To the Editor: Screening for latent tuberculosis infection (LTBI) among recent tuberculosis (TB) contacts is an important component of TB control, particularly in settings with low TB incidence aiming toward pre-elimination (1). However, currently available LTBI diagnostics lack sensitivity and have poor predictive value for incident TB (2–8). Consequently, prevention of one incident TB case requires treatment of many individuals for LTBI. This is true for both interferon-γ release assays (IGRAs) and the tuberculin skin test (TST). A recent evaluation found that QuantiFERON Gold-In-Tube (QFT-GIT; Qiagen) and T-SPOT.TB (Oxford Immunotec) performed similarly to the TST when a Bacillus Calmette–Guerin vaccination-stratified TST cutoff was used (8).
A new-generation QuantiFERON (QuantiFERON-TB Gold Plus; QFT-Plus) was recently launched, adding a second TB antigen tube (TB2) that incorporates short peptides designed to stimulate a CD8+ T-cell response, in addition to the CD4+-response tube (TB1) included in previous versions. The proposed rationale for this is that CD8+ responses have been associated with mycobacterial load and recent TB exposure (9, 10). Initial independent evaluations have suggested that QFT-Plus may have improved test sensitivity in active TB compared with QFT-GIT (11), and that the CD8+-targeted antigen tube response may be associated with proxy measures of the degree of TB exposure among contacts (12). However, no studies have examined the prognostic value of QFT-Plus for predicting incident TB. We aimed to address this key knowledge gap in a prospective cohort of TB contacts in the United Kingdom.
Methods
We recruited adult (≥16 yr old) contacts of pulmonary and extrapulmonary TB index cases from 10 London TB clinics while attending for routine contact screening (July 7, 2015 to November 22, 2016). Participants completed a questionnaire and underwent blood sampling for QFT-Plus (at least 6 weeks from the last known TB exposure). Contacts with evidence of prevalent TB disease (defined as TB diagnosed within 21 days of enrollment, as per previous work [8]) and those who accepted preventive therapy (offered in accordance with contemporary national guidance [13, 14]) were excluded from the analysis. The study was approved by the UK National Research Ethics Service (ref: 14/EM/1208).
Participants were linked to national TB surveillance records held by Public Health England, including all statutory TB notifications, to identify those notified with TB (until December 31, 2017). TB notifications were validated by local record review and included those with culture-confirmed TB or a clinical diagnosis with radiological or histological evidence of TB, for which a clinician had prescribed a full course of anti-TB treatment.
The QFT-Plus results were interpreted according to the manufacturer’s guidance, with TB antigen responses calculated as TB antigen interferon-γ minus unstimulated control interferon-γ. We calculated incidence rates and rate ratios (IRRs) relative to the negative test category, along with sensitivity, specificity, and predictive values, including the full duration of follow-up.
To assess the incremental value of adding the CD8+-stimulating tube in predicting incident TB cases, we compared receiver operating characteristic (ROC) curves and area under the curve (AUC) values obtained using TB1 only, TB2 only, and the maximal TB antigen tube (higher of TB1 and TB2). We also plotted a ROC curve for the calculated difference between the TB1 and TB2 tubes (TB2 − TB1) as a surrogate for the CD8+-specific response, as it has been hypothesized that this may identify contacts with recently acquired Mycobacterium tuberculosis infection, who are at the highest risk of TB disease (12).
Results
We recruited a total of 623 contacts, 532 (85.4%) of whom had QFT-Plus results (89 missing and 2 indeterminate) and were followed for a median 1.93 years (interquartile range [IQR] 1.65–2.21 yr). QFT-Plus results were positive in 180 of the 532 contacts (33.8%) (Table 1), and 39 (21.7%) of these participants commenced preventive therapy. One patient was notified with prevalent TB (3 days after recruitment). A total of 492 participants were therefore included in the analysis. The included and excluded participants had similar baseline characteristics, except that those who commenced preventive therapy were younger than those who did not (Table 1).
Table 1.
