Within-subject interlaboratory variability of QuantiFERON-TB gold in-tube tests

Quantum Dot-Based Nanosensors for In Vitro Detection of Mycobacterium tuberculosis

Despite the existing effective treatment methods, tuberculosis (TB) is the second most deadly infectious disease, its carriers in the latent and active phases accounting for more than 20% of the world population. An effective method for controlling TB and reducing TB mortality is regular population screening aimed at diagnosing the latent form of TB and taking preventive and curative measures. Numerous methods allow diagnosing TB by directly detecting Mycobacterium tuberculosis (M.tb) biomarkers, including M.tb DNA, proteins, and specific metabolites or antibodies produced by the host immune system in response to M.tb. PCR, ELISA, immunofluorescence and immunochemical analyses, flow cytometry, and other methods allow the detection of M.tb biomarkers or the host immune response to M.tb by recording the optical signal from fluorescent or colorimetric dyes that are components of the diagnostic systems. Current research in biosensors is aimed at increasing the sensitivity of detection, a promising approach being the use of fluorescent quantum dots as brighter and more photostable optical tags. Here, we review current methods for the detection of M.tb biomarkers using quantum dot-based nanosensors and summarize data on the M.tb biomarkers whose detection can be made considerably more sensitive by using these sensors.

Enhancing the interferon-γ release assay through omission of nil and mitogen values

PURPOSE

To address the limited utility of the interferon (IFN)-γ release assay (IGRA) caused by its variability and inconsistency.

METHODS

This retrospective cohort study was based on data obtained between 2011 and 2019. QuantiFERON-TB Gold-In-Tube was used to measure IFN-γ levels in nil, tuberculosis (TB) antigen, and mitogen tubes.

RESULTS

Of 9,378 cases, 431 had active TB. The non-TB group comprised 1,513 IGRA-positive, 7,202 IGRA-negative, and 232 IGRA-indeterminate cases. Nil-tube IFN-γ levels were significantly higher in the active TB group (median = 0.18 IU/mL; interquartile range: 0.09-0.45 IU/mL) than in the IGRA-positive non-TB (0.11 IU/mL; 0.06-0.23 IU/mL) and IGRA-negative non-TB (0.09 IU/mL; 0.05-0.15 IU/mL) groups (P < 0.0001). From receiver operating characteristic analysis, TB antigen tube IFN-γ levels had higher diagnostic utility for active TB than TB antigen minus nil values. In a logistic regression analysis, active TB was the main driver of higher nil values. In the active TB group, after reclassifying the results based on a TB antigen tube IFN-γ level of 0.48 IU/mL, 14/36 cases with negative results and 15/19 cases with indeterminate results became positive, while 1/376 cases with positive results became negative. Overall, the sensitivity for detecting active TB improved from 87.2 to 93.7%.

CONCLUSION

The results of our comprehensive assessment can aid in IGRA interpretation. Since nil values are governed by TB infection rather than reflecting background noise, TB antigen tube IFN-γ levels should be used without subtracting nil values. Despite indeterminate results, TB antigen tube IFN-γ levels can be informative.

Screening for Latent Tuberculosis Infection in Adults: Updated Evidence Report and Systematic Review for the US Preventive Services Task Force

Importance

Latent tuberculosis infection (LTBI) can progress to active tuberculosis disease, causing morbidity and mortality.

Objective

To review the evidence on benefits and harms of screening for and treatment of LTBI in adults to inform the US Preventive Services Task Force (USPSTF).

Data Sources

PubMed/MEDLINE, Cochrane Library, and trial registries through December 3, 2021; references; experts; literature surveillance through January 20, 2023.

Study Selection

English-language studies of LTBI screening, LTBI treatment, or accuracy of the tuberculin skin test (TST) or interferon-gamma release assays (IGRAs). Studies of LTBI screening and treatment for public health surveillance or disease management were excluded.

Data Extraction and Synthesis

Dual review of abstracts, full-text articles, and study quality; qualitative synthesis of findings; meta-analyses conducted when a sufficient number of similar studies were available.

Main Outcomes and Measures

Screening test accuracy; development of active tuberculosis disease, transmission, quality of life, mortality, and harms.

Results

A total of 113 publications were included (112 studies; N = 69 009). No studies directly evaluated the benefits and harms of screening. Pooled estimates for sensitivity of the TST were 0.80 (95% CI, 0.74-0.87) at the 5-mm induration threshold, 0.81 (95% CI, 0.76-0.87) at the 10-mm threshold, and 0.60 (95% CI, 0.46-0.74) at the 15-mm threshold. Pooled estimates for sensitivity of IGRA tests ranged from 0.81 (95% CI, 0.79-0.84) to 0.90 (95% CI, 0.87-0.92). Pooled estimates for specificity of screening tests ranged from 0.95 to 0.99. For treatment of LTBI, a large (n = 27 830), good-quality randomized clinical trial found a relative risk (RR) for progression to active tuberculosis at 5 years of 0.35 (95% CI, 0.24-0.52) for 24 weeks of isoniazid compared with placebo (number needed to treat, 112) and an increase in hepatotoxicity (RR, 4.59 [95% CI, 2.03-10.39]; number needed to harm, 279). A previously published meta-analysis reported that multiple regimens were efficacious compared with placebo or no treatment. Meta-analysis found greater risk for hepatotoxicity with isoniazid than with rifampin (pooled RR, 4.22 [95% CI, 2.21-8.06]; n = 7339).

Conclusions and Relevance

No studies directly evaluated the benefits and harms of screening for LTBI compared with no screening. TST and IGRAs were moderately sensitive and highly specific. Treatment of LTBI with recommended regimens reduced the risk of progression to active tuberculosis. Isoniazid was associated with higher rates of hepatotoxicity than placebo or rifampin.

