Medline ® Abstracts for References 2-14

The identification of individuals with latent tuberculosis infection (LTBI) is useful for both fundamental understanding of the pathogenesis of disease and for clinical and public health interventions (i.e., to prevent progression to disease). Basic research suggests there is a pathogenetic continuum from exposure to infection to disease, and individuals may advance or reverse positions within the spectrum, depending on changes in the host immunity. Unfortunately, there is no diagnostic test that resolves the various stages within the spectrum of Mycobacterium tuberculosis infection. Two main immune-based approaches are currently used for identification of LTBI: the tuberculin skin test (TST) and the interferon-gamma release assay (IGRA). TST can use either the conventional purified protein derivative or more specific antigens. Extensive research suggests that both TST and IGRA represent indirect markers of M. tuberculosis exposure and indicates a cellular immune response to M. tuberculosis. The imperfect concordance between these two tests suggests that neither test is perfect, presumably due to both technical and biological reasons. Neither test can accurately differentiate between LTBI and active TB. Both IGRA and TST have low sensitivity in a variety of immunocompromised populations. Cohort studies have shown that both TST and IGRA have low predictive value for progression from infection to active TB. For fundamental applications, basic research is necessary to identify those at highest risk of disease with a positive TST and/or IGRA. For clinical applications, the identification of such biomarkers can help prioritize efforts to interrupt progression to disease through preventive therapy.

BACKGROUND: Until recently, the tuberculin skin test was the only test for detecting latent tuberculosis (TB) infection, but 2 ex vivo interferon-gamma release assays (IGRAs) are now commercially licensed.

PURPOSE: To estimate sensitivity, specificity, and reproducibility of IGRAs (commercial or research versions of QuantiFERON [QFT]and Elispot) for diagnosing latent TB infection in healthy and immune-suppressed persons.

DATA SOURCES: The authors searched MEDLINE and reviewed citations of all original articles and reviews for studies published in English.

STUDY SELECTION: Studies had evaluated IGRAs using Mycobacterium tuberculosis-specific antigens (RD1 antigens) and overnight (16- to 24-h) incubation times. Reference standards had to be clearly defined without knowledge of test results. DATA EXTRACTION AND QUALITY ASSESSMENT: Specific criteria for quality assessment were developed for sensitivity, specificity, and reproducibility.

DATA SYNTHESIS: When newly diagnosed active TB was used as a surrogate for latent TB infection, sensitivity of all tests was suboptimal, although it was higher with Elispot. No test distinguishes active TB from latent TB. Sensitivity of the tuberculin skin test and IGRAs was similar in persons who were categorized into clinical gradients of exposure. Pooled specificity was 97.7% (95% CI, 96% to 99%) and 92.5% (CI, 86% to 99%) for QFT and for Elispot, respectively. Both assays were more specific than the tuberculin skin test in samples vaccinated with bacille Calmette-Guérin. Elispot was more sensitive than the tuberculin skin test in 3 studies of immune-compromised samples. Discordant tuberculin skin test and IGRA reactions were frequent and largely unexplained, although some may be related to varied definitions of positive test results. Reversion of IGRA results from positive to negative was common in 2 studies in which it was assessed.

LIMITATIONS: Most studies used cross-sectional designs with the inherent limitation of no gold standard for latent TB infection, and most involved small samples with a widely varying likelihood of true-positive and false-positive test results. There is insufficient evidence on IGRA performance in children, immune-compromised persons, and the elderly.

CONCLUSIONS: New IGRAs show considerable promise and have excellent specificity. Additional studies are needed to better define their performance in high-risk populations and in serial testing. Longitudinal studies are needed to define the predictive value of IGRAs.

BACKGROUND: Interferon-gamma-release assays (IGRAs) are alternatives to the tuberculin skin test (TST). A recent meta-analysis showed that IGRAs have high specificity, even among populations that have received bacille Calmette-Guérin (BCG) vaccination. Sensitivity was suboptimal for TST and IGRAs.

PURPOSE: To incorporate newly reported evidence from 20 studies into an updated meta-analysis on the sensitivity and specificity of IGRAs.

DATA SOURCES: PubMed was searched through 31 March 2008, and citations of all original articles, guidelines, and reviews for studies published in English were reviewed.

