequently detected in subjects with active TB disease as opposed

Table 9.3  Summary of characteristics of IGRAs

•IGRAs are based on the ex vivo release of the key anti-MTB cytokine interferon gamma (IFN-γ)

•IGRAs are more specific than TST and not affected by previous BCG vaccination

•Sensitivity of IGRAs is at least equivalent to TST

•IGRAs are similar to TST in predicting progression from latent to active TB

•Current IGRAs are unavailable to rule-in/rule-out the diagnosis of active TB

•IGRAs are increasingly incorporated into national guidelines

•IGRAs are unable to differentiate between latent and active TB

•IGRAs should not be used to monitor response to therapy or as a test of cure

to LTBI, or a recent exposure to TB, and decline when patients receive anti-TB treatment. Preliminary data from a multicenter European study suggest that the QFT-Plus retains the same sensitivity as the previous version; however, we are yet to observe any large head to head trials showing superiority.4–7

Currently, both assays use the M.tb-specific antigens, earlysecreted antigen-6 (ESAT-6), and culture filtrate protein-10 (CFP-10). QFT-GIT additionally includes a third antigen Rv2654 or TB7.7. These antigens confer the high specificity of IGRAs; in contrast, TST utilizes PPD, a highly antigenic but relatively nonspecific mixture of over 200 MTB antigens, many of which are shared with the Mycobacterium bovis BCG vaccine leading to false-positive TST results in those who are BCG vaccinated.

ESAT-6 and CFP-10 are proteins encoded in the genomic region of MTB known as region of difference-1 (RD-1). During repeated passages of a virulent strain of M. bovis in vitro through ox bile, Calmette and Guérin developed the non-virulent vaccine strain. Exploration of the M.tb genome during the late twentieth century revealed that this process had resulted in the loss of several virulence-encoding genomic regions including RD-1, rendering BCG unable to produce these proteins. ESAT-6 and CFP-10 are the best understood and form a secreted heterodimer that is part of a type VII secretion system.8,9 Subsequent studies identified them to be highly immunodominant and therefore attractive candidates for a TB blood test.10,11 Only four species of environmental mycobacteria, Mycobacterium kansasii, M. szulgai, M. marinum, and M. riyadhense have RD-1-like regions minimizing false positives. The use of ESAT-6 and CFP-10 has led to the first generation of IGRAs. Over the years, further antigens that are targets of IFN-γ-secreting T-cells in M.tb-infected persons and that are also highly specific for M.tb, either by virtue of being RD1encoded or RD1-dependent for virulence and secretion have been identified.12,13

LATENT TUBERCULOSIS

Latent tuberculosis infection (LTBI) was previously thought to represent a uniform state. However, as research progresses it has become clear that LTBI constitutes a broad spectrum of infection states that differ by the degree of the pathogen replication, host immune response, and inflammation.14 After initial exposure,

M.tb will either be cleared or individuals will become actively or latently infected. The core concept being that in LTBI, patients remain asymptomatic despite an element of persistent immune response to stimulation by M.tb antigens.

As part of the WHO’s aim to eradicate TB strategy there has been a huge research push to understand the natural history and biology of LTBI. Epidemiological studies have shown the majority of reactivated latent disease will do so soon after infection (3–9 months) and almost always within 2 years.2

The most important step when diagnosing LTBI is to first ensure that the patient does not have active TB. The diagnostic process for LTBI should begin with a thorough history (to identify risk factors for TB exposure) and examination with the aim being to identify any clinical evidence of active TB. If patients are symptomatic they should be referred for diagnostic evaluation for potential active TB.

LTBI diagnosis by IGRA

In parallel with national and international guidelines for TB elimination, awareness, diagnosis, and therefore treatment of LTBI has increased in recent years.15–19 When investigating patients for possible LTBI, it must first be remembered that one should only investigate those patients in whom a positive result would lead to treatment.

The evidence-base supporting the clinical utility of IGRA is largest in the diagnosis of LTBI in immunocompetent individuals. Generating the evidence base itself has been challenging as the lack of a gold standard test for LTBI makes it impossible to assess any new diagnostic tool with definable certainty as to sensitivity and specificity, because the TST itself is an inadequate reference standard. Studies that evaluate the utility of the IGRA have therefore taken several alternative approaches to substitute LTBI. Active TB has been used as a surrogate as well as the degree of MTB exposure. However, arguably the most pertinent and the new gold standard involves correlating IGRA responses with the subsequent development of disease. To assess IGRA specificity, studies have analyzed the presence of IGRA negativity in healthy BCG-vaccinated individuals at low risk of TB infection due to the absence of epidemiologic risk factors for TB exposure.

