Comparison of Two Interferon-γ Assays and Tuberculin Skin Test for Tracing Tuberculosis Contacts

Screening for tuberculosis infection among newly arrived asylum seekers: comparison of QuantiFERONTB Gold with tuberculin skin test

Background

QuantiFERON^®TB Gold (QFT) is a promising blood test for tuberculosis infection but with few data so far from immigrant screening. The aim of this study was to compare results of QFT and tuberculin skin test (TST) among newly arrived asylum seekers in Norway and to assess the role of QFT in routine diagnostic screening for latent tuberculosis infection.

Methods

The 1000 asylum seekers (age ≥ 18 years) enrolled in the study were voluntarily recruited from 2813 consecutive asylum seekers arriving at the national reception centre from September 2005 to June 2006. Participation included a QFT test and a questionnaire in addition to the mandatory TST and chest X-ray.

Results

Among 912 asylum seekers with valid test results, 29% (264) had a positive QFT test whereas 50% (460) tested positive with TST (indurations ≥ 6 mm), indicating a high proportion of latent infection within this group. Among the TST positive participants 50% were QFT negative, whereas 7% of the TST negative participants were QFT positive. There was a significant association between increase in size of TST result and the likelihood of being QFT positive. Agreement between the tests was 71–79% depending on the chosen TST cut-off and it was higher for non-vaccinated individuals.

Conclusion

By using QFT in routine screening, further follow-up could be avoided in 43% of the asylum seekers who would have been referred if based only on a positive TST (≥ 6 mm). The proportion of individuals referred will be the same whether QFT replaces TST or is used as a supplement to confirm a positive TST, but the number tested will vary greatly. All three screening approaches would identify the same proportion (88–89%) of asylum seekers with a positive QFT and/or a TST ≥ 15 mm, but different groups will be missed.

Comparison of sensitivities of two commercial gamma interferon release assays for pulmonary tuberculosis

There are few head-to-head comparisons of the commercial gamma interferon release assays (GIRAs). We compared the performance of the T-SPOT.TB and QuantiFERON-TB Gold In-Tube (QFT-IT) assays in patients with culture-proven pulmonary tuberculosis. Blood was drawn for both assays within 14 days of starting antituberculosis treatment. The QFT-IT indeterminate rate was 3.5%; the T-SPOT.TB failure rate was 1.4%. There was poor agreement between the GIRAs (kappa = 0.257) among the 270 patients with valid results for both tests. The sensitivities of the T-SPOT.TB and QFT-IT assays were 94.1 and 83.0%, respectively, with a significant difference in the performance of the assays (P = 0.001 [McNemar test]). Factors independently associated with indeterminate QFT-IT results were an age of >or=60 years (odds ratio [OR] 11.18, 95% confidence interval [CI] = 1.841 to 67.823, P = 0.009), female sex (OR = 7.47, 95% CI = 1.517 to 36.733, P = 0.013) and non-Chinese (i.e., Indian or Malay) race (OR = 7.89, 95% CI = 1.585 to 39.267, P = 0.012). The QFT-IT assay was significantly less sensitive in patients >or=60 years old (OR = 0.41, 95% CI = 0.181 to 0.918, P = 0.030) and in Indian compared to Chinese patients (OR = 0.27, 95% CI = 0.073 to 0.990, P = 0.048). The T-SPOT.TB assay was significantly less sensitive in Malay (OR = 0.23, 95% CI = 0.063 to 0.815, P = 0.023) and Indian patients (OR = 0.09, 95% CI = 0.017 to 0.429, P = 0.003) compared to Chinese patients. The performance of both assays was not significantly altered in diabetics. The diminished sensitivity of the GIRAs in persons of Malay and Indian race merits further study.

Enhancement of diagnostic efficiency by a gamma interferon release assay for pulmonary tuberculosis

This study was designed to examine the use of the QuantiFERON-TB Gold assay as an aid in the diagnosis of active pulmonary tuberculosis (TB) in Brazilian patients. Using the receiver operating characteristic curve, the cutoff was adjusted to >or=0.20 IU/ml. The sensitivity increased to 86%, with 100% specificity. All TB patients with negative sputum smear microscopy and negative culture results were positive using this test.

Improved diagnostic evaluation of suspected tuberculosis

BACKGROUND

The role of new T-cell-based blood tests for tuberculosis in the diagnosis of active tuberculosis is unclear.

