Prospective evaluation of a whole-blood test using Mycobacterium tuberculosis-specific antigens ESAT-6 and CFP-10 for diagnosis of active tuberculosis

[Innovative strategies to diagnose and monitor tuberculosis patients]

Mycobacterium tuberculosis is the single most deadly microorganism worldwide. A third of the world population is thought to have latent tuberculosis and approximately 2 million people die of the disease each year. Short and closely supervised treatment regimens are needed, but it is also essential to develop new strategies to ensure prompt diagnosis of the disease. In particular, cheap methods are needed to tackle tuberculosis from a population perspective. The present article reviews the advances in immunology and molecular strategies for epidemiological diagnosis and monitoring of tuberculosis patients.

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.

Evaluation of Mycobacterium tuberculosis--specific antibody responses in populations with different levels of exposure from Tanzania, Ethiopia, Brazil, and Denmark

BACKGROUND

New, simple, and better-performing diagnostic tools are needed for the diagnosis of tuberculosis (TB). Much effort has been invested in developing an antibody-based test for TB, but to date, no such test has performed with sufficient sensitivity and specificity. A key question remaining is the extent to which the disappointing performance of current tests is associated with a high background prevalence of latent TB.

METHODS

We compared Mycobacterium tuberculosis-specific ESAT-6 and CFP-10 antibody responses in a total of 565 human serum samples from M. tuberculosis-uninfected donors and donors with latent infection, as well as samples from patients with active TB. Our study included samples from 4 countries, representing environments with low, intermediate, and high TB incidences.

RESULTS

We demonstrated significant increases in antibody levels in latently infected contacts, compared with M. tuberculosis-uninfected individuals, and in patients with active TB disease, compared with latently infected contacts. Furthermore, we found a striking increase in the magnitude of the antibody responses in samples obtained from infected Ethiopian individuals (with and without disease), compared with Danish and Brazilian infected individuals; this was presumably the result of higher exposure levels.

CONCLUSIONS

Our study confirms the presence of ESAT-6 and CFP-10 antibodies in patients with TB, and we demonstrate that significant antibody responses are not restricted to active TB disease but can reflect latent infection, particularly in areas with high levels of exposure to M. tuberculosis. This finding is important for the understanding of the poor discriminatory power of current serodiagnostic tests in regions of endemicity, and it may have major implications on the future development of serologic tests.

Diagnosis of tuberculosis by an enzyme-linked immunospot assay for interferon-gamma

*National Taiwan University Hospital, Taipei, Taiwan,

This assay for interferon-γ can rapidly and accurately diagnose active tuberculosis in a disease-endemic area.

We evaluated an enzyme-linked immunospot assay for interferon-γ (T SPOT-TB) for rapid diagnosis of active tuberculosis (TB) in a disease-endemic area. From January to June 2005, patients whose clinical symptoms and radiographic findings were compatible with TB were recruited, and a blood sample was obtained for T SPOT-TB assay within 7 days of microbiologic studies. Sixty-five patients were studied, including 39 (60%) with active TB. Thirty-five (53.8%) patients had underlying medical conditions. Thirty-seven patients had positive cultures for Mycobacterium tuberculosis, and 11 patients had positive cultures for nontuberculous mycobacteria. The sensitivity, specificity, positive predictive value, and negative predictive value of the T SPOT-TB assay were 87.2%, 88.5%, 91.9%, and 82.1%, respectively. The accuracy of this test in diagnosing active TB is >80%, even in an area with a high incidence of nontuberculous mycobacteria disease.

Diagnosis of tuberculosis in an Indian population by an indirect ELISA protocol based on detection of Antigen 85 complex: a prospective cohort study

BACKGROUND

Diagnosis of tuberculosis (TB) remains problematic despite many new advanced diagnostic methods. A reliable and rapid diagnostic test, which could be performed in any standard pathology laboratory, would help to obtain definitive early diagnoses of TB. In the present study we describe a prospective evaluation for demonstrating Antigen (Ag) 85 complex in the sera from TB patients.