Immunocompetent adults

ACTIVE TB AS A SURROGATE MARKER

Results from extensive meta-analyses and systematic reviews conclude that the sensitivity of IGRAs is superior to that of TST. The T-spot has a consistently higher sensitivity than the IGRA or TST in several cohorts (ELISpot 67%–89% vs. ELISA 61%–84% vs. TST 65%–77%).20–24 Based on published meta-analyses, IGRAs have a specificity for LTBI diagnosis of >95% in settings with a low-TB incidence, with specificity not being affected by BCG vaccination.20,23,25 Data on the newer QFT-Plus is more limited. However, performance of this assay using active TB as a surrogate appears to be equivalent to that of QFT-GIT with a computed overall sensitivity of 87.9%.7

TB EXPOSURE AS A SURROGATE

Exposure studies can take the form of direct contact exposure or the assumption of exposure from countries where TB is endemic. Much of the available data using TB exposure as a surrogate for LTBI in immunocompetent adults come from precise, point source outbreak, and contact-tracing investigations. One of the first studies to use this exposure as a proxy for LTBI looked at adult household contacts of TB patients in the UK. It was shown that IGRA responses were significantly correlated (more so than with TST) with intensity of exposure and not affected by prior BCG vaccination.10 Subsequently, this has been replicated in recent migrants to the UK. IGRA test positivity was independently associated, on multivariate analysis, with risk factors for TB infection—namely TB incidence in country of origin and age.26 A recent large prospective study enrolled 10,740 US-based participants at high risk for latent infection (included children, adults, and immunosuppressed). The TST was less specific than either IGRA, particularly among foreign-born populations (TST 70.0% vs. QFT-GIT 98.5% vs. T-SPOT 99.3%), concluding that IGRAs should be strongly preferred over the TST for both foreign-born children and adults.27

Immunocompromised subjects and other special populations

ACTIVE TB AS A SURROGATE FOR LTBI

In general, fewer studies have assessed the performance of IGRAs in HIV-positive individuals who are co-infection with M.tb. The evidence suggests that IGRAs perform better than TST in this population with T-SPOT being superior to QFT-GIT.

Early work from Zambia and South Africa found that the sensitivity of the T-SPOT in HIV-positive adults and children was relatively high and superior to TST.28–30 Although fewer studies have been conducted in HIV-positive subjects in low-TB burden settings, they have also found the sensitivity of the IGRA to be greater than that observed with the TST.27,31–33 Studies conducted in HIV-infected subjects in Zambia, Tanzania, and Italy found that the sensitivity of the QFT-GIT in HIV-positive individuals was higher than that of TST but significantly lower than in HIVnegative individuals.33–35

Two recent systematic reviews and meta-analyses have computed pooled sensitivity figures for the performance of IGRAs in HIV-positive individuals. Cattamanchi estimated that in HIVpositive individuals with active TB, QFT-GIT sensitivity was 61% (95% confidence interval [CI] 47%–75%) while T-SPOT sensitivity was 72% (95% CI 62–81).21 Higher sensitivities were reported in high-income countries, i.e., T-SPOT 94% (95% CI 73%–100%) and QFT-GIT 67% (95% CI 47%–83%).21 By contrast, Santin and colleagues calculated the pooled sensitivities of 61% and 65% for the QFT-GIT and T-SPOT, respectively.36 Ayubi et al. found that in HIV patients, TST negative/QFT-GIT positive discordance was more commonly observed likely reflecting the lower sensitivity of TST in HIV patients.37

The reported effect of CD4 count on IGRA performance is variable. There is some published evidence suggesting that QFTGIT performance is adversely affected by falling CD4 count.35 However, in a large study conducted by Clark et al. in a low-TB

burden setting they found that T-SPOT results were independent of CD4 cell counts in HIV-positive individuals.31

A recent Zambian study of patients with TB–HIV co-infection found an overall sensitivity of 83% for QFT-Plus. Although test sensitivity did not significantly differ according to HIV status, the authors did find that those individuals with a CD4 count <100 were significantly less likely to have a positive result, suggesting that advanced immunosuppression adversely impacts on test performance.38 A recent systematic review of 4,856 HIV-positive subjects found that both TST and IGRAs (T.SPOT and QFT-GIT) performed similarly, although concordance between the tests varied considerably in certain studies indicating that further research was required.39

The consensus of these multiple studies and meta-analyses is that IGRAs perform as well as or better than TST in HIV positive individuals; however they do not perform as well as they do in HIV-negative populations and sensitivity decreases with falling CD4 count.