OBJECTIVE

To compare the performance of 2 interferon-gamma assays and tuberculin skin testing in adults with suspected tuberculosis.

DESIGN

Prospective study conducted in routine practice.

SETTING

2 urban hospitals in the United Kingdom.

PATIENTS

389 adults, predominantly of South Asian and black ethnicity, with moderate to high clinical suspicion of active tuberculosis.

INTERVENTION

Tuberculin skin testing, the enzyme-linked immunospot assay (ELISpot) incorporating early secretory antigenic target-6 and culture filtrate protein-10 (standard ELISpot), and ELISpot incorporating a novel antigen, Rv3879c (ELISpot(PLUS)) were performed during diagnostic assessment by independent persons who were blinded to results of the other test.

MEASUREMENTS

Sensitivity, specificity, predictive values, and likelihood ratios.

RESULTS

194 patients had a final diagnosis of active tuberculosis, of which 79% were culture-confirmed. Sensitivity for culture confirmed and highly probable tuberculosis was 89% (95% CI, 84% to 93%) with ELISpot(PLUS), 85% (CI, 79% to 90%) with standard ELISpot, 79% (CI, 72% to 85%) with 15-mm threshold tuberculin skin testing, and 83% (CI, 77% to 89%) with stratified thresholds of 15 and 10 mm in vaccinated and unvaccinated patients, respectively. The ELISpot(PLUS) assay was more sensitive than tuberculin skin testing with 15-mm cutoff points (P = 0.01) but not with stratified cutoff points (P = 0.10). The ELISpot(PLUS) assay had 4% higher diagnostic sensitivity than standard ELISpot (P = 0.02). Combined sensitivity of ELISpot(PLUS) and tuberculin skin testing was 99% (CI, 95% to 100%), conferring a negative likelihood ratio of 0.02 (CI, 0 to 0.06) when both test results were negative.

LIMITATIONS

Local standards for tuberculin skin testing differed from others used internationally. The study sample included few immunosuppressed patients.

CONCLUSION

The ELISpot(PLUS) assay is more sensitive than standard ELISpot and, when used in combination with tuberculin skin testing, enables rapid exclusion of active infection in patients with moderate to high pretest probability of tuberculosis.

Use of an interferon-gamma release assay to diagnose latent tuberculosis infection in foreign-born patients

BACKGROUND

The tuberculin skin test (TST) has a low specificity in the setting of bacille Calmette-Guérin (BCG) vaccination. Interferon-gamma release assays (IGRAs) appear to be more specific but have not been validated in this population under routine clinical conditions. We sought to validate the routine clinical use of the T-SPOT.TB test (Oxford Immunotec; Oxford, UK), an IGRA, in a predominantly foreign-born population with a high rate of BCG vaccination.

METHODS

We compared the TST and the T-SPOT.TB test in 96 subjects at a New York City Department of Health tuberculosis clinic. We aimed to determine which test better predicted being a close contact of a case of active tuberculosis, a surrogate for latent tuberculosis infection.

RESULTS

A positive T-SPOT.TB test result was strongly associated with being a close contact of a case of active tuberculosis after adjustment for potential confounders (adjusted odds ratio, 2.9; 95% confidence interval, 1.1 to 7.3; p = 0.03). A positive TST result was associated with being a contact only in subjects without BCG vaccination (p = 0.02). The T-SPOT.TB test was more specific for being a close contact than the TST (p < 0.001). Specificity in BCG-vaccinated subjects was 3% for the TST compared with 70% for the T-SPOT.TB test (p < 0.001).

CONCLUSIONS

The T-SPOT.TB test is superior in routine clinical use to the TST for identifying high-risk individuals among foreign-born populations with high rates of BCG vaccination.

Review article: minimizing tuberculosis during anti-tumour necrosis factor-alpha treatment of inflammatory bowel disease

BACKGROUND

Tumour necrosis factor (TNF)-alpha inhibitors are a major advance in the management of inflammatory bowel disease but increase the risk for tuberculosis (TB).

AIM

To examine the reasons for the increase in the risk for TB and the strategies to reduce it.

METHODS

PubMed searches were performed using search terms that included TB and each of the current anti-TNF-alpha biological agents and also TB and Crohn's disease.