STUDY SELECTION: Studies that evaluated QuantiFERON-TB Gold, QuantiFERON-TB Gold In-Tube (both from Cellestis, Victoria, Australia), and T-SPOT.TB (Oxford Immunotec, Oxford, United Kingdom) or its precommercial ELISpot version, when data on the commercial version were lacking. For assessing sensitivity, the study sample had to have microbiologically confirmed active tuberculosis. For assessing specificity, the sample had to comprise healthy, low-risk individuals without known exposure to tuberculosis. Studies with fewer than 10 participants and those that included only immunocompromised participants were excluded.

DATA EXTRACTION: One reviewer abstracted data on participant characteristics, test characteristics, and test performance from 38 studies; these data were double-checked by a second reviewer. The original investigators were contacted for additional information when necessary.

DATA SYNTHESIS: A fixed-effects meta-analysis with correction for overdispersion was done to pool data within prespecified subgroups. The pooled sensitivity was 78% (95% CI, 73% to 82%) for QuantiFERON-TB Gold, 70% (CI, 63% to 78%) for QuantiFERON-TB Gold In-Tube, and 90% (CI, 86% to 93%) for T-SPOT.TB. The pooled specificity for both QuantiFERON tests was 99% among non-BCG-vaccinated participants (CI, 98% to 100%) and 96% (CI, 94% to 98%) among BCG-vaccinated participants. The pooled specificity of T-SPOT.TB (including its precommercial ELISpot version) was 93% (CI, 86% to 100%). Tuberculin skin test results were heterogeneous, but specificity in non-BCG-vaccinated participants was consistently high (97% [CI, 95% to 99%]).

LIMITATIONS: Most studies were small and had limitations, including no gold standard for diagnosing latent tuberculosis and variable TST methods and cutoff values. Data on the specificity of the commercial T-SPOT.TB assay were limited.

CONCLUSION: The IGRAs, especially QuantiFERON-TB Gold and QuantiFERON-TB Gold In-Tube, have excellent specificity that is unaffected by BCG vaccination. Tuberculin skin test specificity is high in non-BCG-vaccinated populations but low and variable in BCG-vaccinated populations. Sensitivity of IGRAs and TST is not consistent across tests and populations, but T-SPOT.TB appears to be more sensitive than both QuantiFERON tests and TST.

BACKGROUND: Variability in interferon-gamma release assays (IGRAs) results for tuberculosis has implications for interpretation of results close to the cut-point, and for defining thresholds for test conversion and reversion. However, little is known about the within-subject variability (reproducibility) of IGRAs. Several national guidelines recommend a two-step testing procedure (tuberculin skin test [TST]followed by IGRA) for the diagnosis of LTBI. However, the effect of a preceding TST on subsequent IGRA results has been reported in studies with apparently conflicting results.

METHODOLOGY/FINDINGS: We conducted a systematic review to synthesize evidence on within-subject variability of IGRA results and the potential boosting effect of TST. We searched several databases and reviewed citations of previous reviews on IGRAs. We included studies using commercial IGRAs, in addition to non-commercial versions of the ELISPOT assay. Four studies, fulfilling our predefined criteria, examined within-subject variability and 13 studies evaluated TST effects on subsequent IGRA responses. Meta-analysis was not considered appropriate because of heterogeneity in study methods,assays, and populations. Although based on limited data, within-subject variability was present in all studies but the magnitude varied (16-80%) across studies. A TST induced "boosting" of IGRA responses was demonstrated in several studies and although more pronounced in IGRA-positive (i.e. sensitized) individuals, also occurred in a smaller but not insignificant proportion of IGRA-negative subjects. The TST appeared to affect IGRA responses only after 3 days and may apparently persist for several months, but evidence for this is weak.

CONCLUSIONS/SIGNIFICANCE: Although reproducibility data are scarce, significant within person IGRA variability has been reported. If confirmed in more studies, this has implications for the interpretation of results close to the cut-point and for definition of conversions and reversions. Although the effect of TST on IGRA results is likely to be inconsequential in IGRA-positive subjects, in IGRA-negative subjects, the interpretation of results may be confounded by a preceding TST if administered more than 3 days prior to an IGRA.