IGRA has been shown to be more sensitive than TST at diagnosing latent TB in pregnancy. In a pregnant HIV-positive population IGRA has proven superior,40 having identified >2-fold more women with LTBI compared to TST in one study.41 This is also true for HIV-negative women in a high-burden setting,42 and suggests that TST often fails to detect LTBI in pregnant and peripartum women.

The performance of IGRAs is increasingly being evaluated in other special populations—especially in iatrogenically immunosuppressed subjects with immune-mediated inflammatory diseases (IMID). We know that these are a very high risk group for progression to TB.43 Most studies have been cross-sectional in design and focused on the concordance between TST and IGRAs and correlating IGRA responses with risk factors for LTBI.44

Evaluation of T-SPOT responses in individuals with IMID has found that concordance between the IGRA and TST is, in general, moderate to poor.45,46 In a US study of 200 patients with rheumatoid arthritis, the T-SPOT and TST showed poor concordance; additionally the T-SPOT was not significantly associated with risk factors for LTBI.47 There is some evidence that the ELISpot (but not TST) is significantly associated with TB exposure.48–50 As with the T-SPOT, concordance between the QIF-GIT and TST in this population has been moderate/poor.51–53 Conversely, an Italian study reported high concordance between TST and QIF-GIT. This was likely due to the low proportion of the population previously BCG vaccinated.54 Two separate studies from Switzerland and Ireland in IMID patients undergoing pre-anti-tumor necrosis factor (TNF) screening found that ELISA positivity was significantly associated with risk factors for LTBI.48,55

Although there is no formal meta-analysis in this area, recent work assessing the performance of the IGRAs in patients with end-stage renal failure has found that the T-SPOT, QFT-GIT, and TST were all significantly associated with clinical risk factors for LTBI.56

Children

Children are more likely to progress to active disease following primary infection, and this is especially true for those <2 years

and <5 years of age. This is likely to happen in the first 12 months following infection, without significant prior symptoms and be serious in nature.57

ACTIVE TB AS A SURROGATE FOR LTBI

Evidence in children is somewhat conflicting. Generally, conclusions are that IGRAs perform better in high-income, low-M. tb burden settings whereas TST performs equally well or slightly superior to IGRA in low-income, high-burden settings. However, results vary depending on which platform is used, the clinical setting, and the inclusion criteria.58–62 The most recent large meta-analysis conducted by Sollai et al. evaluated 37 studies in high-income countries. QFT-GIT (79%) sensitivity was superior to T-SPOT (67%) and TST in high-income settings. However in low-income settings, although T-SPOT was superior to QFT-GIT, both IGRAs did not perform any better than TST.58 In a 2011 metaanalysis, both IGRAs had a higher sensitivity than TST, and IGRA performance was superior to TST in high-, rather than low-/mid- dle-income settings (sensitivity of the QFT-GIT and T.SPOT.TB were 83% and 84%, respectively).59 An Italian meta-analysis found that pooled QFT-GIT sensitivity was higher than that for T-SPOT; however, TST sensitivity was superior to both IGRAs.62 In combining positive results from TST and either IGRA, over 90% of children with culture-confirmed TB were correctly identified.62

TB EXPOSURE A SURROGATE FOR LTBI

In one of the first and largest studies of this type, the T-SPOT was employed in a large school outbreak investigation comprising 535 students.63 In this study, TST and a pre-commercial T-SPOT were compared and agreement in this principally BCG-vaccinated cohort was high (89% concordance, κ = 0.72), T-SPOT correlated better with M.tb exposure (based on proximity and duration of exposure to the index case) than TST.63 Similar findings from other settings have confirmed that the T-SPOT significantly correlates with TB exposure.64–69 QIF and QIF-GIT have also been found to correlate with exposure to M.tb and be unaffected by the BCG status.70–74 Similar findings have been found in other highTB burden settings73,74 as well as low-TB burden settings.27,75 A few studies are in conflict with these results—a South African study revealed no significant relationship between the levels of exposure and QIF-GIT positivity in children at high risk of LTBI.76 In non-BCG-vaccinated children from Australia and Spain (low-TB burden countries), QIF-GIT remained negative in a significant proportion of children with a positive TST, suggesting lower sensitivity than TST in diagnosing LTBI in children.69,71

