Clinical Application of Interferon-γ Release Assays for the Prevention of Tuberculosis in Countries with Low Incidence

How well is the progression to tuberculosis predicted by IGRAs in countries with a low incidence of TB?

Recommendations to use IGRAs in countries with a low incidence of TB are restricted to asymptomatic individuals who have an increased risk for progression towards tuberculosis, and for the purpose of guiding preventive chemotherapy. According to WHO guidelines, individuals at risk include close contacts of patients with active TB, migrants, health care workers, patients with diabetes, and immunocompromised patients with HIV infection, chronic renal failure or receiving immunosuppressive drug therapy, treatment with TNF antagonists or other biological therapeutics [46]. Although IGRAs have a high specificity and good sensitivity for diagnosing latent or active infection with M. tuberculosis, several studies from countries with low incidence of TB have shown that only a very small percentage of individuals with positive test results develop active TB, and the risk for progression differs among the groups considered at risk [8, 33, 36, 42, 43, 74-50]. Moreover, patients with severe immunodeficiency who develop TB may have had negative test results at the time of screening, which emphasizes that the negative predictive value is not sufficient in this situation [8]. The ability of IGRAs to predict progression towards TB in various risk groups from low-incidence countries was extracted from recently published key studies with sample sizes of more than 1,000 individuals, and performance characteristics are summarized in Table 3. We have also included a study in migrant contacts to show comparative data for this risk group. Although the selection of studies was not based on a formal systematic review, the focus on analyses with large sample size mostly derived from multicenter analyses allows for a representative estimation of IGRA performance in current clinical practice. Although the risk for the development of TB is highest in the first year following contact with M. tuberculosis and the follow-up time in these studies was between 2-5 years, some individuals may develop TB at a later time. Therefore, results from these studies may underestimate the long-term risk for the development of tuberculosis.

The number needed to treat (NNT) to prevent one case of TB is a clinically useful parameter based on test results to guide preventive chemotherapy in a given risk group. It is based on the absolute risk reduction (1/absolute risk reduction), and is calculated from the difference in the number of TB cases found on follow-up in individuals with positive test results between groups with and without preventive chemotherapy. Recently, several large studies with more than a 1,000 individuals each have been published that allow a robust estimation of the NNT for various groups considered at risk. A large European multicenter study performed in the TuBerculosis Network European Trialsgroup (TBNET; www.tb-net.org) included 4,513 close contacts of patients with active tuberculosis who were screened with either QFT-G-IT or the T-SPOT.TB assay [36]. The diagnosis of tuberculosis was overall a rare event with only 24 incident cases. The incidence of tuberculosis was substantially lower in contacts with preventive chemotherapy, indicating that treatment was highly effective. Also, the negative predictive value was nearly 100%, emphasizing that tuberculosis is rare in IGRA-negative contacts. Nevertheless, the positive predictive value of a positive IGRA was low (1.9 and 0.7 for QFT-G-IT or the T-SPOT.TB), which shows that the majority of contacts with positive IGRA did not develop TB. Taking these results together and based on this study, the NNT to prevent one case of TB is 37 for QFT-G-IT and 37 for the T-SPOT.TB (Table 3) [36]. Interestingly, these NNTs are very similar to those derived from another contact-tracing study involving 1579 subjects in Germany [47], where the NNT for the QFT-G-IT was 34. Finally, in a large Dutch study, where 4716 contacts were screened using a TST and/or IGRA, the NNT reached 89. Interestingly, if screening was restricted to close contacts, this led to a substantial decrease in the NNT to 30 (Table 3) [48]. Together these findings illustrate that the NNT among contacts is in the range of 30 to 37, and that this NNT-range may be achieved if screening is focused on close contacts (Table 3). The NNT may be considerably higher if individuals from several risk groups are estimated together. This is illustrated by a nation-wide study from Denmark which included 15,980 individuals after contact tracing, screening prior to TNF antagonist treatment, or suspected of having TB.

Table 3. Numbers needed to treat to prevent one case of TB in people with a positive IGRA or TST test result in different risk-groups in low incidence countries.

