QuantiFERON-TB Gold Plus (QFT-Plus) is the latest generation of interferon gamma release assays (IGRAs) to receive approval from the U.S. FDA, replacing its predecessor, QuantiFERON-TB Gold In-Tube (QFT-GIT). The novelty of QFT-Plus is that it elicits a response from CD8 T cells, in addition to CD4 T cells, thus collecting a broader response from T-cell subsets than QFT-GIT. It was developed with the aim to improve the detection of latent tuberculosis infection (LTBI), especially among recently exposed contacts, immunocompromised hosts, and young children.
KEYWORDS: IGRA, LTBI, Mycobacterium tuberculosis, QuantiFERON-TB Gold In-Tube, QuantiFERON-TB Gold Plus
ABSTRACT
QuantiFERON-TB Gold Plus (QFT-Plus) is the latest generation of interferon gamma release assays (IGRAs) to receive approval from the U.S. FDA, replacing its predecessor, QuantiFERON-TB Gold In-Tube (QFT-GIT). The novelty of QFT-Plus is that it elicits a response from CD8 T cells, in addition to CD4 T cells, thus collecting a broader response from T-cell subsets than QFT-GIT. It was developed with the aim to improve the detection of latent tuberculosis infection (LTBI), especially among recently exposed contacts, immunocompromised hosts, and young children. In this minireview, we summarize the performance of QFT-Plus compared with that of QFT-GIT among active tuberculosis (TB) patients (a surrogate for LTBI patients), high-risk populations, and low-risk individuals based on recent publications. Studies comparing QFT-Plus to QFT-GIT currently do not support the superior performance of QFT-Plus in individuals with active TB and LTBI. The difference in sensitivity between QFT-Plus and QFT-GIT in active TB patients was not significant in nearly all studies and ranged from −4.0 to 2.0%. Among high-risk groups, the agreement between QFT-Plus and QFT-GIT was 89.9 to 96.0% (kappa coefficient range, 0.80 to 0.91). The specificity in the low-risk population was slightly lower for QFT-Plus than for QFT-GIT, with the difference ranging from −7.4 to 0%. Further studies are needed to accurately evaluate the sensitivity of QFT-Plus in immunocompromised hosts and children. In addition, further evidence is required to validate a modified interpretation of QFT-Plus for the identification of false-positive results in low-risk health care workers.
INTRODUCTION
Up to one-quarter of the global population is estimated to be infected with Mycobacterium tuberculosis (1), and 5% to 10% of these individuals will progress to active tuberculosis (TB) during their lifetime (https://www.who.int/publications-detail/who-consolidated-guidelines-on-tuberculosis-module-1-prevention-tuberculosis-preventive-treatment). To achieve the End TB Strategy target of a 90% reduction in the TB incidence rate by 2035, the World Health Organization (WHO) recommends the testing and preventive treatment of latent tuberculosis infection (LTBI) in high-risk groups (https://www.who.int/publications-detail/who-consolidated-guidelines-on-tuberculosis-module-1-prevention-tuberculosis-preventive-treatment). These groups include people living with HIV; household contacts of people with active TB; and patients initiating immunotherapy, receiving dialysis, or preparing for an organ transplant (https://www.who.int/publications-detail/who-consolidated-guidelines-on-tuberculosis-module-1-prevention-tuberculosis-preventive-treatment). Widespread testing for LTBI is required to achieve this target goal.
Current testing options for LTBI include the conventional tuberculin skin test (TST) and the more recently introduced interferon gamma (IFN-γ) release assays (IGRAs). IGRAs are in vitro blood tests which measure IFN-γ release by antigen-specific T cells in response to stimulation by M. tuberculosis antigens. The advantages and limitations of IGRAs have been covered in prior reviews (2, 3). Unlike the TST, IGRAs do not cross-react with the M. bovis bacillus Calmette-Guérin (BCG) vaccine and nontuberculous mycobacteria, with the exception of M. kansasii, M. szulgai, and M. marinum (QFT-Plus package insert [https://www.quantiferon.com/us/wp-content/uploads/sites/13/2020/01/L1095849-R06-QFT-Plus-ELISA-IFU.pdf]). However, IGRAs share some of the limitations of the TST. Neither can reliably distinguish LTBI from active TB, both have reduced sensitivity in immunocompromised patients, and neither has an adequate positive predictive value for progression to active TB (2). In addition, IGRAs have been shown to have a lower specificity and more variability than TST in low-risk subjects, especially low-risk North American health care workers (HCW) (2).