Baseline characteristics of the study cohort, stratified by Quantiferon-TB Gold Plus results and provision of preventive therapy
| QFT-Plus Negative* | QFT-Plus Positive |
QFT-Plus Missing† | All | ||
|---|---|---|---|---|---|
| No PT* | PT | ||||
| Age | |||||
| Median (IQR) | 31 (25–43) | 43 (32–54) | 30 (26–35) | 31.5 (23.7–49) | 33 (25–46) |
| Missing | 3 (0.9) | 2 (1.4) | 0 (0) | 1 (1.1) | 6 (1) |
| Sex | |||||
| Male | 165 (46.9) | 76 (53.9) | 24 (61.5) | 37 (40.7) | 302 (48.5) |
| Female | 180 (51.1) | 62 (44) | 15 (38.5) | 51 (56) | 308 (49.4) |
| Missing | 7 (2) | 3 (2.1) | 0 (0) | 3 (3.3) | 13 (2.1) |
| Ethnicity | |||||
| White | 95 (27) | 27 (19.1) | 9 (23.1) | 31 (34.1) | 162 (26) |
| South Asian | 117 (33.2) | 55 (39) | 13 (33.3) | 33 (36.3) | 218 (35) |
| Black African or Caribbean | 67 (19) | 30 (21.3) | 7 (17.9) | 15 (16.5) | 119 (19.1) |
| Other | 63 (17.9) | 24 (17) | 10 (25.6) | 9 (9.9) | 106 (17) |
| Missing | 10 (2.8) | 5 (3.5) | 0 (0) | 3 (3.3) | 18 (2.9) |
| UK born | |||||
| No | 235 (66.8) | 126 (89.4) | 33 (84.6) | 66 (72.5) | 460 (73.8) |
| Yes | 111 (31.5) | 11 (7.8) | 6 (15.4) | 24 (26.4) | 152 (24.4) |
| Missing | 6 (1.7) | 4 (2.8) | 0 (0) | 1 (1.1) | 11 (1.8) |
| Contact type | |||||
| Household | 210 (59.7) | 96 (68.1) | 30 (76.9) | 49 (53.8) | 385 (61.8) |
| Family nonhousehold | 19 (5.4) | 7 (5) | 2 (5.1) | 3 (3.3) | 31 (5) |
| Work or social | 62 (17.6) | 19 (13.5) | 4 (10.3) | 14 (15.4) | 99 (15.9) |
| Other | 13 (3.7) | 3 (2.1) | 2 (5.1) | 2 (2.2) | 20 (3.2) |
| Missing | 48 (13.6) | 16 (11.3) | 1 (2.6) | 23 (25.3) | 88 (14.1) |
| Diabetes | |||||
| No | 318 (90.3) | 120 (85.1) | 38 (97.4) | 83 (91.2) | 559 (89.7) |
| Yes | 20 (5.7) | 18 (12.8) | 0 (0) | 6 (6.6) | 44 (7.1) |
| Missing | 14 (4) | 3 (2.1) | 1 (2.6) | 2 (2.2) | 20 (3.2) |
| HIV | |||||
| No | 331 (94) | 137 (97.2) | 37 (94.9) | 84 (92.3) | 589 (94.5) |
| Yes | 4 (1.1) | 0 (0) | 1 (2.6) | 2 (2.2) | 7 (1.1) |
| Missing | 17 (4.8) | 4 (2.8) | 1 (2.6) | 5 (5.5) | 27 (4.3) |
| Follow-up, yr | |||||
| Median (IQR) | 1.94 (1.64–2.21) | 1.92 (1.66–2.21) | 1.85 (1.67–2.25) | 1.56 (1.25– 2.06) | 1.88 (1.58–2.20) |
| Total | 352 | 141 | 39 | 91 | 623 |
Definition of abbreviations: HIV = human immunodeficiency virus; IQR = interquartile range; PT = preventive therapy; QFT-Plus = QuantiFERON-TB Gold Plus.
Data are presented as n (%) unless stated otherwise.
Included in the primary analysis.
Includes two patients with indeterminate QFT-Plus results.