In this study, 40 incident TB cases were observed, and the PPV of a positive QFT-G-IT was 1.32 only with an estimated NNT of 68 [49]. Interestingly, this corresponds to the higher NNT calculated from a large multicenter study among immunocompromised patients including HIV-infected individuals, patients with chronic renal failure or rheumatoid arthritis, or stem-cell and solid organ transplant recipients [8]. In the combined analysis of all patients, the NNT was 50 for the TST, 80 for the QFT-G-IT, and 64 for the T-SPOT.TB assay. It should be noted that for immunocompromised patients the NNT was higher with the use of IGRAs than with the use of TST. In addition, TB cases also occurred in patients who tested negative with IGRA/TST and were almost exclusively found among HIV-infected patients; all HIV-patients developing TB had ongoing HIV replication [8, 45]. Thus, targeted preventive therapy for immunocompromised hosts in low TB incidence countries is most effective in HIV-infected patients with detectable viral replication (NNT of 8 for the TST, 13 for the QFT-G-IT, and 10 for the T-SPOT.TB assay, Table 3) [45, 52].

Health-care workers represent another group of individuals where serial monitoring has been recommended due to a presumed higher risk for disease progression. However, as emphasized by 3 large studies from Germany and from the United States involving single or serial screening of more than 15,000 individuals, no case of tuberculosis occurred (Table 3) [42, 43, 50], which may suggest that serial screening of health-care workers should not be performed and should be restricted to personnel after close contact with a patient with active TB. Alternative definitions of IGRA conversion and different cutoff points for different risk populations should be developed [53]. These would need to be evaluated in large data sets and modeling studies [43, 54].

Finally, screening is recommended for migrants. A study from the Netherlands analyzed progression towards TB in approximately 300 migrant contacts of cases with active TB [51]. Although progression towards TB in those with negative tests was not reported, the estimated NNT of migrants who had active TB contacts was similar to other contacts with active TB cases (26 for the TST, 37 for the QFT-G-IT, and 30 for the T-SPOT.TB test) (Table 3).

Taken together, a comparison of the NNTs of these studies from countries with a low incidence of TB indicates that the risk for progression differs from one group to another. Moreover, as illustrated by the lowest NNT in close contacts or migrant contacts, or in HIV-infected individuals with ongoing HIV-replication, targeted screening of individuals with additional, clinical risk factors can lead to a substantial decrease in screening load and unnecessary preventive chemotherapy. Similarly, in other groups previously considered at risk such as health-care workers or patients with chronic renal failure, where general LTBI testing was recommended, screening [46] and treatment may now be focused on individuals with additional risk factors.

The benefits of preventive chemotherapy also have to be weighed against age-related drug-associated hepatoxicity [55, 56], and the complexity of such trade-offs might be best judged by data-driven algorithms [57]. Table 4 provides recommendations based on available evidence for the NNT regarding the use of IGRAs and preventive chemotherapy in countries with low incidence of TB. Screening for LTBI is currently also recommended and frequently used in other patients considered at risk such as patients prior to TNF antagonist treatment or receiving other immunotherapies for neoplastic, hematologic, rheumatologic, dermatologic, and other conditions. Similar studies with large sample sizes are also needed to define the NNTs for these groups. Additional information for the use of IGRAs and preventive chemotherapy in countries with high or intermediate incidence of TB, where IGRAs are generally not recommended to screen for LTBI, is provided in an appendix.

Table 4. Recommendations for the use of IGRA-testing* and preventive chemotherapy against active tuberculosis in low-incidence countries according to the number needed to treat to prevent one case of TB.

^a number needed to treat to prevent a case of TB of < 50; ^b number needed to treat to prevent a case of TB of > 50 but justified by severity of TB disease; ^cnumber needed to treat to prevent a case of TB of > 100; at least one additional risk factors for LTBI should be present; ^dnumber needed to treat to prevent a case of TB of >> 100 or ^enot known but perceived to be high; unless other factors leading to strong or conditional recommendations are present; *the best method is to combine IGRA/TST results with a risk/benefit assessment performed by algorithms [57].

Are the proposed IGRA cutoffs valid?

Based on the comparison between healthy controls and patients with culture-confirmed TB, the following cutoffs were established for the 2 commercially available IGRAs: a positive T-SPOT.TB result is defined as antigen-specific induced IFN-γ secretion of more than 5 spot forming cells (SFC) per 250,000 PBMC after subtraction of the negative control reactivity [58], with a borderline zone between 5 and 7 SFC defined for use in the United States. The result of the QFT-G-IT assay is defined as positive above a cutoff of 0.35 IU IFN-γ/ml whole blood after subtraction of the nil control [59, 60]. For the purpose of sensitivity, low cutoffs were assigned in order not to miss any cases.

Reproducibility studies have identified some inter-test variability [54]. This may be due to variability in the individual immune function. Moreover, variations might be due to the time of blood collection [61], delay or differences in processing, or duration of incubation. Important immunological sources might be inadequate disinfection before phlebotomy or previous TST boosting.