METHODS

Indirect ELISA, employing monoclonal antibodies (mAb) against the purified Ag 85 complex, was used to demonstrate Ag 85 complex in sera from TB patients. Serum samples were obtained from 197 different groups of patients: confirmed TB {n = 24}, clinically diagnosed TB {n = 104}, disease controls {n = 49} and healthy controls {n = 20}. Receiver operating curve (ROC) was used to calculate the cut off value and comparison between TB and non-TB groups were done by the chi-square test.

RESULTS

The indirect ELISA method, using an mAb against Ag 85 complex, yielded 82% sensitivity (95% confidence interval [CI] 67 to 93%) and 86% specificity (95% CI, 57 to 98%) for the diagnosis of TB. The serum positivities for Ag 85 complex in cases of confirmed and clinically diagnosed TB patients were 96% (23/24) and 79% (82/104) respectively, while the positivity for patients in the non-tuberculosis group was 14% (10/69).

CONCLUSION

The detection of Ag 85 complex in sera from TB patients by indirect ELISA using mAb against purified Ag 85 complex gives a reliable diagnosis and can be used to develop an immunodiagnostic assay with increased sensitivity and specificity.

Diagnóstico imunológico da tuberculose: problemas e estratégias para o sucesso

A tuberculose continua sendo um grave problema social e de saúde, afetando milhões de pessoas anualmente. A vacina Bacille Calmette-Guerin (BCG), usada no controle profilático, é incapaz de conter a progressão da doença, que usualmente se manifesta através da queda da imunidade celular do indivíduo. O diagnóstico da tuberculose em seus estágios iniciais, aliado à poliquimioterapia, pode contribuir para o controle da disseminação da infecção. Os atuais métodos de diagnóstico apresentam problemas, como: baixa sensibilidade da baciloscopia; longo tempo de realização das culturas microbiológicas; e baixa especificidade do teste cutâneo com o derivado protéico purificado do M. tuberculosis. Novos métodos de diagnóstico que utilizam antígenos específicos (por exemplo, os conhecidos em inglês como o early secreted antigenic target 6-kDa e o culture filtrate protein 10-kDa), estão sendo testados. Os genes que codificam esses antígenos estão localizados na região de diferença 1 do M. tuberculosis, M. africanum e M. bovis, mas estão ausentes no M. bovis (BCG) e na maioria das micobactérias do meio ambiente. Métodos de diagnóstico baseados na produção de interferon-gama por linfócitos T, em resposta a esses antígenos, como o QuantiFERON-TB® e o T SPOT.TB®, estão sendo testados, e superam o teste cutâneo com o derivado protéico purificado nas seguintes características: maior sensibilidade; menor reatividade cruzada devido à vacinação com o BCG ou infecção por micobactérias do meio ambiente; e tempo de execução. A introdução de métodos de diagnóstico mais específicos e sensíveis, assim como um maior entendimento dos mecanismos moleculares e celulares que regulam a interação parasito-hospedeiro, pode contribuir para um eficiente combate à tuberculose.

Usefulness of whole-blood interferon-gamma assay and interferon-gamma enzyme-linked immunospot assay in the diagnosis of active pulmonary tuberculosis

PURPOSES

The aim of this study was to evaluate the usefulness of the whole-blood interferon-gamma assay (enzyme-linked immunosorbent assay [ELISA]) and interferon-gamma enzyme-linked immunospot assay (ELISPOT) based on early secretory antigenic target 6 and culture filtrate protein 10 in the diagnosis of active pulmonary tuberculosis (TB) in routine clinical practice.

METHOD

We conducted a prospective study enrolling 144 participants with suspected pulmonary TB in a tertiary referral hospital in Seoul, South Korea, to investigate the diagnostic sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of these tests. Clinical assessment, tuberculin skin test (TST), whole-blood interferon-gamma ELISA (QuantiFERON-TB Gold [QFT-G]; Cellestis Ltd; Victoria, Australia), and an ELISPOT assay (T SPOT.TB; Oxford Immunotec; Oxford, UK) were performed. Test results were compared with the final confirmed diagnoses.