BACKGROUND: Clinical roles of QuantiFERON-TB Gold (QFT-G)/Gold in-Tube (QFT-G-IT) and T-SPOT.TB in tuberculosis require clarification.

METHODS: MEDLINE and EMBASE were searched for relevant English papers. Summary estimates of likelihood ratios (LR) of QFT-G/QFT-G-IT and T-SPOT.TB for latent tuberculosis infection (LTBI) and tuberculosis disease in adults were obtained by bivariate and univariate random effects meta-analyses after assessing heterogeneity. Probable ranges of prevalence for LTBI and tuberculosis disease were estimated. Critical values of positive LR (PLR) and negative LR (NLR) corresponding to a 90% certainty threshold were calculated over probable prevalence ranges. It was considered reliable to rule in when the best estimated PLR exceeds the corresponding critical value and to rule out when the best estimated NLR is less than the corresponding critical value.

RESULTS: 35 studies involving predominantly immunocompetent adults were identified. Based on bivariate meta-analysis, PLR (95% CI) for LTBI were 7.9 (3.6 to 17.3) for T-SPOT.TB and 48.1 (19.7 to 117.6) and 10.8 (5.3 to 21.8) for QFT-G/QFT-G-IT based on Japanese and other studies, respectively.Corresponding NLR (95% CI) were 0.10 (0.06 to 0.18), 0.11 (0.07 to 0.18) and 0.23 (0.16 to 0.32). PLR (95% CI) for tuberculosis disease were 3.6 (2.3 to 5.6) for QFT-G, 2.1 (1.1 to 4.0) for QFT-G-IT and 4.7 (2.4 to 9.1) and 2.3 (1.3 to 4.0) for T-SPOT.TB based on studies with mean or median age>47. 1 years and<or = 47.1 years, respectively. Corresponding NLR (95% CI) were 0.18 (0.12 to 0.27), 0.38 (0.22 to 0.68), 0.11 (0.06 to 0.20) and 0.20 (0.10 to 0.40). Estimated prevalence ranges were 10-55% for LTBI and 40-60% for tuberculosis disease.

CONCLUSIONS: At a 90% certainty threshold, LTBI is best diagnosed by QFT-G/QFT-G-IT and excluded by T-SPOT.TB or QFT-G/QFT-G-IT; none can diagnose tuberculosis disease, whereas. T-SPOT.TB can exclude tuberculosis disease among middle-aged and older patients.

n 2005, CDC published guidelines for using the QuantiFERON-TB Gold test (QFT-G) (Cellestis Limited, Carnegie, Victoria, Australia) (CDC. Guidelines for using the QuantiFERON-TB Gold test for detecting Mycobacterium tuberculosis infection, United States. MMWR;54[No. RR-15]:49-55). Subsequently, two new interferon gamma (IFN- gamma) release assays (IGRAs) were approved by the Food and Drug Administration (FDA) as aids in diagnosing M. tuberculosis infection, both latent infection and infection manifesting as active tuberculosis. These tests are the QuantiFERON-TB Gold In-Tube test (QFT-GIT) (Cellestis Limited, Carnegie, Victoria, Australia) and the T-SPOT.TB test (T-Spot) (Oxford Immunotec Limited, Abingdon, United Kingdom). The antigens, methods, and interpretation criteria for these assays differ from those for IGRAs approved previously by FDA. For assistance in developing recommendations related to IGRA use, CDC convened a group of experts to review the scientific evidence and provide opinions regarding use of IGRAs. Data submitted to FDA, published reports, and expert opinion related to IGRAs were used in preparing these guidelines. Results of studies examining sensitivity, specificity, and agreement for IGRAs and TST vary with respect to which test is better. Although data on the accuracy of IGRAs and their ability to predict subsequent active tuberculosis are limited, to date, no major deficiencies have been reported in studies involving various populations. This report provides guidance to U.S. public health officials, health-care providers, and laboratory workers for use of FDA-approved IGRAs in the diagnosis of M. tuberculosis infection in adults and children. In brief, TSTs and IGRAs (QFT-G, QFT-GIT, and T-Spot) may be used as aids in diagnosing M. tuberculosis infection. They may be used for surveillance purposes and to identify persons likely to benefit from treatment. Multiple additional recommendations are provided that address quality control, test selection, and medical management after testing. Although substantial progress has been made in documenting the utility of IGRAs, additional research is needed that focuses on the value and limitations of IGRAs in situations of importance to medical care or tuberculosis control. Specific areas needing additional research are listed.