In BCG-vaccinated populations of children, there are lower levels of agreement between IGRAs and TST. This is to be expected as the TST, but not IGRAs, are confounded by BCG vaccination.63,65,69,77

Specificity of IGRAs in LTBI diagnosis

Quantitative estimates of IGRA specificity (for both T-SPOT and QIF-GIT) have been calculated by studying BCG-vaccinated individuals at an ultra-low risk of LTBI due to the absence of epidemiologic risk factors for M.tb exposure. In recent systematic reviews and meta-analyses, the specificity of the QIF-GIT ranged from

96% to 99% and 86% to 93% for the T-SPOT. Both IGRAs have consistently been shown to have a higher specificity compared to the TST in the immunodiagnosis of LTBI—particularly in BCGvaccinated populations. The specificity of the QFT-Plus has been found to be equivalent to that of the QFT-GIT although the evidence base for this test remains limited at present.79

THE PREDICTIVE POWER OF IGRAS

FOR PROGRESSION TO ACTIVE TB

Clinical benefits from chemoprophylaxis for LTBI can only occur if IGRA-positive contacts are truly at increased risk of subsequently progressing to active TB compared with IGRA-negative contacts. The gold standard marker of LTBI is the correlation with risk of progression to active TB disease. In immunocompetent individuals only a small proportion, approximately 5%–10%, will develop TB disease from a latent infection during their lifetime; nearly all within the 2 years post exposure.2 If prophylaxis was provided for all those with LTBI, it would result in an enormous waste of resources, increased risks and side effects associated with the medications themselves and increased likelihood of anti-TB drug resistance, especially when the majority of patients would have never developed active TB. An additional caveat is that chemoprophylaxis is not completely effective. Studies have shown that traditional 6–12 month isoniazid regimens as chemoprophylaxis for LTBI (which have the most abundant evidence for clinical efficacy) have a protective effect of 60% when taken correctly,80 however initiation and completion rates of chemoprophylaxis are frequently suboptimal and vary greatly across different populations.81

A more powerfully predictive test to stratify latently infected individuals by progression risk would much improve targeting of preventative treatment.

Low-TB burden settings

IMMUNOCOMPETENT ADULTS

Both IGRA platforms were first shown to have prognostic power for subsequent development of active TB in two large longitudinal cohort studies in 2008; since then other studies have confirmed these initial observations.22,82 However, the size of the prognostic power of IGRAs in comparison with TST until recently has been a key area of uncertainty. One systematic review of 15 studies with 26,680 individuals concluded that the prognostic power of IGRAs and TST were low but broadly equivalent83 whereas another review of 28 studies including >30,000 individuals found IGRAs to be more predictive than TST.84 Generally, the studies included were of low numbers and very few individuals progressed to active TB. Similarly, a European study from 26 centers in 10 different European countries tested contacts of TB cases with T-SPOT and QFT-GIT for LTBI. Overall, in the group that received no chemoprophylaxis, 16 people developed active disease of 494 with positive IGRAs, 14/421 (3.3%) QFT-GIT positive contacts and 2/73 (2.7%) T-SPOT positive, over a median follow-up of period of 2.5 years.85 Until recently, studies have been small scale with many only testing two strategies.83,84,86–89