RESULTS

Increased susceptibility to TB, often with extrapulmonary or disseminated disease, occurs following treatment with all anti-TNF-alpha biological agents and amounts to a four- to 20-fold increased risk with infliximab. TB usually occurs shortly after anti-TNF-alpha initiation suggesting reactivation of latent infection. Animal studies show that TNF-alpha inhibition impairs inflammatory cell trafficking and granuloma formation. Currently recommended screening for latent TB typically, risk assessment, tuberculin skin testing and chest radiograph used prior to anti-TNF-alpha treatment can reduce TB rates by up to 90% but newer screening interferon gamma assays may enhance screening efficacy. Patients positive on screening who are treated with isoniazid and subsequently receive anti-TNF-alpha treatment still have approximately 19% risk for TB.

CONCLUSIONS

Tuberculosis following treatment with TNF-alpha inhibitors usually results from reactivation of latent disease. Screening reduces the risk substantially but does not completely eliminate it.

Comparison of quantiferon-TB gold with tuberculin skin test for detecting latent tuberculosis infection prior to liver transplantation

Screening for latent tuberculosis infection (LTBI) is recommended prior to organ transplantation. The Quantiferon-TB Gold assay (QFT-G) may be more accurate than the tuberculin skin test (TST) in the detection of LTBI. We prospectively compared the results of QFT-G to TST in patients with chronic liver disease awaiting transplantation. Patients were screened for LTBI with both the QFT-G test and a TST. Concordance between test results and predictors of a discordant result were determined. Of the 153 evaluable patients, 37 (24.2%) had a positive TST and 34 (22.2%) had a positive QFT-G. Overall agreement between tests was 85.1% (kappa= 0.60, p < 0.0001). Discordant test results were seen in 12 TST positive/QFT-G negative patients and in 9 TST negative/QFT-G positive patients. Prior BCG vaccination was not associated with discordant test results. Twelve patients (7.8%), all with a negative TST, had an indeterminate result of the QFT-G and this was more likely in patients with a low lymphocyte count (p = 0.01) and a high MELD score (p = 0.001). In patients awaiting liver transplantation, both the TST and QFT-G were comparable for the diagnosis of LTBI with reasonable concordance between tests. Indeterminate QFT-G result was more likely in those with more advanced liver disease.

Use of whole-blood samples in in-house bulk and single-cell antigen-specific gamma interferon assays for surveillance of Mycobacterium tuberculosis infections

Tests based on the gamma interferon (IFN-gamma) assay (IGA) are used as adjunctive tools for the diagnosis of Mycobacterium tuberculosis infection. Here we compared in-house and commercial whole-blood IGAs to identify a suitable assay for the surveillance of tuberculosis in population studies. The IGAs were selected on the basis of the ease with which they are performed and because they require a small amount of a biological sample and do not require cell purification. Since a "gold standard" for latently M. tuberculosis-infected individuals is not available, the sensitivities and the specificities of the IGAs were determined with samples from patients with clinically diagnosed active tuberculosis and in Mycobacterium bovis BCG-unvaccinated healthy controls. The in-house tests consisted of a bulk assay based on diluted whole blood and a single-cell assay based on IFN-gamma intracellular staining. The commercial assays used were the QuantiFERON-TB-Gold (Q-TB) and the Q-TB in-tube tests. When the purified protein derivative was used as the antigen, in-house whole-blood intracellular staining was found to be highly discriminatory between active tuberculosis patients and BCG-vaccinated healthy controls, whereas the other IGAs did not discriminate between the two categories of patients. When M. tuberculosis-specific antigens were used, a very strong agreement between the results of the Q-TB in-tube assay and the clinical diagnosis was observed, while the Q-TB assay, performed according to the manufacturer's instructions, showed a significantly lower performance. Intriguingly, when the test was performed with RD1 proteins instead of peptides, its sensitivity was significantly increased. The in-house assay with diluted whole blood showed an elevated sensitivity and an elevated specificity, and the results agreed with the clinical diagnosis. Considering that the in-house assay uses 1/20 of the sample compared with the amount of sample used in the commercial IGA, it appears to be particularly promising for use in pediatric studies. Overall, the different assays showed different performance characteristics that need to be considered for surveillance of tuberculosis in population studies.