Comparing QuantiFERON-TB Gold Plus with QuantiFERON-TB Gold in-tube for diagnosis of latent tuberculosis infection among highly TB exposed gold miners in South Africa

Background: QuantiFERON-TB-Gold-in-tube (QFT-GIT) is an interferon-gamma release assay (IGRA) used to diagnose latent tuberculosis infection. Limited data exists on performance of QuantiFERON-TB Gold-Plus (QFT-Plus), a next generation of IGRA that includes an additional antigen tube 2 (TB2) while excluding TB7.7 from antigen tube 1 (TB1), to measure TB specific CD4+ and CD8+ T lymphocytes responses. We compared agreement between QFT-Plus and QFT-GIT among highly TB exposed goldminers in South Africa. Methods: We enrolled HIV-negative goldminers in South Africa, aged ≥33 years with no prior history of TB disease or evidence of silicosis. Blood samples were collected for QFT-GIT and QFT-Plus. QFT-GIT was considered positive if TB1 tested positive; while QFT-Plus was positive if both or either TB1 or TB2 tested positive, as per manufacturer's recommendations. We compared the agreement between QFT-Plus and QFT-GIT using Cohen's Kappa. To assess the specific contribution of CD8+ T-cells, we used TB2-TB1 differential values as an indirect estimate. A cut-off value was set at 0.6. Logistic regression was used to identify factors associated with having TB2-TB1>0.6 difference on QFT-Plus. Results: Of 349 enrolled participants, 304 had QFT-Plus and QFT-GIT results: 205 (68%) were positive on both assays; 83 (27%) were negative on both assays while 16 (5%) had discordant results. Overall, there was 94.7% (288/304) agreement between QFT-Plus and QFT-GIT (Kappa = 0.87). 214 had positive QFT-Plus result, of whom 202 [94.4%, median interquartile range (IQR): 3.06 (1.31, 7.00)] were positive on TB1 and 205 [95.8%, median (IQR): 3.25 (1.53, 8.02)] were positive on TB2. A TB2-TB1>0.6 difference was observed in 16.4% (35/214), with some evidence of a difference by BMI; 14.9% (7/47), 9.8% (9/92) and 25.3% (19/75) for BMI of 18.5-24.9, 18.5-25 and >30 kg/m 2, respectively (P=0.03). Conclusion: In a population of HIV-negative goldminers, QFT-Plus showed high agreement with QFT-GIT, suggesting similar performance.

Variability of interferon-γ release assays in people at high risk of tuberculosis infection or progression to tuberculosis disease living in the United States

OBJECTIVES

Interferon-γ release assays, including T-SPOT.TB (TSPOT) and QuantiFERON Gold In-Tube (QFT), are important diagnostic tools for tuberculosis infection, but little work has been done to study the performance of these tests in populations prioritized for tuberculosis testing in the United States, especially those other than health care personnel.

METHODS

Participants were enrolled as part of a large, prospective cohort of people at high risk of tuberculosis infection or progression to tuberculosis disease. All participants were administered a tuberculin skin test, TSPOT, and QFT test. A subset of participants had their QFT (n = 919) and TSPOT (n = 885) tests repeated when they returned to get their tuberculin skin test read 2 to 3 days later (repeat study). A total of 531 participants had a TSPOT performed twice on the same sample taken at the same time (split study).

RESULTS

The QFT repeat test interpretations were discordant (one test positive and the other negative) for 6.4% of participants (59 of 919), and the TSPOT tests were discordant for 60 of 885 participants in the repeat study (6.8%) and 41 of 531 participants in the split study (7.7%). There was a high degree of variability in the quantitative test results for both QFT and TSPOT, and discordance was not associated with both test results being near the established cut-offs. Furthermore, the proportion of discordance was similar when comparing participants in both the TSPOT repeat and TSPOT split studies.

DISCUSSION

Both QFT and TSPOT were 6% to 8% discordant. The results should be interpreted with caution, particularly when seeing a conversion or reversion in serial testing.

Comparing the performance of QuantiFERON-TB Gold Plus with QuantiFERON-TB Gold in-tube among highly TB exposed gold miners in South Africa

Background: QuantiFERON-TB-Gold-in-tube (QFT-GIT) is an interferon-gamma release assay (IGRA) used to diagnose latent tuberculosis infection. Limited data exists on performance of QuantiFERON-TB Gold-Plus (QFT-Plus), a next generation of IGRA that includes an additional antigen tube 2 (TB2) while excluding TB7.7 from antigen tube 1 (TB1), to measure TB specific CD4+ and CD8+ T lymphocytes responses. We compared the performance of QFT-Plus with QFT-GIT among highly TB exposed goldminers in South Africa. Methods: We enrolled HIV-negative goldminers in South Africa, aged ≥33 years with no prior history of TB disease or evidence of silicosis. Blood samples were collected for QFT-GIT and QFT-Plus. QFT-GIT was considered positive if TB1 tested positive; while QFT-Plus was positive if both or either TB1 or TB2 tested positive, as per manufacturer's recommendations. We compared the performance of QFT-Plus with QFT-GIT using Cohen’s Kappa. To assess the specific contribution of CD8+ T-cells, we used TB2−TB1 differential values as an indirect estimate. A cut-off value was set at 0.6. Logistic regression was used to identify factors associated with having TB2-TB1>0.6 difference on QFT-Plus. Results: Of 349 enrolled participants, 304 had QFT-Plus and QFT-GIT results: 205 (68%) were positive on both assays; 83 (27%) were negative on both assays while 16 (5%) had discordant results. Overall, there was 94.7% (288/304) agreement between QFT-Plus and QFT-GIT (Kappa = 0.87). 214 had positive QFT-Plus result, of whom 202 [94.4%, median interquartile range (IQR): 3.06 (1.31, 7.00)] were positive on TB1 and 205 [95.8%, median (IQR): 3.25 (1.53, 8.02)] were positive on TB2. A TB2-TB1>0.6 difference was observed in 16.4% (35/214), with some evidence of a difference by BMI; 14.9% (7/47), 9.8% (9/92) and 25.3% (19/75) for BMI of 18.5-24.9, 18.5-25 and >30 kg/m2, respectively (P=0.03). Conclusion: In a population of HIV-negative goldminers, QFT-Plus showed high agreement with QFT-GIT, suggesting similar performance.

Lessons Learned from Implementation of an Interferon Gamma Release Assay to Screen for Latent Tuberculosis Infection in a Large Multicenter Observational Cohort Study in Brazil

The interferon gamma release assay (IGRA) has emerged as a useful tool for identifying latent tuberculosis infection (LTBI). This assay can be performed through testing platforms such as the QuantiFERON-TB Gold Plus (QFT-Plus) assay. This in vitro test has been incorporated into several guidelines worldwide and has recently been considered by the World Health Organization (WHO) for the diagnosis of LTBI. The possibility of systematically implementing IGRAs such as the QFT-Plus assay in centers that perform LTBI screening has been accelerated by the decreased availability of the tuberculin skin test (TST) in several countries. Nevertheless, the process to implement IGRA testing in routine clinical care has many gaps. The study utilized the expertise acquired by the laboratory teams of the Regional Prospective Observational Research in Tuberculosis (RePORT)-Brazil consortium during study protocol implementation of LTBI screening of tuberculosis (TB) close contacts. RePORT-Brazil includes clinical research sites from Brazilian cities and is the largest multicenter cohort of TB close contacts in the country to date. Operational and logistical challenges faced during IGRA implementation in all study laboratories are described, as well as the solutions that were developed and led to the successful establishment of IGRA testing in RePORT-Brazil. The descriptions of the problems identified and resolved in this study can assist laboratories implementing IGRAs, in addition to manufacturers of IGRAs providing effective technical support. This will facilitate the implementation of IGRA testing in countries with large TB burdens, such as Brazil.