Healthcare workers (HCWs) are at increased risk of exposure to tuberculosis (TB). Traditionally, screening for latent TB infection (LTBI) is done using the tuberculin skin test (TST). Interferon-gamma release assays (IGRAs) are now increasingly being used for diagnosis of LTBI, but their role in HCW screening is unclear. A systematic review was conducted of all IGRA studies in HCWs to summarise their performance in cross-sectional and serial testing settings. By searching four electronic databases and other sources, all available studies using any one of the commercial IGRA assays in HCWs were retrieved and screened. 50 unique studies were identified which met the inclusion criteria including five from high TB incidence settings. Among 24 cross-sectional studies in low TB incidence settings, the pooled prevalence of positive IGRA using either test was significantly lower than for a positive TST. However, in high-incidence settings (n=2) there were no consistent differences in the prevalence of positive tests. IGRAs showed good correlation with occupational risk factors for TB exposure in low-incidence settings. Only 10 studies assessed use of IGRA for serial testing and all showed large variation in the rates of conversions and reversions, with no data suggesting that IGRAs are better at identifying the incidence of new TB infection than the TST. The use of IGRAs instead of TST for one-time screening may result in a lower prevalence of positive tests and fewer HCWs who require LTBI treatment, particularly in low TB incidence settings. However, the use of IGRAs for serial testing is complicated by lack of data on optimum cut-offs for serial testing and unclear interpretation and prognosis of conversions and reversions. Further longitudinal research will be required to inform guidelines on serial testing using IGRAs.

OBJECTIVE: To determine whether interferon-gamma release assays (IGRAs) improve the identification of HIV-infected individuals who could benefit from latent tuberculosis infection therapy.

DESIGN: Systematic review and meta-analysis.

METHODS: We searched multiple databases through May 2010 for studies evaluating the performance of the newest commercial IGRAs (QuantiFERON-TB Gold In-Tube [QFT-GIT]and T-SPOT.TB [TSPOT]) in HIV-infected individuals. We assessed the quality of all studies included in the review, summarized results in prespecified subgroups using forest plots, and where appropriate, calculated pooled estimates using random effects models.

RESULTS: The search identified 37 studies that included 5736 HIV-infected individuals. In three longitudinal studies, the risk of active tuberculosis was higher in HIV-infected individuals with positive versus negative IGRA results. However, the risk difference was not statistically significant in the two studies that reported IGRA results according to manufacturer-recommended criteria. In persons with active tuberculosis (a surrogate reference standard for latent tuberculosis infection), pooled sensitivity estimates were heterogeneous but higher for TSPOT (72%; 95% confidence interval [CI], 62-81%) than for QFT-GIT (61%; 95% CI, 47-75%) in low-/middle-income countries. However, neither IGRA was consistently more sensitive than the tuberculin skin test in head-to-head comparisons. Although TSPOT appeared to be less affected by immunosuppression than QFT-GIT and the tuberculin skin test, overall, differences among the three tests were small or inconclusive.

CONCLUSIONS: Current evidence suggests that IGRAs perform similarly to the tuberculin skin test at identifying HIV-infected individuals with latent tuberculosis infection. Given that both tests have modest predictive value and suboptimal sensitivity, the decision to use either test should be based on country guidelines and resource and logistic considerations.

BACKGROUND: The utility of interferon gamma release assays (IGRAs) has been assessed in adults, but remains unclear in children. We reviewed the literature on the use of a commercial IGRA in immunocompetent children for the diagnosis of both latent tuberculosis infection (LTBI) and TB disease.

METHODS: We searched PubMed for studies published before January 2010 on the diagnosis of TB in children using an IGRA. We compared the specificity and sensitivity of the tuberculin skin test (TST) and the IGRA for LTBI and conducted a random effects meta-analysis on sensitivity of the IGRA for TB disease.