Use of the QuantiFERON-TB Gold test in Japanese patients with sarcoidosis

BACKGROUND AND OBJECTIVE

Mycobacterium tuberculosis has been proposed as a candidate agent for the cause of sarcoidosis. The QuantiFERON-TB Gold test has a higher specificity for detecting M. tuberculosis infection than the conventional tuberculin skin test. This study aimed to investigate the rate of positive QuantiFERON-TB Gold results in Japanese sarcoidosis patients.

PATIENTS AND METHODS

The QuantiFERON-TB Gold test, an enzyme-linked immunosorbent assay, was used to assess the levels of interferon-gamma resulting from immune responses to M. tuberculosis-specific antigens, namely early secretory antigen target 6 and culture filtrate protein 10, in 90 Japanese sarcoidosis patients.

RESULTS

The QuantiFERON-TB Gold result was positive in 3 of the 90 patients tested.

CONCLUSION

The positivity rate of QuantiFERON-TB Gold was 3.3% in Japanese sarcoidosis patients.

Comparison of two commercially available gamma interferon blood tests for immunodiagnosis of tuberculosis

We evaluated the T-SPOT.TB and Quantiferon-TB Gold In tube (QFN-G-IT) tests for diagnosing Mycobacterium tuberculosis infection. T-SPOT.TB was more sensitive than QFN-G-IT in diagnosing both active and latent infection. Both gamma interferon tests were unaffected by prior Mycobacterium bovis BCG vaccination. Among children who were not BCG vaccinated but had a positive tuberculin skin test, QFN-G-IT was negative in 53.3% of cases, and T-SPOT.TB was negative in 50% of cases.

Comparison of two interferon gamma release assays in the diagnosis of Mycobacterium tuberculosis infection and disease in The Gambia

Background

IFN-γ Release Assays (IGRAs) have been licensed for the diagnosis of latent Mycobacterium tuberculosis infection (LTBI). Their performance may depend on assay format and may vary across populations and settings. We compared the diagnostic performance of an in-house T -cell and commercial whole blood-based IGRAs for the diagnosis of LTBI and TB disease in The Gambia.

Methods

Newly diagnosed sputum smear positive cases and their household contacts were recruited. Cases and contacts were bled for IGRA and contacts had a Mantoux skin test. We assessed agreement and discordance between the tests and categorized a contact's level of M. tuberculosis exposure according to where s/he slept relative to a case: the same room, same house or a different house. We assessed the relationship between exposure and test results by multiple logistic regression.

Results

In 80 newly diagnosed TB cases, the sensitivity of ELISPOT was 78.7% and for QFT-GIT was 64.0% (p = 0.047). Of 194 household contacts 57.1% and 58.8% were positive for ELISPOT and QFT-GIT respectively. The overall agreement between both IGRAs for LTBI in contacts was 71.4% and there was no significant discordance (p = 0.29). There was significant discordance between the IGRAs and TST. Neither IGRA nor TST had evidence of false positive results because of Bacille Calmette Guérin (BCG) vaccination. However, agreement between QFT-GIT and TST as well as discordance between both IGRAs and TST were associated with BCG vaccination. Both IGRAs responded to the M. tuberculosis exposure gradient and were positively associated with increasing TST induration (p = 0.003 for ELISPOT and p = 0.001 for QFT-GIT).

Conclusion

The ELISPOT test is more sensitive than the QFT-GIT for diagnosing TB disease. The two tests perform similarly in the diagnosis of LTBI in TB contacts. Significant discordance between the two IGRAs and between each and the TST remain largely unexplained.

[Interferon-gamma release assays in tuberculosis diagnostics]

Two years ago, CE certified interferon-gamma release assays (IGRA) were launched on the German market (QuantiFERON-TB Gold In-Tube and T-SPOT-TB). Since this time, a multitude of studies have analysed these assays. Guidelines have been elaborated by national expert committees of England, the USA and Switzerland. However, standards of tuberculosis diagnostics and management may vary from country to country. This statement provides practice relevant recommendations for indications, pre-analytics and the interpretation of IGRA test results under different clinical conditions. The IGRA are integrated into existing guidelines for the management of tuberculosis.

Heparin-binding-hemagglutinin-induced IFN-gamma release as a diagnostic tool for latent tuberculosis

BACKGROUND

The detection of latent tuberculosis infection (LTBI) is a major component of tuberculosis (TB) control strategies. In addition to the tuberculosis skin test (TST), novel blood tests, based on in vitro release of IFN-gamma in response to Mycobacterium tuberculosis-specific antigens ESAT-6 and CFP-10 (IGRAs), are used for TB diagnosis. However, neither IGRAs nor the TST can separate acute TB from LTBI, and there is concern that responses in IGRAs may decline with time after infection. We have therefore evaluated the potential of the novel antigen heparin-binding hemagglutinin (HBHA) for in vitro detection of LTBI.