IMPORTANCE The IGRA has emerged as a useful tool for identifying persons with LTBI. Although the implementation of IGRAs is of utmost importance, to our knowledge there is scarce information on the identification of logistical and technical challenges for systematic screening for LTBI on a large scale. Thus, the descriptions of the problems identified and resolved in this study can assist laboratories implementing IGRAs, in addition to manufacturers of IGRAs providing effective technical support. This will facilitate the implementation of IGRA testing in countries with large TB burdens, such as Brazil.

Is the New Interferon-Gamma Releasing Assay Beneficial for the Diagnosis of Latent and Active Mycobacterium tuberculosis Infections in Tertiary Care Setting?

Interferon-Gamma Release Assays (IGRAs) are widely used in the laboratory diagnosis of Mycobacterium tuberculosis (MTB) infections, particularly in the latent form. We compared the performance of a newly developed IGRA, the Standard E TB-Feron ELISA (TBF) with the currently used QuantiFERON-TB Gold Plus assay (QFT-Plus) for the detection of latent tuberculosis infections (LTBIs) in tertiary care settings. We also investigated interferon-gamma (IFN-γ) released by T cell subsets via intracellular cytokine staining (ICS) and flow cytometry. A total of 335 subjects including 40 patients with active tuberculosis (ATB), 75 immunocompromised patients with LTBIs (P-LTBI), 70 health care workers with LTBIs (H-LTBI), and 150 healthy controls (HC) were studied. Overall, 168 subjects (50.1%) and 178 subjects (53.1%) displayed IGRA-positive results in the QFT-Plus and TBF, respectively. The overall concordance rate was 94.0%. The sensitivity and specificity of TBF were 88% and 95%, respectively, while the sensitivity and specificity of QFT-Plus were 90% and 100%, respectively. Twenty discordant results (6.0%) were observed in simultaneously performed QFT-Plus and TBF. Particularly, 13 LTBI subjects previously positive QFT-Plus showed negative results in QFT-Plus performed after enrollment. In TBF, six subjects showed positive results while five were negatively concordant with QFT-plus and two were indeterminate. The overall proportion of IFN-γ releasing CD8+ T lymphocytes was significantly higher in TBF compared to those of QFT-Plus TB1 and TB2 (0.21% vs. 0.01% and 0.02%; p-value < 0.05). The recombinant protein antigens in the TBF stimulated TB-specific CD8+ T cells more efficiently. Therefore, TBF would be a useful alternative to current IGRAs such as the QFT-Plus, particularly in tertiary care settings where the immunocompromised patients are subjected to IGRA tests to differentiate MTB infection. Further strategies to analyze the implications of the discrepancies, particularly near the cutoff values between different IGRAs, are needed.

Comparing the performance of QuantiFERON-TB Gold Plus with QuantiFERON-TB Gold in-tube among highly TB exposed gold miners in South Africa

Background: QuantiFERON-TB Gold in-tube (QFT-GIT) is an interferon-gamma release assay (IGRA) used to diagnose latent tuberculosis infection. Limited data exists on performance of QuantiFERON-TB Gold-Plus (QFT-Plus), a next generation of IGRA that includes an additional antigen tube 2 (TB2) while excluding TB7.7 from antigen tube 1 (TB1), to measure TB specific CD4+ and CD8+ T lymphocytes responses. We compared the performance of QFT-Plus with QFT-GIT among highly TB exposed goldminers in South Africa. Methods: We enrolled HIV-negative goldminers in South Africa, ≥33 years with no prior history of TB disease or evidence of silicosis. Blood samples were collected for QFT-GIT and QFT-Plus. QFT-GIT was considered positive if TB1 tested positive; while QFT-Plus was positive if both or either TB1 or TB2 tested positive, as per manufacturer's recommendations. We compared the performance of QFT-Plus with QFT-GIT using Cohen’s Kappa. To assess the specific contribution of CD8+ T-cells, we used TB2−TB1 differential values as an indirect estimate. A cut-off value was set at 0.6. Logistic regression was used to identify factors associated with having TB2-TB1>0.6 difference on QFT-Plus. Results: Of 349 enrolled participants, 304 had QFT-Plus and QFT-GIT results: 205 (68%) were positive on both assays; 83 (27%) were negative on both assays while 16 (5%) had discordant results. Overall, there was 94.7% (288/304) agreement between QFT-Plus and QFT-GIT (Kappa = 0.87). 214 had positive QFT-Plus result, of whom 202 [94.4%, median interquartile range (IQR): 3.06 (1.31, 7.00)] were positive on TB1 and 205 [95.8%, median (IQR): 3.25 (1.53, 8.02)] were positive on TB2. A TB2-TB1>0.6 difference was observed in 16.4% (35/214), with some evidence of a difference by BMI; 14.9% (7/47), 9.8% (9/92) and 25.3% (19/75) for BMI of 18.5-24.9, 18.5-25 and >30 kg/m2, respectively (P=0.03). Conclusion: In a population of HIV-negative goldminers, QFT-Plus showed a similar performance to QFT-GIT.

Tuberculosis preventive treatment: the next chapter of tuberculosis elimination in India

The End TB Strategy envisions a world free of tuberculosis—zero deaths, disease and suffering due to tuberculosis by 2035. This requires reducing the global tuberculosis incidence from >1250 cases per million people to <100 cases per million people within the next two decades. Expanding testing and treatment of tuberculosis infection is critical to achieving this goal. In high-burden countries, like India, the implementation of tuberculosis preventive treatment (TPT) remains a low priority. In this analysis article, we explore potential challenges and solutions of implementing TPT in India. The next chapter in tuberculosis elimination in India will require cost-effective and sustainable interventions aimed at tuberculosis infection. This will require constant innovation, locally driven solutions to address the diverse and dynamic tuberculosis epidemiology and persistent programme monitoring and evaluation. As new tools, regimens and approaches emerge, midcourse adjustments to policy and practice must be adopted. The development and implementation of new tools and strategies will call for close collaboration between local, national and international partners—both public and private—national health authorities, non-governmental organisations, research community and the diagnostic and pharmaceutical industry. Leading by example, India can contribute to global knowledge through operational research and programmatic implementation for combating tuberculosis infection.