RESULTS: Of 68 studies identified, 20 were included in this review. There was increased specificity of the IGRA for LTBI in children compared with TST, but varying sensitivities. Sensitivity of the IGRA in detecting TB disease in children also varied when compared with TST (meanκscore, 0.57). For all TB cases, the pooled sensitivity was 66% (95% confidence interval[CI], 53%-78%) with heterogeneity (I²= 74.8%). Stratification by background TB incidence highlighted a significantly reduced IGRA sensitivity of 55% (95% CI, 37%-73%) in high incidence settings when compared with low incidence settings, 70% (95% CI, 53%-84%).

CONCLUSIONS: There was no clear evidence that IGRAs should replace TST for detecting LTBI in children. Sensitivity of the IGRA for TB disease was no different from TST, and a significantly reduced IGRA sensitivity was found in high-burden TB settings compared with low-burden TB settings. Further studies are needed to determine the value of IGRAs in LTBI and TB disease diagnosis in children.

PURPOSE OF REVIEW: To provide a narrative synthesis of evidence on interferon-gamma release assays (IGRAs) for the diagnosis of latent tuberculosis infection (LTBI) in individuals with immune-mediated inflammatory disorders (IMIDs).

RECENT FINDINGS: Only a few studies have evaluated IGRAs in IMIDs, and most were small and varied considerably with respect to the use of immunosuppressive medications and types of IMIDs. Current evidence does not clearly suggest that IGRAs are better than tuberculin skin test (TST) in identifying individuals with IMID who could benefit from LTBI treatment. To date, no studies have been done on the predictive value of IGRAs in IMID patients. Important questions remain unanswered as to the impact of immunosuppressive medications and the impact of type of IMID on IGRA performance.

SUMMARY: Despite the lack of clear evidence, there is an increasing tendency for guidelines to prefer IGRA over TST in IMIDs or to recommend both TST and IGRA to enhance sensitivity. We believe the use of eithertest is acceptable for LTBI screening. Clinicians could consider starting with IGRAs in individuals with a history of Bacille Calmette-Guérin (BCG) vaccination after infancy or with repeated BCG vaccinations. When the index of suspicion for LTBI is high, both IGRA and TST could be performed, especially prior to initiating TNF-αinhibitor therapy. Regardless of the test used, it is important to remember that in the face of immune-suppression, both IGRA and TST can be falsely negative and are thus only diagnostic aids - they will need to be interpreted with other clinical and risk factor data.

BACKGROUND: The diagnostic value of interferon-γrelease assays (IGRAs) for active tuberculosis in low- and middle-income countries is unclear.

METHODS: We searched multiple databases for studies published through May 2010 that evaluated the diagnostic performance of QuantiFERON-TB Gold In-Tube (QFT-GIT) and T-SPOT.TB (T-SPOT) among adults with suspected active pulmonary tuberculosis or patients with confirmed cases in low- and middle-income countries. We summarized test performance characteristics with use of forest plots, hierarchical summary receiver operating characteristic (HSROC) curves, and bivariate random effects models.

RESULTS: Our search identified 789 citations, of which 27 observational studies (17 QFT-GIT and 10 T-SPOT) evaluating 590 human immunodeficiency virus (HIV)-uninfected and 844 HIV-infected individuals met inclusion criteria. Among HIV-infected patients, HSROC/bivariate pooled sensitivity estimates (highest quality data) were 76% (95% confidence interval [CI], 45%-92%) for T-SPOT and 60%(95% CI, 34%-82%) for QFT-GIT. HSROC/bivariate pooled specificity estimates were low for both IGRA platforms among all participants (T-SPOT, 61% [95% CI, 40%-79%]; QFT-GIT, 52% [95% CI, 41%-62%]) and among HIV-infected persons (T-SPOT, 52% [95% CI, 40%-63%]; QFT-GIT, 50% [95% CI, 35%-65%]). There was no consistent evidence that either IGRA was more sensitive than the tuberculin skin test for active tuberculosis diagnosis.

CONCLUSIONS: In low- and middle-income countries, neither the tuberculin skin test nor IGRAs have value for active tuberculosis diagnosis in adults, especially in the context of HIV coinfection.