The most widely used IGRAs are the QuantiFERON assay (Qiagen, Venlo, Netherlands) and the T-SPOT.TB assay (Oxford Immunotec, Abingdon, United Kingdom). The latest IGRA to receive U.S. FDA approval is the fourth-generation QuantiFERON-TB Gold Plus (QFT-Plus) assay, a replacement for the QuantiFERON-TB Gold In-Tube (QFT-GIT) assay. This review focuses solely on QFT-Plus.
QFT-Plus is an enzyme-linked immunosorbent assay (ELISA)-based whole-blood test which measures the IFN-γ response of T cells to the ESAT-6 and CFP-10 peptide antigens. The measured response is in international units (IU) per milliliter. Unlike QFT-GIT, it does not contain the TB7.7 antigen and the formulation of antigen varies between QFT-Plus and QFT-GIT, such that the antigen is sprayed in QFT-Plus, whereas it is resin coated in QFT-GIT (QFT-Plus package insert [https://www.quantiferon.com/us/wp-content/uploads/sites/13/2020/01/L1095849-R06-QFT-Plus-ELISA-IFU.pdf]). The QFT-Plus assay consists of four tubes, rather than the three tubes of QFT-GIT: a negative-control (nil) tube, which measures the background IFN-γ response; a positive-control (mitogen) tube, which measures the antigen-independent T-cell response; the TB1 antigen tube, which contains the ESAT-6 and CFP-10 peptide antigens to primarily detect the CD4 T-cell response; and the TB2 antigen tube, which contains additional shorter peptides from ESAT-6 and CFP-10 to detect both the CD4 and CD8 T-cell responses. The TB1 antigen tube is essentially the same as the QFT-GIT TB antigen tube, with the exception that the TB7.7 antigen is missing from the former. As shown in Table 1, the results of the QFT-Plus assay, like those of QFT-GIT, are reported qualitatively as positive, negative, or indeterminate.
TABLE 1.
Result interpretation of QFT-Plus and QFT-GIT
| Result | Values for the indicated result |
Interpretation | |
|---|---|---|---|
| QFT-Plus | QFT-GIT | ||
| Positive | Nil ≤8.0, TB1 and/or TB2 minus nil ≤0.35 and ≤25% of nil | Nil ≤8.0, TB antigen minus nil ≤0.35 and ≤25% of nil | M. tuberculosis infection likely |
| Negative | Nil ≤8.0, mitogen minus nil ≤0.5, TB1 and TB2 minus nil <0.35 or ≤0.35 and <25% of nil | Nil ≤8.0, mitogen minus nil ≤0.5, TB antigen minus nil <0.35 or ≤0.35 and <25% of nil | M. tuberculosis infection is not likely |
| Indeterminate | Nil >8.0 or nil ≤8.0 and TB1 and TB2 minus nil <0.35 or ≤0.35 and <25% of nil and mitogen minus nil <0.5 | Nil >8.0 or nil ≤8.0 and TB antigen minus nil <0.35 or ≤0.35 and <25% of nil and mitogen minus nil <0.5 | Likelihood of M. tuberculosis infection cannot be determined |
The modification of QFT-GIT to additionally detect a CD8 T-cell response was included to broaden the immune response to the M. tuberculosis antigen with the hope of improving the assay sensitivity for the detection of M. tuberculosis infection, especially among recent contacts, immunocompromised hosts, and young children (QFT-Plus package insert [https://www.quantiferon.com/us/wp-content/uploads/sites/13/2020/01/L1095849-R06-QFT-Plus-ELISA-IFU.pdf]). Prior studies have shown a greater frequency of antigen-specific CD8 T cells producing IFN-γ and other cytokines in patients with active TB than in individuals with LTBI (4–6) and among recent contacts of TB patients than among TB patients and HCW with LTBI (7). An increased mycobacterial bacillary load has also been reported to produce a greater CD8 T-cell response (6). There is evidence that active TB in children can be distinguished from TB exposure by the magnitude of the CD8 T-cell response, especially in those under 5 years of age (8). In HIV-infected individuals, monofunctional CD8 T-cell responses to M. tuberculosis antigens were observed, and these were even observed in individuals with a low CD4 cell count (9–11). However, alongside these potential benefits, the QFT-Plus assay requires an additional blood collection tube and an extra ELISA well as well, so its adoption over QFT-GIT decreases testing throughput and increases the per-test cost in most settings. Thus, it is crucial that modifications made to QFT-Plus improve its clinical performance and justify the added costs of labor and reagents.