METHODOLOGY AND PRINCIPAL FINDINGS

HBHA was compared to purified protein derivative (PPD) and ESAT-6 in IGRAs on lymphocytes drawn from 205 individuals living in Belgium, a country with low TB prevalence, where BCG vaccination is not routinely used. Among these subjects, 89 had active TB, 65 had LTBI, based on well-standardized TST reactions and 51 were negative controls. HBHA was significantly more sensitive than ESAT-6 and more specific than PPD for the detection of LTBI. PPD-based tests yielded 90.00% sensitivity and 70.00% specificity for the detection of LTBI, whereas the sensitivity and specificity for the ESAT-6-based tests were 40.74% and 90.91%, and those for the HBHA-based tests were 92.06% and 93.88%, respectively. The QuantiFERON-TB Gold In-Tube (QFT-IT) test applied on 20 LTBI subjects yielded 50% sensitivity. The HBHA IGRA was not influenced by prior BCG vaccination, and, in contrast to the QFT-IT test, remote (>2 years) infections were detected as well as recent (<2 years) infections by the HBHA-specific test.

CONCLUSIONS

The use of ESAT-6- and CFP-10-based IGRAs may underestimate the incidence of LTBI, whereas the use of HBHA may combine the operational advantages of IGRAs with high sensitivity and specificity for latent infection.

Tuberculosis antigen-specific immune responses can be detected using enzyme-linked immunospot technology in human immunodeficiency virus (HIV)-1 patients with advanced disease

There are limited data on the efficacy of T cell-based assays to detect tuberculosis (TB) antigen-specific responses in immune-deficient human immunodeficiency virus (HIV) patients. The aim of this study is to determine whether TB antigen-specific immune responses can be detected in patients with HIV-1 infection, especially in those with advanced disease (CD4 T cell count < 300 cells/microl). An enzyme-linked immunospot (ELISPOT) assay, which detects interferon (IFN)-gamma secreted by T cells exposed to TB antigens, was used to assess specific immune responses in a prospective study of 201 HIV-1-infected patients with risk factors for TB infection, attending a single HIV unit. The performance of the ELISPOT assay to detect TB antigen-specific immune responses is independent of CD4 T cell counts in HIV-1 patients. The sensitivity and specificity of this assay for the diagnosis of active tuberculosis does not differ significantly from values obtained in immunocompetent subjects. The negative predictive value of the TB ELISPOT test is 98.2%. A positive predictive value of 86% for the diagnosis of active tuberculosis was found when the combined number of early secretory antigen target-6 (ESAT-6) and culture filtrate protein-10 (CFP-10) IFN-gamma spots to CD4 T cell count ratio was > 1.5. TB antigen-specific immune responses can be detected in HIV patients with low CD4 T cell counts using ELISPOT technology in a routine diagnostic laboratory and is a useful test to exclude TB infection in immune-deficient HIV-1 patients. A combination of TB antigen-specific IFN-gamma responses and CD4 T cell counts has the potential to distinguish active tuberculosis from latent infection.

U.S. medical resident familiarity with national tuberculosis guidelines

Background

The ability of medical residents training at U.S. urban medical centers to diagnose and manage tuberculosis cases has important public health implications. We assessed medical resident knowledge about tuberculosis diagnosis and early management based on American Thoracic Society guidelines.

Methods

A 20-question tuberculosis knowledge survey was administered to 131 medical residents during a single routinely scheduled teaching conference at four different urban medical centers in Baltimore and Philadelphia. Survey questions were divided into 5 different subject categories. Data was collected pertaining to institution, year of residency training, and self-reported number of patients managed for tuberculosis within the previous year. The Kruskal-Wallis test was used to detect differences in median percent of questions answered correctly based on these variables.

Results

The median percent of survey questions answered correctly for all participating residents was 55%. Medical resident knowledge about tuberculosis did not improve with increasing post-graduate year of training or greater number of patients managed for tuberculosis within the previous year. Common areas of knowledge deficiency included the diagnosis and management of latent tuberculosis infection (median percent correct, 40.7%), as well as the interpretation of negative acid-fast sputum smear samples.