Comparison of the QuantiFERON-TB Gold Plus and QuantiFERON-TB Gold In-Tube Interferon Gamma Release Assays in Patients at Risk for Tuberculosis and in Health Care Workers

The QuantiFERON-TB Gold Plus (QFT-Plus; Qiagen, Germantown, MD) interferon gamma release assay (IGRA) received FDA clearance in 2017 and will replace the prior version of the assay, the QFT-Gold In-Tube (QFT-GIT). Here, we compared performances of the QFT-Plus assay and the QFT-GIT version in a diverse patient population, including patients undergoing evaluation for or follow-up of latent tuberculosis infection (LTBI; n = 39) or active TB infection (n = 3), and in health care workers (HCWs; n = 119) at Mayo Clinic (Rochester, MN). Compared to the QFT-GIT, the QFT-Plus assay showed 91.2% (31/34) positive, 98.4% (124/126) negative, and 96.6% (156/161) overall qualitative agreement among the 161 enrolled subjects, with a Cohen's kappa value of 0.91 (excellent interrater agreement). Among the 28 patients diagnosed with LTBI at the time of enrollment, the QFT-GIT and QFT-Plus assays agreed in 24 (85.7%) patients; in all four discordant patients, the positivity of the QFT-GIT or QFT-Plus IGRA was associated with low-level interferon gamma (IFN-γ) reactivity, ranging from 0.36 IU/ml to 0.66 IU/ml. Additionally, we document a high degree of correlation between IFN-γ levels in the QFT-GIT TB antigen tube and each of the two QFT-Plus TB antigen tubes, as well as between the QFT-Plus TB1 and TB2 tubes (Pearson's correlation coefficients [R] > 0.95). Overall, we show comparable results between the QFT-GIT and QFT-Plus assays in our study population composed of subjects presenting with a diverse spectrum of TB infections. Our findings suggest that the necessary transition to the QFT-Plus assay will be associated with a minimal difference in assay performance characteristics.

Incorporation of Social Determinants of Health in the Peer-Reviewed Literature: A Systematic Review of Articles Authored by the National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention

OBJECTIVES

Social determinants of health (SDHs) are the complex, structural, and societal factors that are responsible for most health inequities. Since 2003, the National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention (NCHHSTP) has researched how SDHs place communities at risk for communicable diseases and poor adolescent health. We described the frequency and types of SDHs discussed in articles authored by NCHHSTP.

METHODS

We used the MEDLINE/PubMed search engine to systematically review the frequency and type of SDHs that appeared in peer-reviewed publications available in PubMed from January 1, 2009, through December 31, 2014, with a NCHHSTP affiliation. We chose search terms to identify articles with a focus on the following SDH categories: income and employment, housing and homelessness, education and schooling, stigma or discrimination, social or community context, health and health care, and neighborhood or built environment. We classified articles based on the depth of topic coverage as "substantial" (ie, one of ≤3 foci of the article) or "minimal" (ie, one of ≥4 foci of the article).

RESULTS

Of 862 articles authored by NCHHSTP, 366 (42%) addressed the SDH factors of interest. Some articles addressed >1 SDH factor (366 articles appeared 568 times across the 7 categories examined), and we examined them for each category that they addressed. Most articles that addressed SDHs (449/568 articles; 79%) had a minimal SDH focus. SDH categories that were most represented in the literature were health and health care (190/568 articles; 33%) and education and schooling (118/568 articles; 21%).

CONCLUSIONS

This assessment serves as a baseline measurement of inclusion of SDH topics from NCHHSTP authors in the literature and creates a methodology that can be used in future assessments of this topic.

Practice Guidelines for Clinical Microbiology Laboratories: Mycobacteria

Mycobacteria are the causative organisms for diseases such as tuberculosis (TB), leprosy, Buruli ulcer, and pulmonary nontuberculous mycobacterial disease, to name the most important ones. In 2015, globally, almost 10 million people developed TB, and almost half a million patients suffered from its multidrug-resistant form. In 2016, a total of 9,287 new TB cases were reported in the United States. In 2015, there were 174,608 new case of leprosy worldwide. India, Brazil, and Indonesia reported the most leprosy cases. In 2015, the World Health Organization reported 2,037 new cases of Buruli ulcer, with most cases being reported in Africa. Pulmonary nontuberculous mycobacterial disease is an emerging public health challenge. The U.S. National Institutes of Health reported an increase from 20 to 47 cases/100,000 persons (or 8.2% per year) of pulmonary nontuberculous mycobacterial disease among adults aged 65 years or older throughout the United States, with 181,037 national annual cases estimated in 2014. This review describes contemporary methods for the laboratory diagnosis of mycobacterial diseases. Furthermore, the review considers the ever-changing health care delivery system and stresses the laboratory's need to adjust and embrace molecular technologies to provide shorter turnaround times and a higher quality of care for the patients who we serve.

Optimization and Interpretation of Serial QuantiFERON Testing to Measure Acquisition of Mycobacterium tuberculosis Infection

RATIONALE

Conversion from a negative to positive QuantiFERON-TB test is indicative of Mycobacterium tuberculosis (Mtb) infection, which predisposes individuals to tuberculosis disease. Interpretation of serial tests is confounded by immunological and technical variability.

OBJECTIVES

To improve the consistency of serial QuantiFERON-TB testing algorithms and provide a data-driven definition of conversion.

METHODS

Sources of QuantiFERON-TB variability were assessed, and optimal procedures were identified. Distributions of IFN-γ response levels were analyzed in healthy adolescents, Mtb-unexposed control subjects, and patients with pulmonary tuberculosis.

MEASUREMENTS AND MAIN RESULTS

Individuals with no known Mtb exposure had IFN-γ values less than 0.2 IU/ml. Among individuals with IFN-γ values less than 0.2 IU/ml, 0.2-0.34 IU/ml, 0.35-0.7 IU/ml, and greater than 0.7 IU/ml, tuberculin skin test positivity results were 15%, 53%, 66%, and 91% (P < 0.005), respectively. Together, these findings suggest that values less than 0.2 IU/ml were true negatives. In short-term serial testing, "uncertain" conversions, with at least one value within the uncertainty zone (0.2-0.7 IU/ml), were partly explained by technical assay variability. Individuals who had a change in QuantiFERON-TB IFN-γ values from less than 0.2 to greater than 0.7 IU/ml had 10-fold higher tuberculosis incidence rates than those who maintained values less than 0.2 IU/ml over 2 years (P = 0.0003). By contrast, "uncertain" converters were not at higher risk than nonconverters (P = 0.229). Eighty-seven percent of patients with active tuberculosis had IFN-γ values greater than 0.7 IU/ml, suggesting that these values are consistent with established Mtb infection.

CONCLUSIONS

Implementation of optimized procedures and a more rigorous QuantiFERON-TB conversion definition (an increase from IFN-γ <0.2 to >0.7 IU/ml) would allow more definitive detection of recent Mtb infection and potentially improve identification of those more likely to develop disease.