In 2018 the UK PREDICT TB study was published, examining the prognostic value of IGRAs and TST in predicting the development of active TB in those with recent M.tb contacts, arrival to the UK from a high-burden area in the last 5 years, or travel frequently to high-burden countries. It is the largest study to directly compare the predictive power of the two IGRAs and TST for subsequent development of incident TB. This multicenter UK study prospectively compared the predictive value of TST, QFTGIT, and T-SPOT in almost 9,610 subjects with TB exposure (half contacts and half recent migrants), of whom approximately 20% were deemed to have LTBI at enrollment based on IGRA and TST. They used three different thresholds for positivity of a TST result: TST-5 mm was considered positive if BCG vaccination was absent whereas TST-15 was considered positive if previously vaccinated. During follow-up, 97 incident cases of TB developed. Of those who completed all three tests and follow–up, 77/6,380 patients progressed to active TB; of these 47/77 (61%) had a positive QIFGIT, 52/77 (68%) a positive T-SPOT, 57/77 (74%) positive TST-15, and 64/77 (83%) a positive TST-5. To identify the most suitable screening test to assess for the progression of disease: a high proportion of tested individuals should be classified as test negative and therefore require no further monitoring; there should be a low rate of progression to TB in those who tested negative, indicating the ability of the test to correctly rule out future disease. All tests had a good negative predictive value (NPV) T-SPOT and TST-15 (99.5%) and QFT (99.4%). The investigators found that the annual incidence of TB in those who tested positive was highest for T-SPOT followed by TST (using a 15 mm cut-off in BCGvaccinated persons) and then QFT-GIT with positive predictive values (PPVs) of 4.2%, 3.5%, and 3.3% respectively. In total, 1,235 people had a positive T-SPOT of whom 52 went on to develop active TB. This means the number needed to treat (NNT) to prevent one case of TB progression is 24 for T-SPOT-TB, 29 for TST, and 31 for QFT-GIT. All three of these tests were significantly better predictors of progression than using lower TST cut-offs (5 and 10 mm).90 If the intention is to identify as many individuals as possible with a positive test result in a high risk cohort who will progress to active TB (e.g., among household contacts of patients with smear-positive pulmonary TB), TST-5 is the preferable test (as currently recommended in US Centers for Disease Control and Prevention guidelines).91 However, although you will then identify the majority of those who go on to develop active TB there will be a huge proportion with a positive test who do not develop active disease.

PREDICT represents the first head-to-head, large-scale comparison of three different testing strategies in a low incidence TB country. It is clear that in immunocompetent adults, when negative, the current IGRAs act as a good rule-out test for the development of TB, however they are still relatively poor at predicting the future development of TB and tests. Higher PPVs are required to affect a step-change in TB control and prevention. Enhancing the prognostic power of IGRAs, finding additional biomarkers or clinical risk scores to identify those that will progress to active TB thereby reducing the NNT to prevent a single case of progression to active TB is therefore an urgent research priority as highlighted by the WHO.19,92

INDIVIDUALS WITH HIV

In immunocompromised individuals, especially in patients with HIV infection, the NPV of an immunodiagnostic test is insufficient to rule out the future development of TB.

A systematic review of the predictive power of IGRAs in immunocompromised individuals found that the majority of the immunocompromised cohort that progressed to active TB were HIV-positive, therefore the number of HIV-infected persons with a positive test needed to treat was lower (range 14.0–25.5) and TB cases were only found in patients with ongoing detectable viral loads.93 It should be noted however that although very few, incident TB cases did occur in individuals with negative test results especially among patients with HIV infection. In all the studies included in the meta-analysis the NPV for both IGRAs was

>99%.85,93,94

A large study from Europe recruited immunocompromised patients from 17 European healthcare facilities, 10 of 768 HIVpositive individuals progressed to active TB in the time period and this was poorly predicted by TST or IGRAs.93

In an Austrian prospective longitudinal study, 830 HIV-1- infected patients underwent testing with the QFT-GIT assay. The cohort was reviewed at 3-month intervals to monitor for any development of active TB. Median follow-up time was 19 months. Active TB occurred in three patients and exclusively in patients with a positive QFT-GIT assay result at a rate of 3/37 (8.1%).95

CHILDREN

There are no large studies looking at the use of IGRA to specifically predict progression to active TB in children in low-burden countries. Given the high-risk nature of LTBI in children, if children are found to have a positive IGRA they are offered chemoprophylaxis; therefore, there is very limited available data.

OTHER AT-RISK POPULATIONS

Within immunocompromised groups it is evident that there are different amounts of progression to active TB dependent on numerous clinical and biochemical markers.