Conclusion

Many medical residents lack adequate knowledge of recommended guidelines for the management of tuberculosis. Since experience during training influences future practice pattterns, education of medical residents on guidelines for detection and early management of tuberculosis may be important for future improvements in national tuberculosis control strategies.

Interferon- Release Assays Improve the Diagnosis of Tuberculosis and Nontuberculous Mycobacterial Disease in Children in a Country with a Low Incidence of Tuberculosis

BACKGROUND

Diagnosis of childhood tuberculosis (TB) is challenging. The widely used tuberculin skin test (TST) may produce -positive results because of cross-reactivity with nontuberculous mycobacteria or bacille Calmette-Guerin vaccination, resulting in unnecessary treatment. Two recently developed interferon- gamma release assays (IGRAs) show good diagnostic accuracy for active TB in adults; pediatric data are limited, particularly in areas with a low incidence of TB. We assessed the diagnostic accuracy of IGRAs for TB in children in an area with a low incidence of TB.

METHODS

In a hospital-based study, the diagnostic accuracy of the TST and 2 IGRAs (T SPOT-TB [T-SPOT; Oxford Immunotec] and QuantiFERON-TB Gold In-Tube [QFT-IT; Cellestis]) were assessed in a cohort of 73 children (median age, 39 months); 28 children with bacteriologically confirmed TB were compared with children without TB (23 with bacteriologically confirmed nontuberculous mycobacterial lymphadenitis and 22 with other nonmycobacterial respiratory tract infections).

RESULTS

The specificity for TB of QFT-IT was 100% (95% confidence interval [CI], 91%-100%), and the specificity of T-SPOT was 98% (95% CI, 87%-100%), both of which were considerably higher than the specificity of TST (58%; 95% CI, 42%-73%). The specificity of the TST was 10.5% (95% CI, 1%-33%) in children with nontuberculous mycobacterial lymphadenitis and was 100% (95% CI, 83%-100%) in children with other nonmycobacterial respiratory tract infections. The sensitivity of both QFT-IT and T-SPOT was 93% (95% CI, 77%-99%), and the sensitivity of the TST was 100% (95% CI, 88%-100%). Agreement between the IGRAs was 95.6% ( kappa =0.91); 6.8% of the IGRAs showed indeterminate results.

CONCLUSIONS

Both IGRAs showed high diagnostic value in bacteriologically confirmed childhood TB. Their advantage in this study, when performed in addition to the TST, was the ability to distinguish -positive TST results caused by nontuberculous mycobacterial disease, thereby reducing overdiagnosis of TB and guiding clinical management.

Follow-up study of tuberculosis-exposed supermarket customers with negative tuberculin skin test results in association with positive gamma interferon release assay results

We report a follow-up study of 29 subjects with negative tuberculin skin test (TST) results in association with positive gamma interferon release assay (IGRA) results, mainly due to responses to CFP-10 in the T-SPOT.TB assay, during a contact investigation. One year later, 12/29 subjects (41%) had converted to positive TST results in association with negative IGRA results.

Current issues on molecular and immunological diagnosis of tuberculosis

Laboratory diagnosis of tuberculosis (TB) traditionally relies on smear microscopy and culture of Mycobacterium tuberculosis from clinical samples. With recent advances in technology, there have been numerous efforts to develop new diagnostic tests for TB that overcome the low sensitivity and specificity and long turnover time associated with current diagnostic tests. Molecular biological tests based on nucleic acid amplification have brought an unprecedented opportunity for the rapid and specific detection of M. tuberculosis from clinical specimens. With automated sequencing analysis, species identification of mycobacteria is now easier and more accurate than with conventional methods, and rapid detection of mutations in the genes associated with resistance to TB drugs provides early information on the potential drug resistance for each clinical isolate or for clinical samples. In addition, immunological tests for the detection of M. tuberculosis antigens and antibodies to the antigens have been explored to identify individuals at risk of developing TB or with latent TB infection (LTBI). The recent introduction of commercial IFN-gamma assay kits for the detection of LTBI provides a new approach for TB control even in areas with a high incidence of TB. However, these molecular and immunological tools still require further evaluation using large scale cohort studies before implementation in TB control programs.