RESULTS

Active pulmonary TB was diagnosed in 67 of 144 participants (47%). Sensitivities of QFT-G and T SPOT.TB for active pulmonary TB were 89% (95% confidence interval [CI], 79 to 96%) and 92% (95% CI, 83 to 97%), respectively; and specificities were 49% (95% CI, 37 to 61%) and 47% (95% CI, 36 to 59%). NPVs of QFT-G (84%; 95% CI, 69 to 93%) and T SPOT.TB (87%; 95% CI, 73 to 96%) were higher than that of TST (64%; 95% CI, 51 to 76%) [p = 0.001 and p < 0.001, respectively].

CONCLUSION

High NPVs of QFT-G and T SPOT.TB for the diagnosis of active TB suggest the supplementary role of these tests for the diagnostic exclusion of active TB, although the low PPV limits their usefulness in routine clinical practice in South Korea, where the prevalence of latent TB infection is considerable.

Extended culture enhances sensitivity of a gamma interferon assay for latent Mycobacterium tuberculosis infection

To test the hypothesis that prolonged culture would enhance the sensitivity of latent tuberculosis detection by a gamma interferon release assay, blood samples from 33 household contacts of Gambian tuberculosis patients were stimulated with Mycobacterium tuberculosis-specific antigens. After 24 h of culture, 66% were positive, compared to 93% after 6 days of culture.

Les nouveaux tests immunologiques dans le diagnostic de la tuberculose (TB or not TB)

INTRODUCTION

Targeted testing and treatment of individuals with latent tuberculosis infection (LTBI), at high risk of progression to active tuberculosis (ATB), are key elements in the battle against tuberculosis, both in France and in many parts of the world. Though the finding of tubercle bacilli is the essential examination for the diagnosis of ATB, there is no indisputable test for LTBI.

BACKGROUND

The help currently given to the diagnosis of LTBI by the degree of positivity of the tuberculin skin test (TST) is limited, both operationally and logistically, in populations vaccinated with BCG or sensitised by atypical mycobacteria, and by its low sensitivity in those immuno-suppressed persons who are at greatest risk of progression. Moreover the TST has other operational limitations linked to return visits, repeat testing causing a boosting effect and subjective interpretation. A new approach follows the availability of two biological tests for the diagnosis of LTBI (QuantiFERON-TB and T-SPOT-TB) that measure the in-vitro production of interferon gamma (IFN-gamma) by the blood mononuclear cells in response to M. tuberculosis specific antigens (ESAT-6 and CFP10). This revue analyses the published studies, undertaken with varying numbers of patients, that evaluate the diagnostic accuracy of these two tests in comparison with TST. However, validation is handicapped by the lack of a "gold standard" for the diagnosis of LTBI. These studies demonstrate similar levels of specificity for the two biological tests. They are statistically higher than those for TST, particularly in populations vaccinated by BCG. On the other hand, their sensitivity was at least equivalent to that of TST and, in certain studies, superior with T-SPOT-TB. Finally, several studies in contacts have been undertaken with the aim of measuring the concordance between these biological tests and TST. The essential finding is of a very good correlation between positivity of the biological tests and the degree of exposure of the contacts. These tests have additional operational advantages over TST: completed in one visit, results available in 24 hours, absence of inter and intra observer divergence, detection of potential immuno-depression and avoidance of boosting by repeat testing.

VIEWPOINT

Currently, however, these biological tests present several operational limits: lower sensitivity in severe disease, incomplete data in immuno-suppressed subjects and in children, lack of predictive value for future development of ATB, lack of distinction between LTBI and ATB. Numerous clinical studies are under way, in France and elsewhere, in order to reduce these limitations and to allow the appropriate incorporation of these tests into protocols for the diagnosis of tuberculosis.

CONCLUSIONS

These two biological tests should, in the near future, replace or complement TST in the diagnosis of recent LTBI, leading to their optimal incorporation into the decision making processes of the national plans for the control of tuberculosis.

Les nouveaux tests diagnostiques de la tuberculose

INTRODUCTION

A major challenge in tuberculosis (TB) control is the diagnosis and the treatment of latent tuberculosis infection.