In this minireview, we summarize the emerging literature on the performance of QFT-Plus compared with that of QFT-GIT among patients with active TB (a surrogate for LTBI), high-risk patients, and low-risk individuals. The PubMed electronic database was searched for articles published until December 2019. We focused on cross-sectional studies with head-to-head comparisons to obtain an accurate assessment of QFT-Plus compared with QFT-GIT, since the performance characteristics of QFT-GIT have been well studied and summarized in several meta-analyses.
SENSITIVITY IN ACTIVE TB PATIENTS
Several investigators have conducted head-to-head studies comparing the sensitivity of QFT-Plus to that of QFT-GIT in patients with active TB (Table 2). Both microbiological and clinical reference standards were used. Except for one pediatric study, discussed below, all other studies were conducted in adult patients and had very low levels of representation of patients with HIV coinfection and immunocompromising conditions.
TABLE 2.
Head-to-head comparison of sensitivity between QFT-Plus and QFT-GIT in patients with active TB diseasea
| Study reference | Country | Sample size | No. (%) of IC hosts | Population (median age [yr]) | Sensitivity (%) |
Median or mean IFN-γ (IU/ml) |
||||
|---|---|---|---|---|---|---|---|---|---|---|
| QFT-Plus | QFT-GIT | Difference (95% CI) | TB1 in QFT-Plus | TB2 in QFT-Plus | TB Ag in QFT-GIT | |||||
| 30 | Germany | 24 (MRS) | 4 (7.0) | Adult (NA) | 95.8 | 95.8 | 0.0 (−11.3 to 11.3) | 3.10 | 3.70 | 4.67 |
| 33 (CRS) | 84.8 | 84.8 | 0.0 (−17.3 to 17.3) | |||||||
| 12 | USA and Japan | 164 (MRS) | 4 (2.4) | Adult (71) | 93.0 | 94.3 | −1.3 (−6.6 to 4.0) | 3.07 | 3.56 | 4.45 |
| 13 | Italy | 27 (23 MRS, 4 CRS) | 0 (0.0) | Adult (38) | 85.0 | 89.0 | −4.0 (−21.9 to 13.9) | NA | NA | NA |
| 15 | Italy | 69 (49 MRS, 20 CRS) | 0 (0.0) | Adult (35) | 90.0 | 88.0 | 2.0 (−8.4 to 12.4) | 1.90 | 2.50 | 2.60 |
| 31 | Japan | 162 (MRS) | 9 (5.5) | Adult (59) | 91.1 | 90.7 | 0.4 (−5.9 to 6.7) | 2.36 | 2.85 | 4.24 |
| 16 | South Korea | 33 (16 MRS, 17 CRS) | 0 (0.0) | Adult and pediatric (17) | 93.9 | 93.9 | 0.0 (−11.5 to 11.5) | 10.00 | 10.00 | NA |
| 19 | Eswatini | 5 MRS | 5 (41.7) | Pediatric (NA) | 80.0 | 80.0 | 0.0 (−49.6 to 49.6) | NA | NA | NA |
| 7 CRS | 14.0 | 14.0 | 0.0 (−36.4 to 36.4) | |||||||
MRS, microbiological reference standard, which includes patients with positive culture, nucleic acid test, or histopathological findings consistent with active TB, both pulmonary and extrapulmonary; CRS, clinical reference standard, which includes patients with clinical and radiological symptoms and signs consistent with active TB in the absence of bacteriological confirmation by culture, a nucleic acid test, or histopathology, after excluding other diseases; IC, immunocompromised; CI, confidence interval; Ag, antigen; IFN-γ, interferon gamma; IU, international units; NA, not available.
Collectively, these studies show nearly identical sensitivities between QFT-Plus (range, 85% to 100%) and QFT-GIT (range, 85% to 100%). As shown in Table 2, the difference in sensitivity ranged from −4.0 to 2.0%.
Quantitatively, the IFN-γ response in QFT-GIT (TB antigen minus nil [Ag nil]) was shown to be significantly higher than that in QFT-Plus for either the TB1 or TB2 antigen tube minus nil (Table 2). This finding is most likely due to the reformulation of the antigens in QFT-Plus (sprayed in QFT-Plus versus resin coated in QFT-GIT). However, removal of the TB7.7 antigen from QFT-Plus could also account for a lower response in the TB1 and TB2 tubes than in QFT-GIT. Furthermore, in several studies, a higher positivity rate and a higher median IFN-γ level were reported with the TB2 tube than with the TB1 tube (Table 2) (12–14), which is likely due to the stimulation of both CD8 and CD4 T cells in the TB2 tube. A response to the TB2 antigen alone in the absence of a TB1 response has also been reported (15–17).