Ten patients with incident TB were notified during follow-up (median 222 days after recruitment; range 90–688). Among these patients, the median age was 27 (IQR 21–33), three (30%) were female, the majority (7/10; 70%) were of black African or South Asian ethnicity, and all were non-UK born. All 10 patients completed at least 6 months of TB therapy. Two of these cases (20.0%) were pulmonary in site (both were culture confirmed), and eight were exclusively extrapulmonary (two of which were culture confirmed). One participant with TB was diabetic; the remaining patients with TB were not immunocompromised, and none were infected with human immunodeficiency virus. The TB incidence rates (per 1,000 person-years) were 30.6 (95% confidence interval [CI], 15.3–61.1) and 3.0 (95% CI, 0.8–12.1) in the QFT-Plus–positive and –negative groups, respectively (IRR, 10.1 [95% CI, 2.2–47.7]). The sensitivity of QFT-Plus for incident TB was 80.0% (95% CI, 44.4–97.5). The positive predictive value (PPV) and negative predictive value were 5.7% (95% CI, 2.5–10.9) and 99.4% (95% CI, 98.0–99.9), respectively. The characteristics of QFT-Plus for predicting microbiologically confirmed TB cases are reported in Table 2.
Table 2.
Incidence rates, rate ratios, and predictive values for incident tuberculosis during follow-up, stratified by Quantiferon-TB Gold Plus results
| QFT-Plus Positive | QFT-Plus Negative | |
|---|---|---|
| No. of TB cases (microbiologically confirmed and/or clinically diagnosed) | 8 | 2 |
| Participants | 140 | 352 |
| Person-years | 261.6 | 663.0 |
| Incidence rate per 1,000 person-years | 30.6 (15.3–61.1) | 3.0 (0.8–12.1) |
| Incidence rate ratio | 10.1 (2.2–47.7) |
|
| Positive predictive value | 5.7 (2.5–10.9) |
|
| Negative predictive value | 99.4 (98–99.9) |
|
| Sensitivity | 80.0 (44.4–97.5) |
|
| Specificity | 72.6 (68.4–76.5) |
|
| |
||
| No. of TB cases (microbiologically confirmed only) | 3 | 1 |
| Incidence rate per 1,000 person-years | 11.5 (3.7–35.6) | 1.5 (0.2–10.7) |
| Incidence rate ratio | 7.6 (0.8–73.1) |
|
| Positive predictive value | 2.1 (0.4–6.1) |
|
| Negative predictive value | 99.7 (98.4–100) |
|
| Sensitivity | 75.0 (19.4–99.4) |
|
| Specificity | 71.9 (67.7–75.9) | |
Definition of abbreviations: QFT-Plus = QuantiFERON-TB Gold Plus; TB = tuberculosis.
ROC curves for prediction of incident TB during all follow-up were similar for the TB1, TB2, and maximal TB antigen responses (AUC 0.80–0.82; Figure 1A). TB2 minus TB1, however, did not discriminate TB progressors from nonprogressors (AUC 0.44 [95% CI, 0.20–0.68]). There was a very strong correlation between the TB1 and TB2 interferon-γ responses (r = 0.993; P < 0.001; Figure 1B).
Figure 1.
(A) Receiver operating characteristic curves showing the performance of QuantiFERON-TB Gold Plus for predicting incident tuberculosis during the duration of follow-up, stratified by antigen tube interferon-γ responses. TB max = higher of the CD4+-response tube (TB1) and CD8+-response tube (TB2). AUC = area under the curve (95% confidence interval). (B) Scatterplot showing association of interferon-γ responses in the TB1 and TB2 tubes.
Discussion
We found that the performance of QFT-Plus appeared to be comparable to that previously reported in evaluations of QFT-GIT and T-SPOT.TB, with an IRR of 10.1, 80% sensitivity for detection of incident TB, and an overall PPV for incident TB of 5.7% (8). Interferon-γ responses in the TB1 and TB2 tubes were strongly correlated, and ROC curves showed a minimal difference between them for predicting incident TB. As a result, the calculated difference between TB1 and TB2 responses, as a proxy for the CD8-specific response, did not predict incident TB. However, despite our sample size of 492 recent TB contacts, the number of TB progressors was small, reflecting a low progression risk even among contacts. Thus, a larger-scale study is indicated to investigate subtle differences in the relative prognostic contributions of the TB1 and TB2 antigen tubes.
This is the first evaluation of the prognostic value of the QFT-Plus test. The prospective design allowed the collection of detailed clinical, demographic, and laboratory data. We recruited participants while they were receiving routine contact-tracing services, to ensure that the study population would be representative of TB contacts. Therefore, our findings are likely generalizable to other low-incidence settings globally. Moreover, follow-up was robust through linkage to national surveillance records using a validated matching algorithm (15), minimizing the risk of missing incident TB cases.