Optimization and Interpretation of Serial QuantiFERON Testing to Measure Acquisition of Mycobacterium tuberculosis Infection

RATIONALE

Conversion from a negative to positive QuantiFERON-TB test is indicative of Mycobacterium tuberculosis (Mtb) infection, which predisposes individuals to tuberculosis disease. Interpretation of serial tests is confounded by immunological and technical variability.

OBJECTIVES

To improve the consistency of serial QuantiFERON-TB testing algorithms and provide a data-driven definition of conversion.

METHODS

Sources of QuantiFERON-TB variability were assessed, and optimal procedures were identified. Distributions of IFN-γ response levels were analyzed in healthy adolescents, Mtb-unexposed control subjects, and patients with pulmonary tuberculosis.

MEASUREMENTS AND MAIN RESULTS

Individuals with no known Mtb exposure had IFN-γ values less than 0.2 IU/ml. Among individuals with IFN-γ values less than 0.2 IU/ml, 0.2-0.34 IU/ml, 0.35-0.7 IU/ml, and greater than 0.7 IU/ml, tuberculin skin test positivity results were 15%, 53%, 66%, and 91% (P < 0.005), respectively. Together, these findings suggest that values less than 0.2 IU/ml were true negatives. In short-term serial testing, "uncertain" conversions, with at least one value within the uncertainty zone (0.2-0.7 IU/ml), were partly explained by technical assay variability. Individuals who had a change in QuantiFERON-TB IFN-γ values from less than 0.2 to greater than 0.7 IU/ml had 10-fold higher tuberculosis incidence rates than those who maintained values less than 0.2 IU/ml over 2 years (P = 0.0003). By contrast, "uncertain" converters were not at higher risk than nonconverters (P = 0.229). Eighty-seven percent of patients with active tuberculosis had IFN-γ values greater than 0.7 IU/ml, suggesting that these values are consistent with established Mtb infection.

CONCLUSIONS

Implementation of optimized procedures and a more rigorous QuantiFERON-TB conversion definition (an increase from IFN-γ <0.2 to >0.7 IU/ml) would allow more definitive detection of recent Mtb infection and potentially improve identification of those more likely to develop disease.

Evaluation of QuantiFERON-TB Gold-Plus in Health Care Workers in a Low-Incidence Setting

Although launched in 2015, little is known about the accuracy of QuantiFERON-TB Gold-Plus (QFT-Plus) for diagnosis of latent M. tuberculosis infection (LTBI). Unlike its predecessor, QFT-Plus utilizes two antigen tubes to elicit an immune response from CD4⁺ and CD8⁺ T lymphocytes. We conducted a cross-sectional study in low-risk health care workers (HCWs) at a single U.S. center to compare QFT-Plus to QuantiFERON-TB Gold in-tube (QFT). A total of 989 HCWs were tested with both QFT and QFT-Plus. Risk factors for LTBI were obtained from a questionnaire. QFT-Plus was considered positive if either antigen tube 1 (TB1) or TB2 tested positive, per the manufacturer's recommendations, or if both TB1 and TB2 tested positive, using a conservative definition. Results were compared using Cohen's kappa and linear regression, respectively. Agreement of QFT with QFT-Plus was high, at 95.6% (95% confidence interval [CI], 94.3 to 96.9; kappa, 0.57). The majority of discordant results between QFT and QFT-Plus TB1 (84.8%) and QFT and QFT-Plus TB2 (88.6%) fell within the range of 0.2 to 0.7 IU/ml. The positivity rate in 626 HCWs with no identifiable risk factors and no self-reported history of positive LTBI tests was 2.1% (CI, 1.0 to 3.2) and 3.0% (CI, 1.7 to 4.3) with QFT and QFT-Plus, respectively. A conservative definition of a QFT-Plus-positive result yielded a positivity rate of 1.0% (CI, 0.2 to 1.7; P value of 0.0002 versus QFT-Plus and 0.07 versus QFT). On follow-up testing, of 11 HCWs with discordant QFT-Plus results, 90.9% (10/11) had a negative QFT result. The QFT-Plus assay showed a high degree of agreement with QFT in U.S. HCWs. A conservative interpretation of QFT-Plus eliminated nearly all nonreproducible positive results in low-risk HCWs. Larger studies are needed to validate the latter finding and to more clearly define conditions under which a conservative interpretation can be used to minimize nonreproducible positive results in low-risk populations.

Poor agreement between diagnostic tests for latent tuberculosis infection among HIV-infected persons in Hong Kong

BACKGROUND AND OBJECTIVE

The tuberculin skin test (TST), T-Spot.TB (T-Spot) and QuantiFERON-TB Gold-In Tube (QFT) were compared in diagnosing latent tuberculosis infection (LTBI) among human immunodeficiency virus (HIV)-infected persons.

METHODS

Human immunodeficiency virus-infected persons without previous history of tuberculosis or LTBI were simultaneously tested by TST, T-Spot and QFT annually and followed up for tuberculosis.

RESULTS

Among 110 HIV-infected subjects with 85% previous TST screening coverage, 75% on anti-retroviral therapy, well-preserved median CD4 count (414/μL) and low median viral load (<75/μL), baseline TST, T-Spot and QFT were positive in 5.5%, 5.6% and 4.9%, respectively, with almost complete discordance of positive results. Among 91 (83%), 66 (60%) and 26 (24%) subjects successfully undergoing the first, second and third annual retesting, TST, T-Spot and QFT were, respectively, positive in 11/123 (8.9%), 13/173 (7.5%) and 21/182 (11.5%) on retesting, with similar discordance of positive results. There was no significant association with the concurrent CD4 count or viral load. Conversion occurred in 11/123 (8.9%), 8/160 (5.0%) and 18/168 (10.7%) of TST, T-Spot and QFT, respectively, and none was associated with changes in CD4 count or viral load. More than half of the positive T-SPOT and QFT results reverted to negative on follow-up. None of these tests picked up the single case of culture-confirmed tuberculosis observed after 798 person-years of follow-up.

CONCLUSION

Major discordance in positive results, high reversion rates and low tuberculosis incidence among test-positive subjects cast serious doubt on the utility of the currently available LTBI tests in the annual screening of HIV-infected persons in an intermediate tuberculosis burden area.

Negative Tuberculin Skin Test and Prediction of Reversion of QuantiFERON Interferon Gamma Release Assay in US Healthcare Workers

QuantiFERON tuberculosis tests (QFT) reverted in (612) 77% of 1,094 low-risk healthcare workers (HCW) testing less than 1.16 IU/mL. Of HCW testing greater than 1.1 IU/mL, 33 (59%) of 56 with negative tuberculin skin tests (TST) reverted vs 8 (6%) of 125 with positive TSTs. Retesting low-risk QFT-positive and TST-negative HCW is prudent.