STATE OF THE ART

At the time, the diagnosis is based on tuberculin skin test (TST). TST is not specific, has poor sensitivity and is not easy to perform.

PERSPECTIVES

Two interferon-based tests for the diagnosis of tuberculosis have just been licensed. These tests have some advantages on TST. They require only a blood sample and their results are not dependent on the examinator. Their specificity is higher than TST because they don't cross-react with BCG vaccination and with most of the environmental Mycobacterium species. Their sensitivity is higher for the diagnosis of active tuberculosis too. For latent tuberculosis, the interferon-gamma assays show a better correlation with the exposure to Mycobacterium tuberculosis than TST. The ability to detect TB of the two tests seem to be reduced in immunocompromised patients, specially in medically ones.

CONCLUSIONS

Interferon-gamma assays seems to be useful tools in TB detection, but these good results have to be confirmed in larger studies with unselected patients.

The TB pandemic: an old problem seeking new solutions

Tuberculosis (TB) continues to kill more than 2 million people globally each year. Annual TB case notification rates have risen up to fourfold since the mid-1980s, with the highest rate of 1000/100,000 around Cape Town, South Africa. There is an urgent need for novel diagnostic methods and preventive vaccines to control this epidemic. The rising incidence of TB has been attributed to HIV co-infection especially in developing countries. The threat of drug resistance arising from ineffective TB treatment programmes is looming and could potentially lead to loss of any gains made in controlling the disease globally.

Chemotherapy and diagnosis of tuberculosis

Since after the first streptomycin 1944 trials, anti-tuberculous chemotherapy research has been focused upon establishing drug combination regimens capable of overcoming drug resistance and amenable to ambulatory treatment in resource strapped countries. The first milestone being the 1959 Madras trial comparing home and sanatorium treatment in South India. Subsequently, the MRC trials led Fox and Mitchison to indicate rifampicin, isoniazid and pyrazinamide as the first line drugs for short course, 6 month, regimens and the 1982 Hong Kong Chest Service trials established intermittent therapy as the ambulatory treatment standard for directly observed therapy (DOT). The rising of the HIV epidemic at the beginning of the 1980s has refuelled tuberculosis spread in Africa and Asia and contributed to the expansion of drug-resistant tuberculosis worldwide making the development of new drugs and drug regimens for ambulatory treatment a top priority. Led by biotechnological advances, molecular biology has been brought into TB laboratory diagnosis for the highly sensitive and specific rapid identification of Mycobacterium tuberculosis in biological samples. The field of immunological diagnosis of TB infection, dominated since the early 1900s by the intradermal tuberculin reaction has been put back in motion by the discovery of M. tuberculosis-specific proteins and peptides, now employed in blood tests of high sensitivity and specificity for the diagnosis of latent TB which may help with the identification of contacts at higher risk of active disease and the eradication of epidemic cases.

Avantages et limites des tests sanguins in vitro lymphocytes T/interféron gamma comparativement au test intradermique à la tuberculine pour le diagnostic de tuberculose☆

The development of in vitro blood tests that measure the delayed hypersensitivity reaction developed after contact with Mycobacterium tuberculosis will change progressively the diagnosis of M. tuberculosis infection. These blood assays (Quantiferon TB Gold, Cellestis, Australia; T-SPOT.TB, Oxford Immunotec, United Kingdom) use specific, complex M. tuberculosis antigens (ESAT-6 and CFP-10), whereas the intra-dermal Mantoux test is done with tuberculin, a complex mixture of more than 200 antigens. ESAT-6 and CFP-10 are absent from all the BCG vaccine strains used throughout the world. Significant improvement in the specificity with equivalent or increased sensitivity of the in vitro tests compared to the Mantoux test will lead eventually to replacement of the latter.

Usefulness of QuantiFERON TB-2G, a diagnostic method for latent tuberculosis infection, in a contact investigation of health care workers

OBJECTIVE

To evaluate the usefulness of QuantiFERON TB-2G (QFT-2G) for detecting latent tuberculosis infection (LTBI) in a contact investigation of health care workers.