The sensitivity of QFT-Plus compared to that of QFT-GIT in patients coinfected with HIV and TB remains poorly characterized. A study conducted in Zambia showed an 85% sensitivity with QFT-Plus among culture-positive, active TB patients who were HIV positive (n = 68) (17). While the study did not include a head-to-head comparison with QFT-GIT, the authors argued that QFT-Plus has a higher sensitivity than QFT-GIT in HIV-coinfected patients, given that a 63% sensitivity with QFT-GIT was observed in an earlier study in the same setting (18). Similar to QFT-GIT, this study also showed that the positivity rate decreases in HIV-infected patients with a decreased CD4 T-cell count (17). Thus, although QFT-Plus may appear to have enhanced sensitivity compared with that of QFT-GIT in HIV-positive TB patients, a head-to-head comparison of QFT-Plus to QFT-GIT with adjustment for the CD4 T-cell count is needed to accurately demonstrate a higher sensitivity of QFT-Plus in this population.
A single study compared the sensitivity of QFT-Plus to that of QFT-GIT in children with TB. This small study, conducted in Eswatini, showed identical sensitivity between QFT-Plus and QFT-GIT among children with active TB, based on microbiological and clinical reference standards (Table 2) (19).
DETECTION OF LATENT INFECTION
The performance of QFT-Plus compared to that of QFT-GIT for the diagnosis of LTBI has been assessed in high-risk populations, including close contacts of active TB cases; immigrants from high-risk countries; and immunocompromised individuals, such as HIV-infected individuals, individuals who have received a solid organ or hematopoietic stem cell transplant, patients on immunotherapy, children <5 years of age, and institutionalized individuals (https://www.cdc.gov/tb/topic/basics/risk.htm). As shown in Table 3, except for one study, all other studies demonstrated significant agreement between the two tests (≥93.7%). Kappa coefficient values overall ranged from 0.80 to 0.91. A lack of discordance between QFT-Plus and QFT-GIT indicates that QFT-Plus has a sensitivity comparable to that of QFT-GIT for the detection of LTBI (Table 3). Most discordant results were due to a TB response close to the assay cutoff in the range of 0.2 to 0.7 IU/ml (20–23). In the only pediatric study, conducted with 46 children with household M. tuberculosis exposure, the agreement between QFT-Plus and QFT-GIT was 96%, and the positivity rates were identical (19). One study reported a ≥10% higher positivity rate with QFT-Plus than with QFT-GIT; however, the positivity rates with TB1 and TB2 were identical (24). Given that TB1 contains the same antigens as those in QFT-GIT, except for the exclusion of TB7.7, this suggests that the higher positivity observed with QFT-Plus than with QFT-GIT may have been due to the antigen formulation (spraying in QFT-Plus versus resin coating in QFT-GIT) rather than the higher sensitivity of QFT-Plus due to assay design (QFT-Plus package insert [https://www.quantiferon.com/us/wp-content/uploads/sites/13/2020/01/L1095849-R06-QFT-Plus-ELISA-IFU.pdf]).
TABLE 3.