A limitation of this study is that the provision of preventive therapy to a subset of the QFT-Plus–positive patients could have led to selection bias. However, although the patients who received preventive therapy were younger than those who did not (reflecting national policy at the time of the study [13, 14]), other characteristics were similar between the two groups, suggesting that the impact of this bias was likely small. Second, the TB contacts included both pulmonary and extrapulmonary index cases, reflecting national contact screening policy during the study period (13). The PPV of QFT-Plus may be higher among populations that include only pulmonary TB contacts, owing to a higher pretest probability of incident TB. However, previous evaluations of QFT-GIT and T-SPOT.TB also included extrapulmonary contacts, which allows the current findings to be put into this context (8). Third, we did not perform serial testing (before and after exposure), so we were unable to assess QFT-Plus conversions over time, which may provide a more reliable measure of recent M. tuberculosis infection. This reflects the reality of contact screening practices—the ability of assays to accurately stratify TB risk from a single baseline test is therefore a key attribute. The absence of serial testing also means that participants who developed incident TB may have been reexposed to M. tuberculosis during the interval between testing and disease onset, although the overall risk of exposure in the United Kingdom (a low-TB-incidence setting) is likely small. Fourth, QFT-Plus results were missing or indeterminate for 91 of 623 patients (14.6%), in keeping with the proportion of missing results for other IGRAs in our recent evaluation (8). However, these patients’ characteristics were similar to those of the overall study population, suggesting that the risk of subsequent selection bias was likely small. Finally, we included both microbiologically confirmed and clinically diagnosed TB cases in our outcome definition, in keeping with previous IGRA evaluations (3–8). The rationale for this is that extrapulmonary TB, which accounts for a large proportion of TB cases in foreign-born people living in the United Kingdom (16), is often challenging to prove microbiologically. However, all patients who received a diagnosis of TB during the study received a full course of TB therapy, and none were denotified. It is therefore likely that these represented true TB cases, with a low risk of outcome misclassification.
In summary, in this first evaluation of the predictive value of QFT-Plus for incident TB, we found that its performance was comparable to that of other commercial IGRAs. Better biomarkers are required to transform management of TB contacts.
Supplementary Material
Acknowledgments
Acknowledgment
The authors thank the study administrators, laboratory staff who performed the tests, and clinical and nursing colleagues who contributed to participant recruitment. They also thank Qiagen for donating the QFT-Plus test kits, although it had no other role in the study design, conduct, analysis, or interpretation.
Footnotes
Supported by the UK National Institute for Health Research (NIHR) (DRF-2018-11-ST2-004 to R.K.G., and SRF-2011-04-001 and NF-SI-0616-10037 to I.A.). M.N. is supported by the Wellcome Trust (207511/Z/17/Z) and by NIHR Biomedical Research Funding to University of College London and University College London Hospitals. This paper presents independent research supported by the NIHR. The views expressed are those of the authors and not necessarily those of the National Health Service, the NIHR, or the Department of Health and Social Care.
Author Contributions: R.K.G. performed the analyses presented and wrote the first draft of the manuscript. I.A. conceived and designed the study with support from M.L., M.N., and C.J. J.S. and I.A. led recruitment and follow-up of participants with all site principal investigators, including H.K., M.L., and S.L. A.I. recruited and followed participants. All authors approved the final submitted version of the manuscript.
Author disclosures are available with the text of this letter at www.atsjournals.org.