Infect. Control Hosp. Epidemiol. 2016;37(4):478–482

Interferon Gamma Release Assays for Latent Tuberculosis: What Are the Sources of Variability?

Interferon gamma release assays (IGRAs) are blood-based tests intended for diagnosis of latent tuberculosis infection (LTBI). IGRAs offer logistical advantages and are supposed to offer improved specificity over the tuberculin skin test (TST). However, recent serial testing studies of low-risk individuals have revealed higher false conversion rates with IGRAs than with TST. Reproducibility studies have identified various sources of variability that contribute to nonreproducible results. Sources of variability can be broadly classified as preanalytical, analytical, postanalytical, manufacturing, and immunological. In this minireview, we summarize known sources of variability and their impact on IGRA results. We also provide recommendations on how to minimize sources of IGRA variability.

Serial QuantiFERON-TB Gold In-Tube assay and tuberculin skin test to diagnose latent tuberculosis in household Mexican contacts: conversion and reversion rates and associated factors using conventional and borderline zone definitions

A cohort of 123 adult contacts was followed for 18‐24 months (86 completed the follow-up) to compare conversion and reversion rates based on two serial measures of QuantiFERON (QFT) and tuberculin skin test (TST) (PPD from TUBERSOL, Aventis Pasteur, Canada) for diagnosing latent tuberculosis (TB) in household contacts of TB patients using conventional (C) and borderline zone (BZ) definitions. Questionnaires were used to obtain information regarding TB exposure, TB risk factors and socio-demographic data. QFT (IU/mL) conversion was defined as <0.35 to ≥0.35 (C) or <0.35 to >0.70 (BZ) and reversion was defined as ≥0.35 to <0.35 (C) or ≥0.35 to <0.20 (BZ); TST (mm) conversion was defined as <5 to ≥5 (C) or <5 to >10 (BZ) and reversion was defined as ≥5 to <5 (C). The QFT conversion and reversion rates were 10.5% and 7% with C and 8.1% and 4.7% with the BZ definitions, respectively. The TST rates were higher compared with QFT, especially with the C definitions (conversion 23.3%, reversion 9.3%). The QFT conversion and reversion rates were higher for TST ≥5; for TST, both rates were lower for QFT <0.35. No risk factors were associated with the probability of converting or reverting. The inconsistency and apparent randomness of serial testing is confusing and adds to the limitations of these tests and definitions to follow-up close TB contacts.

Reproducibility of interferon gamma (IFN-γ) release Assays. A systematic review

RATIONALE

Interferon gamma (IFN-γ) release assays for latent tuberculosis infection result in a larger-than-expected number of conversions and reversions in occupational screening programs, and reproducibility of test results is a concern.

OBJECTIVES

Knowledge of the relative contribution and extent of the individual sources of variability (immunological, preanalytical, or analytical) could help optimize testing protocols.

METHODS

We performed a systematic review of studies published by October 2013 on all potential sources of variability of commercial IFN-γ release assays (QuantiFERON-TB Gold In-Tube and T-SPOT.TB). The included studies assessed test variability under identical conditions and under different conditions (the latter both overall and stratified by individual sources of variability). Linear mixed effects models were used to estimate within-subject SD.

MEASUREMENTS AND MAIN RESULTS

We identified a total of 26 articles, including 7 studies analyzing variability under the same conditions, 10 studies analyzing variability with repeat testing over time under different conditions, and 19 studies reporting individual sources of variability. Most data were on QuantiFERON (only three studies on T-SPOT.TB). A considerable number of conversions and reversions were seen around the manufacturer-recommended cut-point. The estimated range of variability of IFN-γ response in QuantiFERON under identical conditions was ±0.47 IU/ml (coefficient of variation, 13%) and ±0.26 IU/ml (30%) for individuals with an initial IFN-γ response in the borderline range (0.25-0.80 IU/ml). The estimated range of variability in noncontrolled settings was substantially larger (±1.4 IU/ml; 60%). Blood volume inoculated into QuantiFERON tubes and preanalytic delay were identified as key sources of variability.

CONCLUSIONS

This systematic review shows substantial variability with repeat IFN-γ release assays testing even under identical conditions, suggesting that reversions and conversions around the existing cut-point should be interpreted with caution.

In vitro immunomodulation for enhancing T cell-based diagnosis of Mycobacterium tuberculosis infection

Interferon-gamma release assays have limited sensitivity for detecting latent tuberculosis infection. In this study, we determine if the addition of immunomodulators to the QuantiFERON-TB Gold In-Tube (QFT-GIT) increased test sensitivity without compromising specificity. We prospectively compared QFT-GIT results with and without incubation with 2 immunomodulators (lipopolysaccharide [LPS] and polyinosine-polycytidylic acid [PolyIC]) in 2 cohorts-113 culture-confirmed tuberculosis (TB) subjects in Hanoi, Vietnam, and 226 documented QFT-GIT-negative, low TB risk health care workers undergoing annual TB screening at a US academic institution. Sensitivity of the tests in TB subjects was 84.1% with the standard QFT-GIT and 85.8% and 74.3% after incubation with LPS and PolyIC, respectively. Specificity in low TB risk health care workers was 100% with the standard QFT-GIT by design and 86.7% with LPS and 63.3% with PolyIC. In conclusion, use of the 2 immunomodulators did not improve sensitivity of the QFT-GIT in TB patients and reduced specificity in low-risk health care workers.

The dynamics of QuantiFERON-TB gold in-tube conversion and reversion in a cohort of South African adolescents

RATIONALE

Interferon-γ release assays are used to diagnose tuberculosis infection. In developed countries, high rates of reversion following conversion have been described.

OBJECTIVES

To assess QuantiFERON TB Gold In-Tube test (QFT) conversion and reversion dynamics in a tuberculosis-endemic setting.

METHODS

Adolescents aged 12-18 years residing near Cape Town were recruited. Tuberculin skin tests (TSTs) and QFTs were performed at baseline and after 2 years of follow up. Half of the participants had TST and QFT performed at additional time points. Participants were observed for incident tuberculosis disease for up to 5 years.

MEASUREMENTS AND MAIN RESULTS

Among 5,357 participants, 2,751 (51.4%) and 2,987 (55.8%) had positive QFT and TST results, respectively, at baseline. Annualized QFT and TST conversion risks were 14.0 and 13.0%, respectively, and reversion risks were 5.1 and 4.1%, respectively. Concordance was excellent for conversions (κ = 0.74), but poor for reversions (κ = 0.12). Among recent QFT converters, the magnitude of the QFT value was strongly inversely associated with risk of reversion (P < 0.0001). When longitudinal QFT data were analyzed in a cross-sectional manner, the annual risk of infection was 7.3%, whereas inclusion of reversions in the analysis showed that the actual risk of infection was 14.0%. Incident tuberculosis was 8-fold higher among QFT reverters than in participants with all negative QFT results (1.47 vs. 0.18 cases/100 person-years, P = 0.011).