METHODS

The investigated subjects were 190 subjects among the health care workers who were examined between January 2005 and June 2006. Background information, including a past history of tuberculosis (TB) or tuberculin skin test (TST) or BCG vaccination, and contact score (infectivity x contact duration) were investigated. The TST and QFT-2G test were performed on all subjects.

RESULTS

In 109 subjects with a negative TST history, the TST results were converted to positive in 38 subjects. While the TST was positive in 48 subjects (25%), the QFT-2G test was positive in only five subjects (3%). The correlation of the QFT-2G with TST results was not significant. There was no relationship between contact score and the TST result. Twenty-nine subjects had TST positive responses (22%), but there were no QFT-2G positive responses in subjects with a mild contact score. Sixteen subjects had TST positive responses (31%), but two subjects showed positive QFT-2G results (4%) in the moderate contact score group. However, the positive response rate of the TST and QFT-2G test was the same percentage in the severe contact score group (33%).

CONCLUSION

The QFT-2G test showed a significant relationship with the contact score when compared with the TST. If the subjects with LTBI in the moderate contact score group were selectively excluded, the contact investigation in the mild contact score group may not be necessary. If there would have been many subjects with the QFT-2G positive responses in the moderate contact score group, we think that the QFT-2G test must be performed even in the mild contact score group.

Clinical Reevaluation of the QuantiFERON TB-2G Test as a Diagnostic Method for Differentiating Active Tuberculosis from Nontuberculous Mycobacteriosis

INTRODUCTION

We reevaluated the usefulness of a whole-blood interferon-gamma enzyme-linked immunosorbent assay (QuantiFERON TB-2G [QFT-TB]; Cellestis) in obtaining a differential diagnosis between active tuberculosis (TB) and nontuberculous mycobacteriosis (NTM).

METHODS

The subjects were 50 healthy volunteers, 50 patients with active TB, and 100 patients with NTM who satisfied the diagnostic guidelines of the American Thoracic Society from April 2005 through June 2006. The tuberculin skin test (TST) and the QFT-TB test were performed for all subjects. The QFT-TB test was performed every 2 months.

RESULTS

Of the healthy volunteers, 64% had a negative TST result and 94% had a negative QFT-TB test result. Of the patients with active TB, 64% had a positive TST result and 4% had a negative QFT-TB test result. Of the patients with pulmonary Mycobacterium avium complex disease, 60% had a positive TST result and 7% had a positive QFT-TB test result. The QFT-TB test had a mean sensitivity of 86% and a mean specificity of 94%. The QFT-TB test results for patients with active TB transiently decreased during treatment involving antituberculous drugs. The rate of positive QFT-TB test results was 86% at the initiation of treatment, 48% 6 months later, and 33% 12 months later.

CONCLUSIONS

We confirmed that the QFT-TB test is a useful diagnostic method for differentiating active pulmonary TB from NTM, compared with the TST. However, because it is possible that the effect of the QFT-TB test may be long lasting after treatment and may not be resolved over time, even with treatment, as in this study, it may not provide any level of certainty regarding cure of infection.

QuantiFERON-TB Gold: state of the art for the diagnosis of tuberculosis infection?

Tuberculosis (TB) remains a major threat to global health. The recently launched Global Plan to Stop Tuberculosis 2006-2015 highlights the need for accurate, simple and low-cost diagnostic tests for the detection of TB infection. For the first time in decades, new diagnostic tools have emerged that may facilitate this goal. The discovery of Mycobacterium tuberculosis-specific immunodominant antigens has led to the development of interferon gamma-release assays that have been shown to have high sensitivity and specificity for TB disease. This review focuses on the QuantiFERON-TB Gold tests and addresses the potential strengths and limitations of the current assays, summarizes the available evidence for their use and identifies areas of future research and development. Although representing an advance in TB diagnostics, with the potential to have a significant impact on global TB control, many issues remain unanswered. The cost of the tests and laboratory requirements may limit their use in developing countries. Most importantly, additional studies are needed in TB-endemic regions, particularly in high-risk persons such as children and individuals who are also co-infected with HIV.