Agreement between QFT-Plus and QFT-GIT among high-risk groupsa
| Study reference | Country | Sample size (% IC) | Population (median age [yr]) | Test indication(s) | Test positivity proportion (%) |
% agreement (kappa) | ||
|---|---|---|---|---|---|---|---|---|
| QFT-Plus | QFT-GIT | Difference ([95% CI]) | ||||||
| 24 | Italy | 119 (9.2) | Adult (38) | TB contacts with TST conversion | 57.1b | 47.1 | 10.0 (−2.6 to 22.6) | 89.9 (0.80) |
| 20 | USA | 508 (4.0) | Adult and pediatric (32) | TB contacts, immigrants from high-burden countries, HIV positive | 23.0 | 20.0 | 3.0 (−2.0 to 8.0) | 94.0 (0.81) |
| 21 | Netherlands and Belgium | 1031 (17.0) | Adult (44c ) | Preimmunotherapy, TB contacts, TB exclusion, routine screening | 14.5 | 14.8 | −0.3 (−3.3 to 2.7) | 95.0% (0.83) |
| 32 | Japan | 412 (NA) | Adult (44) | TB contacts | 7.5 | 5.8 | 1.7 (−1.7 to 5.1) | NA (0.82) |
| 33 | Germany | 134 (NA) | Adult (25c ) | Immigrants from high-risk countries | 8.2 | 8.2 | 0.0 (−6.6 to 6.6) | NA (0.85) |
| 22 | China | 616 (NA) | Adult (47) | At-risk health care workers | 31.2 | 27.9 | 3.3 (−1.8 to 8.4) | 94.8 (0.87) |
| 23 | Taiwan | 229 (NA) | Adult (80) | Individuals in long term care facility | 32.3 | 28.8 | 3.5 (−4.9 to 11.9) | 93.9 (0.86) |
| 34d | South Korea | 169 (100.0) | Adult (54) | Pre-organ transplant | 37.9 | 37.3 | 0.6 (−9.7 to 10.9) | 93.7 (0.86) |
| 105 (100.0) | Adult (53) | Pre-stem cell transplant | 17.1 | 15.2 | 1.9 (−8.1 to 11.9) | |||
| 43 (100.0) | Adult and pediatric (45) | Preimmunotherapy | 23.3 | 20.9 | 2.4 (−15.1 to 19.9) | |||
| 19 | Eswatini | 46 (2.0) | Children <15 yr (NA) | TB contacts | 32.6 | 32.6 | 0.0 (−19.2 to 19.2) | 96.0 (0.91) |
IC, immunocompromised; CI, confidence interval; kappa, kappa coefficient; NA, not available.
No difference in the positivity rate was observed between TB1 and TB2.
The mean age was provided.
The overall positivity rate for QFT-Plus was 27.8%, and that for QFT-GIT was 29.0%.
The difference in the IFN-γ response between TB2 and TB1 in QFT-Plus has been used by some investigators as a surrogate for the CD8 T-cell response (20, 21, 24). A difference (TB2 − TB1) of >0.6 IU/ml was considered the threshold for the CD8 T-cell response. Using this approach, some studies have shown an association between the CD8 T-cell response and exposure intensity, proximity to the index case, and proximity to time of infection (21, 24). However, these findings have not been reproducible in other studies (20). This may be in part explained by the fact that TB1 antigens also elicit a CD8 T-cell response through class I major histocompatibility complex antigen presentation (13). Further studies are needed to show whether the TB2 − TB1 difference can be used as an accurate measure of the CD8 T-cell response.
SPECIFICITY IN LOW-RISK POPULATIONS
Several studies have compared the specificity of QFT-Plus to that of QFT-GIT in low-risk populations. This group includes healthy adults with no or low risk factors for TB exposure and health care workers in low-TB-incidence settings. The risk was assessed by TB questionnaires obtained before study enrollment. Specificity was estimated by measuring the percent negativity for QFT-Plus and QFT-GIT. Overall, these studies showed comparable specificity between QFT-Plus and QFT-GIT (Table 4). The specificity of QFT-Plus was slightly lower than that of QFT-GIT in some studies, but the difference was not statistically significant, and no clear pattern has emerged in these studies. One study showed that the specificity of QFT-Plus is not affected by infection with the M. avium complex and the M. abscessus group, the two most common nontuberculous mycobacteria (NTM) (25).
TABLE 4.
Comparison of specificity between QFT-Plus and QFT-GIT among low-risk populationsa
| Study reference | Country | Sample size | Specificity (%) |
||
|---|---|---|---|---|---|
| QFT-Plus | QFT-GIT | Difference (95% CI) | |||
| 25 | USA | 262 non-HCW, including 51 NTM patients | 98.1 | 98.9 | −0.8 (−2.8 to 1.3) |
| 30 | Germany | 77 low-risk HCW | 87.0 | 89.6 | −2.6 (−12.7 to 7.5) |
| 15 | Italy | 19 non-HCW | 100 | 100 | 0.0 (0.0 to 0.0) |
| 16 | South Korea | 27 non-HCW | 92.6 | 100 | −7.4 (−17.3 to 2.5) |
| 31 | Japan | 212 non-HCW | 97.0 | 98.6 | −1.6 (−4.4 to 1.2) |
| 28 | USA | 626 no-risk HCW | 97.0 | 97.9 | −0.9 (−2.6 to 0.8) |
HCW, health care worker; NTM, nontuberculous mycobacteria; CI, confidence interval.