CONCLUSIONS

In this tuberculosis-endemic setting, annual risk of infection was extremely high, whereas QFT and TST conversion concordance was higher and QFT reversion rates were lower than reported in low-burden settings.

Childhood BCG vaccination does not influence control of Mycobacterium tuberculosis growth by human bronchoalveolar lavage cells

BACKGROUND

Childhood vaccination with Mycobacterium bovis Bacille Calmette-Guérin (BCG) reduces the risk of infection with Mycobacterium tuberculosis and the risk of severe forms of tuberculosis in children. The protection of adults from pulmonary tuberculosis is doubtful. This study evaluated the effect of the vaccination on the growth of M. tuberculosis human bronchoalveolar mononuclear cells (BALMC).

METHODS

Healthy, adult healthcare workers who were regularly exposed to M. tuberculosis, household tuberculosis contacts, and cured tuberculosis patients were recruited in a multicentre study conducted in Germany. BALMC were co-cultured with different strains of M. tuberculosis in growth inhibition assays (MGIAs).

RESULTS

MGIAs on BALMC were conducted in 90 contact persons (known vaccination status, n = 75) and 62 former tuberculosis patients (known status, n = 22). Growth rates for M. tuberculosis H37Rv in BALMC were independent of the vaccination status, both in healthy contacts and in cured tuberculosis patients. This finding was validated in growth inhibition assays using two different Haarlem M. tuberculosis outbreak strains. Subgroup analyses based on the Interferon-gamma release assay status found no impact of the vaccination on mycobacterial growth.

CONCLUSIONS

This study suggests that M. bovis BCG vaccination does not alter the anti-mycobacterial capacity of BALMC as assessed in ex-vivo growth inhibition assays.

IFN-γ release assay conversions and reversions. Challenges with serial testing in U.S. health care workers

RATIONALE

IFN-γ release assays (IGRAs) including the QuantiFERON-TB gold in-tube test (QFT-GIT) are increasingly used in place of the tuberculin skin test (TST) in surveillance programs for Mycobacterium tuberculosis infection in the United States. However, data on conversions, reversions, and predictive value of QFT in such programs for health care workers (HCWs) are limited.

OBJECTIVES

The purpose of this study is to assess long-term reproducibility and conversion and reversion rates of QFT-GIT among HCWs who underwent serial testing at a tertiary care center in the United States.

METHODS

Retrospective chart review of HCWs at the Central Arkansas Veterans Healthcare System (CAVHS) who underwent serial testing with QFT-GIT as a part of their employee screening between November 1, 2008 and January 31, 2011.

MEASUREMENTS AND MAIN RESULTS

A total of 2,303 HCWs had at least 2 QFT-GITs 1 year apart. The initial QFT-GIT was positive for 69 and 2 were indeterminate. Of these 69 HCWs, 31 (45%) reverted on repeat testing, and 25 of 31 (80.6%) HCWs who reverted had a negative look-back TST. Of the 2,232 HCWs with an initial negative QFT-GIT, 71 (3.2%) converted on repeat testing. A third QFT-GIT assay was performed in 41 of the 71 converters and 90% (37 of 41) reverted back to negative. Only two HCWs had TST and QFT-GIT conversion.

CONCLUSIONS

Poor IGRA reproducibility and a low predictive value of QFT-GIT conversions indicate that QFT-GIT with current interpretation criteria should not be used for serial screening of U.S. HCWs. Negative TSTs have higher reproducibility than QFT-GIT for serial testing of HCWs in low tuberculosis incidence settings.

Gamma interferon release assays for detection of Mycobacterium tuberculosis infection

Identification and treatment of latent tuberculosis infection (LTBI) can substantially reduce the risk of developing active disease. However, there is no diagnostic gold standard for LTBI. Two tests are available for identification of LTBI: the tuberculin skin test (TST) and the gamma interferon (IFN-γ) release assay (IGRA). Evidence suggests that both TST and IGRA are acceptable but imperfect tests. They represent indirect markers of Mycobacterium tuberculosis exposure and indicate a cellular immune response to M. tuberculosis. Neither test can accurately differentiate between LTBI and active TB, distinguish reactivation from reinfection, or resolve the various stages within the spectrum of M. tuberculosis infection. Both TST and IGRA have reduced sensitivity in immunocompromised patients and have low predictive value for progression to active TB. To maximize the positive predictive value of existing tests, LTBI screening should be reserved for those who are at sufficiently high risk of progressing to disease. Such high-risk individuals may be identifiable by using multivariable risk prediction models that incorporate test results with risk factors and using serial testing to resolve underlying phenotypes. In the longer term, basic research is necessary to identify highly predictive biomarkers.

The use of an interferon-gamma release assay to screen for pediatric latent tuberculosis infection in the eastern region of the Emirate of Abu Dhabi

OBJECTIVES

Intense migration to the United Arab Emirates from tuberculosis (TB) high-endemic areas presents a particular risk to the population. Screening for latent tuberculosis infection (LTBI) usually involves risk assessment, the tuberculin skin test (TST), and interferon-gamma release assay (IGRA). This study investigated the use of an IGRA to screen for LTBI and compared its performance with a risk assessment questionnaire.

METHODS

This prospective cross-sectional study was conducted at seven Ambulatory Healthcare Services facilities in Abu Dhabi. Participants (88% Emiratis) were pediatric patients presenting for routine care. The QuantiFERON-TB Gold In-Tube test was performed and the parents completed a questionnaire assessing TB risk factors.

RESULTS

Six-hundred and ninety-nine subjects (median age 8.7 years, interquartile range 9.2 years) were enrolled; 669 (96%) agreed to testing. Four patients had a positive IGRA; one had previously been treated for TB, resulting in three patients with LTBI. The estimated LTBI prevalence was 0.45% (95% confidence interval 0.09-1.3). A household contact from a TB high-endemic area was reported in 44%, travel to a TB high-endemic area in 10%, and contact with someone with a chronic cough in 7%, a TB case in 3%, a TST-positive case in 2%, and an IGRA-positive case in 2%. Fifty percent of participants had at least one risk factor. The risk assessment did not predict a positive IGRA.

CONCLUSIONS

The questionnaire yielded a risk of TB exposure of 50%, however the LTBI prevalence, as defined by the IGRA, was low (0.45%).