Low sensitivity of a whole-blood interferon-gamma release assay for detection of active tuberculosis

The sensitivity of an interferon-gamma assay (Quantiferon-TB Gold; Cellestis) was evaluated for the detection of tuberculosis among 242 persons with suspected tuberculosis in San Francisco, California. Thirty-seven subjects had culture-confirmed tuberculosis. Excluding 1 indeterminate result, 23 (64%; 95% confidence interval, 48%-78%) of 36 subjects had positive results using the QuantiFERON-TB Gold assay. The 64% sensitivity suggests that the QuantiFERON-TB Gold assay should not be used alone to exclude active tuberculosis.

Cellular recognition of Mycobacterium tuberculosis ESAT-6 and KatG peptides in systemic sarcoidosis

Sarcoidosis is an enigmatic disease with a pathology similar to that of tuberculosis. We detected Th-1 immune responses to Mycobacterium tuberculosis ESAT-6 and KatG peptides from peripheral blood mononuclear cells from 15/26 sarcoidosis, 1/24 purified-protein-derivative-negative (PPD-) (P < 0.0001, Fisher's exact test), and 7/8 PPD-positive (PPD+) subjects (P = 0.21). This finding provides immunologic links between mycobacteria and systemic sarcoidosis.

An Update on the Diagnosis of Tuberculosis Infection

Targeted testing and treatment of individuals with latent tuberculosis infection at increased risk of progression to active disease is a key element of tuberculosis control. This strategy is limited by the poor specificity of the tuberculin skin test in populations vaccinated with bacille Calmette-Guérin and its low sensitivity in immunosuppressed persons, who are at highest risk of progression. Two blood tests (T-SPOT.TB and QuantiFERON-TB Gold), based on detection of IFN-gamma released by T cells in response to M. tuberculosis-specific antigens, may offer an improvement on the skin test. However, validation is challenging due to the lack of a diagnostic gold standard. This critical appraisal of published evidence summarizes the diagnostic accuracy of the new tests. The blood tests have operational advantages over the skin test because no return visit is required, results are available by the next day, and repeated testing does not cause boosting. Both tests are significantly more specific than the skin test in populations vaccinated with bacille Calmette-Guérin. The data suggest that T-SPOT.TB may be more sensitive than the skin test. Data in groups at high risk of progression to disease are scarce, and more research is needed in these populations, but it is clear that T-SPOT.TB performs better than the skin test in young children and HIV-infected people with active tuberculosis. Incorporation of these tests into programs for targeted testing of latent tuberculosis infection will reduce false-positive and false-negative results inherent in tuberculin testing, equipping clinicians with more accurate tools for tuberculosis control and elimination in the 21st century.

Selected RD1 peptides for active tuberculosis diagnosis: comparison of a gamma interferon whole-blood enzyme-linked immunosorbent assay and an enzyme-linked immunospot assay

We recently set up a gamma interferon (IFN-gamma) enzyme-linked immunospot assay (ELISPOT), using selected early secreted antigenic target 6 (ESAT-6) peptides, that appears specific for active tuberculosis (A-TB). However, ELISPOT is difficult to automate. Thus, the objective of this study was to determine if the same selected peptides may be used in a technique more suitable for routine work in clinical laboratories, such as whole-blood enzyme-linked immunosorbent assay (WBE). For this purpose, 27 patients with A-TB and 41 control patients were enrolled. Our WBE, using the already described selected peptides from ESAT-6 plus three new ones from culture filtrate protein 10, was performed, and data were compared with those obtained by ELISPOT. Using our selected peptides, IFN-gamma production, evaluated by both WBE and ELISPOT, was significantly higher in patients with A-TB than in controls (P < 0.0001). Statistical analysis showed a good correlation between the results obtained by WBE and ELISPOT (r = 0.80, P < 0.001). To substantiate our data, we compared our WBE results with those obtained by QuantiFERON-TB Gold, a whole-blood assay based on region of difference 1 (RD1) overlapping peptides approved for TB infection diagnosis. We observed a slightly higher sensitivity with QuantiFERON-TB Gold than with our WBE (89% versus 81%); however, our test provided a better specificity result (90% versus 68%). In conclusion, results obtained by WBE based on selected RD1 peptides significantly correlate with those generated by ELISPOT. Moreover, our assay appears more specific for A-TB diagnosis than QuantiFERON-TB Gold, and thus it may represent a complementary tool for A-TB diagnosis for routine use in clinical laboratories.