References
- 1.World Health Organization. Latent TB infection: updated and consolidated guidelines for programmatic management. Geneva: WHO; 2018 [accessed 2018 Aug 15]. Available from: http://www.who.int/tb/publications/2018/latent-tuberculosis-infection/en/
- 2.Pai M, Denkinger CM, Kik SV, Rangaka MX, Zwerling A, Oxlade O, et al. Gamma interferon release assays for detection of Mycobacterium tuberculosis infection. Clin Microbiol Rev. 2014;27:3–20. doi: 10.1128/CMR.00034-13. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Zellweger JP, Sotgiu G, Block M, Dore S, Altet N, Blunschi R, et al. TBNET. Risk assessment of tuberculosis in contacts by IFN-γ release assays. A Tuberculosis Network European Trials Group study. Am J Respir Crit Care Med. 2015;191:1176–1184. doi: 10.1164/rccm.201502-0232OC. [DOI] [PubMed] [Google Scholar]
- 4.Sloot R, Schim van der Loeff MF, Kouw PM, Borgdorff MW. Risk of tuberculosis after recent exposure. A 10-year follow-up study of contacts in Amsterdam. Am J Respir Crit Care Med. 2014;190:1044–1052. doi: 10.1164/rccm.201406-1159OC. [DOI] [PubMed] [Google Scholar]
- 5.Haldar P, Thuraisingam H, Patel H, Pereira N, Free RC, Entwisle J, et al. Single-step QuantiFERON screening of adult contacts: a prospective cohort study of tuberculosis risk. Thorax. 2013;68:240–246. doi: 10.1136/thoraxjnl-2011-200956. [DOI] [PubMed] [Google Scholar]
- 6.Erkens CGM, Slump E, Verhagen M, Schimmel H, Cobelens F, van den Hof S. Risk of developing tuberculosis disease among persons diagnosed with latent tuberculosis infection in the Netherlands. Eur Respir J. 2016;48:1420–1428. doi: 10.1183/13993003.01157-2016. [DOI] [PubMed] [Google Scholar]
- 7.Diel R, Loddenkemper R, Niemann S, Meywald-Walter K, Nienhaus A. Negative and positive predictive value of a whole-blood interferon-γ release assay for developing active tuberculosis: an update. Am J Respir Crit Care Med. 2011;183:88–95. doi: 10.1164/rccm.201006-0974OC. [DOI] [PubMed] [Google Scholar]
- 8.Abubakar I, Drobniewski F, Southern J, Sitch AJ, Jackson C, Lipman M, et al. PREDICT Study Team. Prognostic value of interferon-γ release assays and tuberculin skin test in predicting the development of active tuberculosis (UK PREDICT TB): a prospective cohort study. Lancet Infect Dis. 2018;18:1077–1087. doi: 10.1016/S1473-3099(18)30355-4. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Day CL, Abrahams DA, Lerumo L, Janse van Rensburg E, Stone L, O’rie T, et al. Functional capacity of Mycobacterium tuberculosis-specific T cell responses in humans is associated with mycobacterial load. J Immunol. 2011;187:2222–2232. doi: 10.4049/jimmunol.1101122. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10.Nikolova M, Markova R, Drenska R, Muhtarova M, Todorova Y, Dimitrov V, et al. Antigen-specific CD4- and CD8-positive signatures in different phases of Mycobacterium tuberculosis infection. Diagn Microbiol Infect Dis. 2013;75:277–281. doi: 10.1016/j.diagmicrobio.2012.11.023. [DOI] [PubMed] [Google Scholar]
- 11.Barcellini L, Borroni E, Brown J, Brunetti E, Codecasa L, Cugnata F, et al. First independent evaluation of QuantiFERON-TB Plus performance. Eur Respir J. 2016;47:1587–1590. doi: 10.1183/13993003.02033-2015. [DOI] [PubMed] [Google Scholar]
- 12.Barcellini L, Borroni E, Brown J, Brunetti E, Campisi D, Castellotti PF, et al. First evaluation of QuantiFERON-TB Gold Plus performance in contact screening. Eur Respir J. 2016;48:1411–1419. doi: 10.1183/13993003.00510-2016. [DOI] [PubMed] [Google Scholar]
- 13.National Institute for Health and Care Excellence. Tuberculosis: clinical diagnosis and management of tuberculosis, and measures for its prevention and control. London: NICE; March 2011 [accessed 2015 Jan 22] Available from: http://www.nice.org.uk/guidance/cg117/resources/guidance-tuberculosis-pdf. [PubMed]
- 14.National Instititute for Health and Care Excellence. Tuberculosis. London: NICE; January 2016 [updated 2019 Sep; accessed 2017 Sep 5]. Available from: https://www.nice.org.uk/guidance/ng33#.
- 15.Aldridge RW, Shaji K, Hayward AC, Abubakar I. Accuracy of probabilistic linkage using the enhanced matching system for public health and epidemiological studies. PLoS One. 2015;10:e0136179. doi: 10.1371/journal.pone.0136179. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 16.Kruijshaar ME, Abubakar I. Increase in extrapulmonary tuberculosis in England and Wales 1999-2006. Thorax. 2009;64:1090–1095. doi: 10.1136/thx.2009.118133. [DOI] [PubMed] [Google Scholar]
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