Interferon-γ release assays and tuberculin skin testing for diagnosis of latent tuberculosis infection in healthcare workers in the United States

RATIONALE

IFN-γ release assays (IGRAs) are alternatives to tuberculin skin testing (TST) for diagnosis of latent tuberculosis infection. Limited data suggest IGRAs may not perform well for serial testing of healthcare workers (HCWs).

OBJECTIVES

Determine the performance characteristics of IGRAs versus TST for serial testing of HCWs.

METHODS

A longitudinal study involving 2,563 HCWs undergoing occupational tuberculosis screening at four healthcare institutions in the United States, where the average tuberculosis case rate ranged from 4 to 9 per 100,000 persons. QuantiFERON-TB Gold In-Tube (QFT-GIT), T-SPOT.TB (T-SPOT), and TST were performed at baseline and every 6 months for 18 months between February 2008 and March 2011.

MEASUREMENTS AND MAIN RESULTS

A total of 2,418 HCWs completed baseline testing, which was positive for 125 (5.2%) by TST, 118 (4.9%) by QFT-GIT, and 144 (6.0%) by T-SPOT. A baseline positive TST with negative IGRAs was associated with bacillus Calmette-Guérin (BCG) vaccination (odds ratio: 25.1 [95% confidence interval: 15.5, 40.5] vs. no BCG). Proportions of participants with test conversion during the study period were 138 of 2,263 (6.1%) for QFT-GIT, 177 of 2,137 (8.3%) for T-SPOT, and 21 of 2,293 (0.9%) for TST (P < 0.001 for QFT-GIT vs. TST and for T-SPOT vs. TST; P = 0.005 for QFT-GIT vs. T-SPOT). Of the QFT-GIT and T-SPOT converters, 81 of 106 (76.4%) and 91 of 118 (77.1%), respectively, were negative when retested 6 months later. There was negative/positive discordance for 15 of 170 (8.8%) participants by QFT-GIT and for 19 of 151 (12.6%) by T-SPOT when blood was drawn 2 weeks later.

CONCLUSIONS

Most conversions among HCWs in low TB incidence settings appear to be false positives, and these occurred six to nine times more frequently with IGRAs than TST; repeat testing of apparent converters is warranted.

Variability of the QuantiFERON®-TB gold in-tube test using automated and manual methods

BACKGROUND

The QuantiFERON®-TB Gold In-Tube test (QFT-GIT) detects Mycobacterium tuberculosis (Mtb) infection by measuring release of interferon gamma (IFN-γ) when T-cells (in heparinized whole blood) are stimulated with specific Mtb antigens. The amount of IFN-γ is determined by enzyme-linked immunosorbent assay (ELISA). Automation of the ELISA method may reduce variability. To assess the impact of ELISA automation, we compared QFT-GIT results and variability when ELISAs were performed manually and with automation.

METHODS

Blood was collected into two sets of QFT-GIT tubes and processed at the same time. For each set, IFN-γ was measured in automated and manual ELISAs. Variability in interpretations and IFN-γ measurements was assessed between automated (A1 vs. A2) and manual (M1 vs. M2) ELISAs. Variability in IFN-γ measurements was also assessed on separate groups stratified by the mean of the four ELISAs.

RESULTS

Subjects (N = 146) had two automated and two manual ELISAs completed. Overall, interpretations were discordant for 16 (11%) subjects. Excluding one subject with indeterminate results, 7 (4.8%) subjects had discordant automated interpretations and 10 (6.9%) subjects had discordant manual interpretations (p = 0.17). Quantitative variability was not uniform; within-subject variability was greater with higher IFN-γ measurements and with manual ELISAs. For subjects with mean TB Responses ±0.25 IU/mL of the 0.35 IU/mL cutoff, the within-subject standard deviation for two manual tests was 0.27 (CI95 = 0.22-0.37) IU/mL vs. 0.09 (CI95 = 0.07-0.12) IU/mL for two automated tests.

CONCLUSION

QFT-GIT ELISA automation may reduce variability near the test cutoff. Methodological differences should be considered when interpreting and using IFN-γ release assays (IGRAs).

Trajectories of tuberculosis-specific interferon-gamma release assay responses among medical and nursing students in rural India

Background: Interferon gamma release assays (IGRAs) have been shown to be highly dynamic tests when used in serial testing for TB infection. However, there is little information demonstrating a clear association between TB exposure and IGRA responses over time, particularly in high TB incidence settings.

Objectives: To assess whether QuantiFERON-TB Gold In-Tube (QFT) responses are associated with occupational TB exposures in a cohort of young health care trainees in India.

Methods: All medical and nursing students at Mahatma Gandhi Institute of Medical Sciences were approached. Participants were followed up for 18 months; QFT was performed 4 times, once every 6 months. Various modeling approaches were used to define IFN-gamma trajectories and correlations with TB exposure.

Results: Among 270 medical and nursing trainees, high rates of conversions (6.3–20.9%) and reversions (20.0–26.2%) were found depending on the definitions used. Stable converters were more likely to have had TB exposure in hospital pre-study. Recent occupational exposures were not consistently associated with QFT responses over time.

Conclusion: IFN-gamma responses and rates of change could not be explained by occupational exposure investigated. High conversion and subsequent reversion rates suggest many health care workers (HCWs) would revert in the absence of treatment, either by clearing the infection naturally or due to fluctuations in the underlying immunological response and/or poor assay reproducibility. QFT may not be an ideal diagnostic test for repeated screening of HCWs in a high TB incidence setting.

Repeat IGRA testing in Canadian health workers: conversions or unexplained variability?

BACKGROUND

Although North American hospitals are switching from tuberculin testing (TST) to interferon-gamma release assays (IGRAs), data are limited on the association between occupational exposure and serial QuantiFERON-TB Gold In-Tube (QFT) results in healthcare workers (HCWs).

METHODS

In a cohort of Canadian HCWs, TST and QFT were performed at study enrolment (TST1 and QFT1) and 1 year later (TST2 and QFT2). Conversion and reversion rates were estimated, and correlation with TB exposure was assessed.

RESULTS

Among 258 HCWs, median age was 36.8 years, 188/258 (73%) were female and 183/258 (71%) were Canadian-born. In 245 subjects with a negative QFT1 we found a QFT conversion rate of 5.3% (13/245, 95% CI 2.9-8.9%). Using more stringent definitions, QFT conversion rates ranged from 2.0 to 5.3%. No TST conversions were found among the 241 HCWs with negative TST1, and no measure of recent TB exposure was associated with QFT conversions. In the 13 HCWs with a positive QFT1, 62% reverted.

CONCLUSION

Using the conventional QFT conversion definition, we found a higher than expected rate of conversion. Recent occupational exposures were not associated with QFT conversions, and no TST conversions occurred in this cohort, suggesting the 'conversions' may not reflect new TB infection.