An in-house RD1-based enzyme-linked immunospot-gamma interferon assay instead of the tuberculin skin test for diagnosis of latent Mycobacterium tuberculosis infection

Identification of individuals infected with Mycobacterium tuberculosis is essential for the control of tuberculosis (TB). The specificity of the currently used tuberculin skin test (TST) is poor because of the broad antigenic cross-reactivity of purified protein derivative (PPD) with BCG vaccine strains and environmental mycobacteria. Both ESAT-6 and CFP-10, two secretory proteins that are highly specific for M. tuberculosis complex, elicit strong T-cell responses in subjects with TB. Using an enzyme-linked immunospot (ELISPOT)-IFN-gamma assay and a restricted pool of peptides derived from ESAT-6 and CFP-10, we have previously demonstrated a high degree of specificity and sensitivity of the test for the diagnosis of TB. Here, 119 contacts of individuals with contagious TB who underwent TST and the ELISPOT-IFN-gamma assay were consecutively recruited. We compared the efficacy of the two tests in detecting latent TB infection and defined a more appropriate TST cutoff point. There was little agreement between the tests (k = 0.33, P < 0.0001): 53% of the contacts with a positive TST were ELISPOT negative, and 7% with a negative TST were ELISPOT positive. Furthermore, respectively 76 and 59% of the ELISPOT-negative contacts responded in vitro to BCG and PPD, suggesting that most of them were BCG vaccinated or infected with nontuberculous mycobacteria. The number of spot-forming cells significantly correlated with TST induration (P < 0.0001). Our in-house ELISPOT assay based on a restricted pool of highly selected peptides is more accurate than TST for identifying individuals with latent TB infection and could improve chemoprophylaxis for the control of TB.

Diagnosis of latent Mycobacterium tuberculosis infection: is the demise of the Mantoux test imminent?

Tuberculosis is responsible for more then 2 million deaths worldwide each year and vies with HIV as the world's most fatal infectious disease. In many developing countries, attempts to control the spread of infection rely solely on identification and treatment of those with active disease, ignoring subclinical infection. However, in developed countries, large efforts are also expended to identify and give prophylactic drugs to people with latent tuberculosis infection. Until recently, the 100-year-old tuberculin skin test (Mantoux) has been the only available diagnostic test for latent tuberculosis infection, despite its many well-known limitations. Advances in scientific knowledge have led to the development of tests for tuberculosis that measure the production of interferon-gamma by T-cells stimulated in vitro with Mycobacterium tuberculosis-specific antigens. These interferon-gamma tests are highly specific and unaffected by prior Bacille Calmette-Guérin vaccination or immune reactivity to most atypical mycobacteria. They are more sensitive than the tuberculin skin test in detecting people with active tuberculosis, and their results correlate more closely with M. tuberculosis exposure risk factors than the tuberculin skin test in people likely to have latent tuberculosis infection. Science has caught up with one of the oldest diagnostic tests still in use worldwide, and the adoption of new, tuberculosis-specific interferon-gamma-based tests should move us one step closer to better control of this insidious pathogen.

Rapid diagnosis of smear-negative tuberculosis by bronchoalveolar lavage enzyme-linked immunospot

RATIONALE

In a large proportion of patients with active pulmonary tuberculosis (pTB), acid-fast bacilli smear results for sputum and bronchial secretions are negative. Detectable growth of Mycobacterium tuberculosis (MTB) in cultures takes several weeks and MTB-specific DNA amplification results on sputum and bronchial secretions are variable in these patients.

OBJECTIVE

We investigated whether a rapid diagnosis of pTB can be established by enumeration of MTB-specific mononuclear cells from bronchoalveolar lavage (BAL) fluid in routine clinical practice.