Recently, a review of the predictive power of IGRAs in immunocompromised individuals (chronic renal failure, solid organ transplantation, stem cell transplantation, and rheumatoid arthritis) demonstrated with a positive IGRA (n = 1,537) the NNT to prevent one active case ranged from approximately 50 to 80. However, the majority of progression occurred in the HIVpositive cohort. In patients with chronic renal failure, rheumatoid arthritis, stem cell, or solid organ transplant recipients no patients progressed to active TB.93,96

The TST and commercial IGRAs have important limitations and poor-predictive value for active TB in patients with advanced chronic kidney disease (CKD). Several studies have explored the diagnostic accuracy of the TST and IGRAs by examining the association between positivity and risk factors for LTBI.56,97 Although IGRAs perform superiorly to TST in the diagnosis of LTBI in those with diabetes,98 on immunosuppressive therapy such as anti-TNF-α,44 transplant and rheumatoid arthritis patients they are not as accurate in immunocompetent individuals.93

In three large-scale occupational studies no healthcare workers with LTBI (defined as a positive IGRA) developed active TB during 2 years of follow-up. Therefore, this would suggest they are no longer risk groups in low-incidence countries.99–101

High-TB burden setting

The predictive value of IGRAs and TST for the development of TB in high-burden countries has been less studied. Studies in different high-risk areas have had conflicting outcomes as to whether IGRAs offer any improvement from the traditional TST testing in the prediction of active TB disease.

IMMUNOCOMPETENT ADULTS

In a recent Indian study, Sharma et al. recruited a large cohort of household contacts of pulmonary TB patients (adults and children). These contacts were prospectively followed-up for 2 years for the development of active TB after getting baseline QFT-GIT assay and TST done. Seventy-six of 1,511 household contacts developed active TB. No significant association of baseline TST and QFT-GIT assay response with subsequent development of active TB was shown.102

In a large meta-analysis, Rangaka and colleagues looked at the prognostic ability of the IGRAs. The study included 15 longitudinal studies that mostly took place in high-TB burden settings with a combined sample size of 26,680. In total, 66/601 patients were QIF-GIT positive with 6/41 who received no chemoprophylaxis developing active TB over the study period giving a PPV of 14.6%. No patients with a negative QIF-GIT developed active TB.83

One of the first studies in a high-burden setting took place in The Gambia exploring the predictive values of T-SPOT vs. TST in contacts of smear-positive cases of TB. It included a very mixed population (children, adults, and HIV-positive/negative). Initial T-SPOT and TST were each positive in just over half of cases who progressed to active TB (71% were positive in one or other test), concluding that neither TST nor T-SPOT were effective enough at reliably predicting disease progression in this setting.103

INDIVIDUALS WITH HIV

TB is the most frequent cause of AIDS-related deaths worldwide; this is despite progress in access to antiretroviral therapy.104 TB is responsible for approximately 400,000 deaths in people living with HIV in 2016 (one-third of all HIV deaths).15

In a study of HIV-infected individuals in Korea (TB rate 110/100,000), 11 patients developed active TB during 238 personyears. Patients with positive T-SPOT responses had a higher TB incidence rate; however, this was not statistically significant (20% [6/30] vs. 6.02% [5/83], p = 0.052). Independent risk factors associated with the progression of TB were low CD4T cell counts, previous history of TB treatment, and positive T-SPOT results. Advanced HIV-infected patients who had a positive T-SPOT had a higher rate of progression to TB in this intermediate TB-endemic area.105

In a South African study, the rate of progression to TB in an HIV population who received no chemoprophylaxis did not differ significantly between the IGRA-negative and -positive groups (positive 3.3/100 person-years, negative 3.9/100 person-years).106

The WHO currently states people living with HIV who have a positive test for LTBI benefit more from preventive treatment than those who have a negative LTBI test; LTBI testing can be used, where feasible, to identify such individuals. However, prophylactic treatment may be given without LTBI testing for people living with HIV or child household contacts aged <5 years supporting the fact that these tests are generally more unreliable in these cohorts.15

PREGNANCY AND HIV INFECTION

Data on HIV, pregnancy, and LTBI progression in high-burden countries is somewhat limited. An Indian study enrolled 252 pregnant HIV-infected women in linked cross-sectional/longitudinal studies: 71 (28%) had a positive QFT-GIT but only 27 (10%) had a positive TST (p < 0.005); 5/252 (2%) women developed active TB, all within 1 year postpartum. All five had a positive QFTGIT but only one had a positive TST. Based on this, the sensitivity of the QFT-GIT for development of postpartum TB was 100%, but the specificity was 27% and the PPV was only 7%. In those women who developed active TB, three women had samples from pregnancy and delivery showing a 2.9-fold decrease in IFN-γ.40 These latter findings would support the hypothesis of a weakening of T-cell-mediated immune response in pregnancy thus leading to the higher risk of active TB observed in late pregnancy and postpartum.107