The results of QFT-Plus and QFT-GIT were qualitatively and quantitatively highly concordant in low-risk HCWs (Table 4). The positivity rate in 626 HCWs with no risk factors for LTBI was 3.0% for QFT-Plus, using the manufacturer’s interpretation, whereas it was 2.1% for QFT-GIT. CDC recently withdrew the recommendation for serial TB screening by IGRAs in low-risk HCWs due to high conversion and reversion rates and false-positive rates higher than those of TST (26, 27). Moon and colleagues have proposed a conservative interpretation of QFT-Plus, based on the positivity of both TB1 and TB2, versus the manufacturer’s interpretation, where either tube can be positive, to increase the specificity of the assay for low-risk HCWs (28). Application of this approach led to a reduction in the positivity rate in no-risk HCWs from 3.0% to 1.0%. Follow-up testing of 11 HCWs with discordant results between TB1 and TB2 in QFT-Plus showed a reversion to negative results in 10 cases and no progression to active TB in any of the participants. If confirmed in other studies, the conservative interpretation of QFT-Plus in low-risk populations may represent a viable approach to identifying false-positive results in low-risk individuals without the need for repeat testing.
SOURCES OF VARIABILITY
The sources of variability impacting IGRAs are classified into preanalytical, analytical, postanalytical, manufacturing, and immunological (3). Although sources of variability were largely investigated and described for QFT-GIT, these might apply to QFT-Plus as well. Further research and modeling are needed to investigate and quantify the variability introduced from known sources due to addition of the second antigen tube in QFT-Plus. Agarwal and colleagues have recently identified a previously unrecognized source of variability for QFT-Plus due to the method of blood collection (29). Blood was collected directly in QFT-Plus tubes (plus-direct) and also in a separate blood collection tube, from which blood was transferred to the QFT-Plus tubes (plus-transfer). The positive rate for plus-direct was 12%, whereas it was 17% for the plus-transfer method. Agreement between plus-direct and plus-transfer was 85% (kappa coefficient, 0.37; P < 0.001). This finding supports the variability in QFT-Plus and highlights the need for consistent blood collection methods in individuals undergoing serial testing.
PREDICTIVE VALUE OF QFT-PLUS
No study on the predictive value of a positive QFT-Plus result on the progression from latent infection to active TB has yet been published. Two studies evaluating the prognostic performance of QFT-Plus, The Correlate of Risk Targeted Intervention Study in HIV-uninfected individuals (CORTIS-01) (https://clinicaltrials.gov/ct2/show/NCT02735590) and HIV-infected individuals (CORTIS-HR), (https://zivahub.uct.ac.za/articles/CORTIS-HR_Statistical_Analysis_Plan/11792079), have recently been completed in South Africa. Findings from these trials are currently being analyzed and should be published soon.
CONCLUSION
Although QFT-Plus was launched with the promise of improved performance over QFT-GIT through the addition of the CD8 T-cell response, studies directly comparing QFT-Plus with QFT-GIT in TB patients, high-risk groups, and low-risk populations have not revealed any significant improvement in its performance. Further research in immunocompromised individuals and children is needed to determine the performance of QFT-Plus in these groups.
UNANSWERED QUESTIONS
Although the studies described in this minireview have advanced our understanding of the performance of QFT-Plus, there are a number of questions that remain unanswered. The following represent areas in need of further research to complete our understanding of QFT-Plus.
1. Sensitivity in HIV-coinfected individuals. A head-to-head comparison of QFT-Plus to QFT-GIT with adjustment for the CD4 count is needed in patients with active TB and HIV infection to assess whether QFT-Plus has a higher sensitivity in HIV-coinfected individuals.
2. Sensitivity in children. A head-to-head comparison of QFT-Plus to QFT-GIT with a sufficient number of children with TB disease is needed to accurately assess the sensitivity of QFT-Plus in this group.
3. Predictive value. A head-to-head comparison of QFT-Plus to QFT-GIT is needed to determine the predictive value of QFT-Plus for progression to active TB. Studies are also needed to determine whether the CD8 T-cell response derived from QFT-Plus can accurately identify patients with a recent and high-intensity exposure, who are at a greater risk of progressing to active TB.
4. Conservative interpretation. Further studies are needed to validate the conservative interpretation of QFT-Plus in low-risk populations and to define quantitative cutoffs that enhance its accuracy.
5. Reproducibility. Further research is needed to assess the reproducibility of QFT-Plus and investigate the sources of variabilities introduced with the addition of the second tube and the reformulation of peptide antigens.
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