Discrepancy between Mycobacterium tuberculosis-specific gamma interferon release assays using short and prolonged in vitro incubation

The sensitivities of various gamma interferon release assays (IGRAs) for the detection of past latent Mycobacterium tuberculosis infection are not known. In this study, we aimed to assess the effects of various IGRA formats and in vitro incubation periods on test outcome. The results of the tuberculin skin test (TST) were compared with those of the QuantiFERON-TB Gold in-tube (QFT-GIT) test, an overnight enzyme-linked immunospot assay (ELISPOT), and a 6-day lymphocyte stimulation test (LST) by using the same M. tuberculosis-specific peptides and samples from 27 TST-positive persons with a history of exposure to M. tuberculosis, 4 patients cured of tuberculosis (TB), and 9 TST-negative controls. Among the TST-positive persons, the LST was more frequently positive (92%; P < 0.01) than either the QFT-GIT test (33%) or ELISPOT (46%). While good agreement was observed between the QFT-GIT test and ELISPOT (kappa = 0.71) and between TST and LST (kappa = 0.78), the agreement between TST or LST, on the one hand, and the QFT-GIT test or ELISPOT, on the other, was poor. These data indicate that the QFT-GIT test and overnight ELISPOT are less sensitive for the detection of past latent TB than the 6-day LST. The observed discrepancies between these IGRAs are most likely related to differences in incubation periods. Whether TST-positive persons with positive LST results but negative QFT-GIT and ELISPOT results are at risk for the development of TB needs to be elucidated before short-incubation IGRAs can be used for the screening of individuals for latent TB before immunosuppressive treatment.

The prognosis of latent tuberculosis: can disease be predicted?

In humans, Mycobacterium tuberculosis persists for long periods in a clinically latent state, creating a huge reservoir of 'silent' tuberculosis (TB) (roughly one-third of the global population) from which new cases continually arise. A prognostic marker for active TB would enable targeted treatment of the small fraction of infected individuals who are most at risk of developing contagious TB, contributing greatly to TB control efforts. Here, we propose that TB-specific interferon-gamma release assays might be useful for identifying individuals with progressive infections who are likely to develop the disease. This might provide an unprecedented advantage for TB control, namely targeted preventive therapy for individuals who are most at risk of developing active contagious TB.

Comparison of Mantoux and QuantiFERON TB Gold tests for diagnosis of latent tuberculosis infection in Army personnel

The tuberculin skin test (TST) was compared with QuantiFERON-TB Gold in-tube (QFT-GIT) test for the diagnosis of tuberculosis in non-Mycobacterium bovis BCG-vaccinated military personnel. Among subjects positive by TST, 44.4% of recruits were positive by QFT-GIT compared with 11.5% subjects tested after missions abroad, suggesting that most TST conversions in the latter group were caused by nontuberculous mycobacteria.

Comparison of an interferon-gamma release assay with tuberculin skin testing in HIV-infected individuals

RATIONALE

Although interferon (IFN)-gamma release assays are approved for the diagnosis of latent tuberculosis infection (LTBI), limited data exist regarding their performance in HIV infection.

OBJECTIVES

To compare tuberculin skin test (TST) results to the commercial IFN-gamma release assay QuantiFERON-TB Gold In-Tube (QFT) for the diagnosis of LTBI in HIV-infected adults.

METHODS

A total of 294 HIV-infected subjects sampled from two San Francisco cohorts underwent TST, using 5 TU of purified protein derivative, and QFT, measuring IFN-gamma response to Mycobacterium tuberculosis-specific RD-1 antigens.

MAIN RESULTS

Of 294 participants, 205 (70%) returned for an evaluable TST. Concordance between QFT and TST was 89.3% (kappa=0.37, p=0.007). However, in subjects with positive test results by either TST or QFT, only 28% (8/29) had positive test results by both modalities. TST-positive/QFT-negative discordant results were found in 5.1% of subjects and TST-negative/QFT-positive discordance in 5.6%. Indeterminate QFT results occurred in 5.1%, all due to a failure to respond to the phytohemagglutinin-positive control. Subjects with a CD4(+) count of less than 100 cells/mm(3) had a relative risk of an indeterminate result of 4.24 (95% confidence interval, 1.55-11.61; p=0.003) compared with those with a CD4(+) count of 100 or more.

CONCLUSIONS

Overall concordance between QFT and TST in HIV infection was high, but agreement among subjects with positive tests by either modality was low.