A Kenyan study explored the predictive value of T-SPOT in 327 pregnant women with HIV without previous TB of whom nine developed TB within 1 year of delivery. They quantified the positivity of T-SPOT classifying the number of IFN-γ spot forming cells (SFCs). IFN-γ ≥6 SFCs showed a sensitivity (78%) and specificity (55%) with a PPV of 5.9%. In women with CD4 <250 cells/µL, sensitivity and specificity of IFN-γ ≥6 SFCs was 89% and 63%, respectively, with a PPV of 19.2%; however, this study was limited by few cases of active TB, absence of TST for comparison, and absence of TB diagnostic confirmation.108

These studies are small and require validation with welldesigned, large, prospective studies.

CHILDREN

A number of large studies have been conducted in high-TB burden areas of South Africa examining the predictive value of IGRA vs. TST. In a study of adolescents aged 12–18 years, 5,244 participants were followed-up for a median of 2.4 years. Positive TST and QFTGIT tests were moderately sensitive predictors of progression to microbiologically confirmed TB disease. There was no significant difference in the predictive ability of the tests.

In a large prospective cohort of young children (2,512 HIVuninfected young children aged 18–24 weeks were enrolled) from a TB-endemic community in South Africa, they found high rates of QFT-GIT conversion and high incidence of TB disease. The PPVs and NPVs using the manufacturer’s cut-off of 0.35 IU/mL were 8% and 99%, respectively. They found an increased risk of incident TB disease following QFT-GIT conversion in those with IFN-γ values (>4.00 IU/mL, >40-fold that of QFT-GIT non-converters and 11-fold that of QFT converters with IFN-γ value between 0.35 and 4.00 IU/mL); however, this then missed a significant proportion of those who went on to develop active TB.109

Limitations of current available diagnostics

TST

The TST has several disadvantages; it demonstrates poor specificity in BCG-vaccinated populations, gives false positives in patients with non-tuberculous mycobacterium,110 and has limited sensitivity in immunosuppressed persons and young children. In addition to this, the requirement for two clinical visits (the second to read the result) can make it logistically difficult.

A further factor confounding interpretation of TST results is the phenomenon of potentiation or boosting. This occurs when the skin test is repeated resulting in an amplified response due to progressive sensitization; this complicates interpretation and can lead to false positives in serially tested subjects.

IGRAS

Although both IGRAs have significant advantages over TST, their limitations have driven the research agenda forward to define next-generation IGRAs, T-cell signatures, and novel biomarkers. Although the advent of IGRAs represents a significant advance on the technologies of the twentieth century, they have several limitations, which necessitate further research to improve our ability to diagnose and treat TB.

Although IGRAs have improved diagnostic sensitivity and specificity compared to TST in many populations, IGRAs are not a gold standard test and therefore unable to rule-in or rule-out active TB or LTBI. To date, the magnitude of the IFN-γ responses does not appear to reflect bacillary burden or disease activity. As a result, IGRAs cannot be used to distinguish between active and LTBI, as a tool for monitoring treatment response or as a test of cure following completion of anti-tuberculous therapy.

The use of IGRAs in active TB diagnostics is beyond the scope of this chapter; however, there has been widespread conflicting information regarding this. The recently published large-scale IDEA study has definitively proven that currently available IGRAs as they stand do not have a useful role as rule-in or rule-out tests in routine clinical practice.111

IGRAs, as is the case with TST, reflect immune priming and test positivity may remain lifelong due to the presence of longlived memory T-cells. Therefore, a positive IGRA result is very difficult to interpret in subjects with a past history of active TB and previously treated latent TB.

As discussed extensively currently IGRAs appear to be useful as a rule-out test for the future development of active TB but lack adequate PPV.90

Kinetics of IGRA responses over time

CONVERSION

Conversion, when applied to TST and IGRA, is defined as the development of a positive result following new infection.112 There is no large-scale evidence comparing time to conversion with TST following a point-source exposure precisely defined in time and place. Small studies have suggested that the conversion time is greatly varied between individuals. The majority of contacts