These variations have most impact if quantitative IGRA results are close to the cutoffs. Serial examinations showed conversion rates between 2.8% and 8.3% and very high reversion rates between 37.3% and 64.8% [42, 43, 50, 62]. This has led some authors to propose borderline definition for positivity to avoid unnecessary chest-X-ray or preventive treatment [44].

Although both IGRAs give quantitative results, clinical decisions are based on qualitative results. One study suggested that an increase in the cutoff value may increase the positive predictive value to predict disease progression and decrease the NNT to prevent a single case [47]. However, these results were not confirmed in a larger study [36], where the currently recommended cutoffs for test positivity provided the best discrimination between those who did and did not progress to active tuberculosis [36]. While these cutoffs appear to be optimal for the TB risk evaluation among contacts, it must be acknowledged that a larger proportion of patients with immunodeficiencies who progress to TB, may have negative IGRA test results at the time of screening [8].

Are IGRAs tools to monitor tuberculosis treatment responses or surrogates for vaccine effectiveness?

The IGRAs measure the response of circulating effector T-lymphocytes to viable and dormant mycobacteria [63], and results are assumed to correlate with the antigenic load present in the body [37, 39, 64]. Treatment and decrease of antigenic load therefore should result in a decrease in IGRAs reactivity, which could conceivably be used for treatment monitoring [64-66].

Active TB

Few studies have tried to correlate the changes in IFN-γ responses in IGRAs with established markers of treatment response or outcome [37, 39, 40, 67]. Overall, no clear correlation of IGRA response with treatment-monitoring parameters could be found. This is probably attributable in part to significant within-subject variability in sequential measurements due to both exogenous (eg, relating to test procedure) and endogenous (eg, co-infections, time from exposure) factors [68].

LTBI

While there are established markers for treatment monitoring in active TB (ie sputum and smear conversion), no biomarker is currently available for the monitoring of latent TB treatment success. By definition, no mycobacterial product is identifiable in LTBI with current techniques, and therefore a reliance on immune-based tests for monitoring is even greater [69]. However, a significant change using an IGRA has been proven difficult to detect probably because (i) IFN-γ is produced in low quantity and mostly in close proximity to the antigen [70, 71]; (ii) changes in IFN-γ response independent of treatment have been observed, which might reflect self-clearance or transition of bacteria into dormancy [72]; (iii) an initial increase in IFN-γ response due to increased antigen stimulation from destruction of mycobacteria can be observed early in treatment [73]. Overall it can be stated that IGRAs do not appear to be a promising biomarker for treatment monitoring in LTBI.

Recently IGRAs were used to describe vaccine-induced cytokine immune responses following a MVA-85 vaccine-specific stimulation in neonates [41]. In this study, induction of a vaccine-specific IGRA response did not correlate with improved capacity to control mycobacterial growth and to protect individuals from active TB.

Special considerations in children

As in adults, IGRAs cannot differentiate between LTBI and active disease in children. Preventive therapy programs optimize cost-effectiveness by targeting populations at greatest risk of TB infection and disease. Prior to initiating preventive therapy, children must complete screening to exclude TB, but confirmation of LTBI is not mandatory. Among young household contacts in TB high-burden settings (in whom the risk of LTBI may be > 40% [74]), it is most cost-effective to provide preventive therapy to all child contacts younger than 5 years without testing for LTBI [75]. This approach may also be reasonable for child close contacts in countries with low incidence of TB. However, in most TB low-burden settings, documentation of a positive IGRA or TST result guides the use of preventive therapy in child contacts [76-78].

Microbiological confirmation of child TB is rare due to the challenges of sputum collection, the paucibacillary nature of the disease, and the low yield of diagnostic tests [79]. A systematic review estimates that 75% of children receive TB treatment based on bacteriologically unconfirmed, clinically diagnosed TB [80]. IGRAs are commonly used in children living in upper income, low TB-burden countries, and the results influence clinical decision making [81]. Pragmatic use of IGRAs and TST in children have emerged through clinical practice and informed US guidelines [77]. In low-risk, BCG-vaccinated or NTM-exposed children (in whom positive TST results are expected due to adaptive immunity against other mycobacteria), high specificity is desired which makes the use of IGRAs preferable. In young or immunocompromised children with significant risk of TB progression, testing strategies optimize sensitivity by using both an IGRA and a TST; a positive result with either test is considered evidence of LTBI. Guidelines in the United Kingdom recommend TST as the first test of LTBI, with use of IGRAs only in children with a negative TST or at times when the TST is unavailable or impractical [78]. Because several studies have shown that IGRAs are more likely to be indeterminate or negative in children younger than 5 years, current US CDC guidelines recommend the use of TST rather than IGRA in children younger than 5 years [76]. The WHO preferentially recommends the TST [82] and recommends against the use of IGRAs in TB high-burden settings [83].

Conclusions

As the risk for tuberculosis depends on host susceptibility and pathogen exposure, the definition of risk groups for targeted LTBI screening varies substantially over time and is dependent on the geographic region.

While health-care workers in the United States and Western Europe used to be considered a high-risk group for TB as the prevalence of the disease was thought to be high, recent studies have shown that the risk of TB for these workers is low, justifying screening by IGRAs only when a special exposure has occurred, similar to household contacts [42-44]. In people with HIV-infection in countries with a low prevalence of TB, the risk for TB is dramatically reduced in patients who experience sustained undetectable levels of viral replication [42, 44]. It is presently unclear whether all HIV-infected patients in these countries should be regularly screened by IGRAs or whether screening should be restricted to the minority who are antiretroviral therapy (ART) naive, are failing HIV-therapy, or have had recent known M. tuberculosis exposure. More than 25% of patients with chronic renal failure from countries with low incidence of TB have positive IGRA responses despite no substantially increased risk for the development of TB [8]. These patients should be screened for LTBI in high-incidence countries where they have an increased risk for M. tuberculosis exposure. They should not be routinely screened in low-incidence countries where a positive test result in these patients is not related to a substantial risk for the development of tuberculosis.

In order to provide evidence-based recommendations that are valid for specific geographic regions, cohorts of putative risk groups for TB must be followed longitudinally to evaluate the performance of immunodiagnostic testing, the risk for tuberculosis, and the effect of preventive chemotherapy in different groups and regions. Consensus must be reached on the NNT to prevent one case of tuberculosis. This consensus should guide national and international management recommendations. The recommendations that are provided in this review (Table 4) allow guidance using sub-group stratified best evidence.

Without knowledge about the NNT to prevent a case of tuberculosis in different groups and regions there is both the risk of undertreatment and overtreatment and lack of acceptance of preventive therapies [85].

In order to achieve the goal of TB elimination in low-burden countries, prevention of TB must be improved substantially. Without a vaccine that protects individuals exposed to M. tuberculosis better than M. bovis BCG, TB prevention strategies must rely on infection control measures and preventive chemotherapy for those individuals with LTBI, who have the highest risk for the progression to active TB [86]. Unfortunately, none of the presently available tests for LTBI offer a high prognostic value. In countries with low incidence for TB, immunodiagnosis is presently the most effective measure to identify people with LTBI who should receive preventive chemotherapy to avoid progression to TB. Considering multiple factors when determining individual risk of future TB is attempted in adult algorithms such as the Online TST/IGRA interpreter, which can likely maximize the prognostic capability of available and future diagnostic tools [57].

A recent study in a large cohort of adolescents from Africa has identified a 16 gene transcriptomic signature from the peripheral blood that was highly predictive for the development of TB and that appears superior to IGRAs in predicting disease development [87]. Promising results were validated in 2 independent African cohorts suggesting that the NNT to prevent one case of TB could be lowered to < 10 by transcript analysis. However, it is presently unclear how well this method will predict the development of TB in individuals from different risk groups in countries with low incidence of TB and how specific such a test will be. Large multinational and multicohort studies are needed to address this question.

While using IGRAs as screening tools for providing preventive chemotherapy for people with positive test results is effective for the individual risk reduction for TB in people from high-risk groups, the effect of screening programs on the overall incidence of TB in low-incidence countries is likely to be low. At times of high global mobility, elimination of TB in low-incidence countries will not be possible by an IGRA-based prevention approach as long as a steep gradient of the burden of TB continues to exist between different geographic regions of the world [88].

Acknowledgments

FIND (CMD) has received funding by the CDC under grant agreement 5U2GPS002746-05. CL is supported by the German Center for Infection Research (DZIF). CL reports personal fees from Chiesi, personal fees from Gilead, personal fees from Abbvie, personal fees from MSD, personal fees from Becton Dickinson, personal fees from Janssen, outside the submitted work. BK reports personal fees from Oxford Immunotec, outside the submitted work. MS reports support for investigator initiated research by Qiagen and Oxford Immunotec (test kits free of charge). In addition, MS has a patent pending on cytokine profiling for detection of active tuberculosis. AM and CMD have nothing to disclose.

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