METHODS

Patients presenting to a tertiary hospital with medical histories and pulmonary infiltrates compatible with tuberculosis, and negative acid-fast bacilli smear results (three) from sputum, were prospectively enrolled in this study. An MTB-specific enzyme-linked immunospot assay (ELISPOT [T-SPOT.TB; Oxford Immunotec, Abingdon, UK]) with early antigenic target-6 (ESAT-6) and culture filtrate protein-10 (CFP-10) peptides was performed on peripheral blood mononuclear cells (PBMCs) and mononuclear cells from the BAL fluid (BALMCs).

MEASUREMENTS AND MAIN RESULTS

Of 37 patients, 12 were found to have smear-negative pTB and 25 were found to have an alternative diagnosis. Patients with tuberculosis had a median number of 17 ESAT-6-specific cells and 24.5 CFP-10-specific cells per 200,000 PBMCs and 37.5 ESAT-6-specific cells and 49.5 CFP-10-specific cells per 200,000 cells in the BAL fluid. Control patients had a median of 1 ESAT-6-specific cell and 1 CFP-10-specific cell per 200,000 PBMCs and no ESAT-6- and CFP-10-specific cells per 200,000 cells in the BAL fluid (p < 0.0001). All patients with TB but none of the control subjects had more than 5 spot-forming cells per 200,000 BALMCs with either peptide in the BAL fluid ELISPOT.

CONCLUSION

Smear-negative pulmonary tuberculosis can be diagnosed rapidly by identification of MTB-specific cells in the BAL fluid.

New tools and emerging technologies for the diagnosis of tuberculosis: part I. Latent tuberculosis

Nearly a third of the world's population is estimated to be infected with Mycobacterium tuberculosis. This enormous pool of latently infected individuals poses a major hurdle for global tuberculosis (TB) control. Currently, diagnosis of latent TB infection (LTBI) relies on the tuberculin skin test (TST), a century-old test with known limitations. In this review, the first of a two-part series on new tools for TB diagnosis, recent advances in the diagnosis of LTBI are described. The biggest advance in recent years has been the development of in vitro T-cell-based interferon-gamma release assays (IGRAs) that use antigens more specific to M. tuberculosis than the purified protein derivative used in the TST. Available research evidence on IGRAs suggests they have higher specificity than TST, better correlation with surrogate markers of exposure to M. tuberculosis in low-incidence settings, and less cross-reactivity due to BCG vaccination than the TST. IGRAs also appear to be at least as sensitive as the purified protein derivative-based TST for active TB. In the absence of a gold standard for LTBI, sensitivity and specificity for LTBI are not well defined. Besides high specificity, other potential advantages of IGRAs include logistical convenience, avoidance of poorly reproducible measurements, such as skin induration, need for fewer patient visits and the ability to perform serial testing without inducing the boosting phenomenon. Overall, due to its high specificity, IGRAs may be useful in low-endemic, high-income settings where cross-reactivity due to BCG might adversely impact the utility of TST. However, despite the growing evidence supporting the use of IGRAs, several unresolved and unexplained issues remain. The review concludes by highlighting areas where evidence is lacking, and provides an agenda for future research. Active TB and drug resistance are discussed in Part II; 423-432 of this issue.

Towards an immunodiagnostic test for leprosy

In addition to multidrug therapy, elimination of leprosy requires improved diagnostic methods. Using a comparative genomics approach, 17 potential protein antigens (MLP) that are restricted to Mycobacterium leprae, or of limited distribution, were produced and tested for antigen-specific immune responses on leprosy patients, healthy contacts of leprosy patients, and tuberculosis patients in Mali and Bangladesh, as well as on non-endemic controls. T-cell antigenicity of MLP was confirmed by IFN-gamma production in whole-blood assays with the highest responses observed in paucibacillary leprosy patients and healthy contacts. Four MLP behaved well in both countries and induced significantly different responses between the study groups. Peptides carrying T cell epitopes from one of the antigens gave promising results in restimulation assays in mice and immune responses were not influenced by prior exposure to BCG or environmental mycobacteria. This study provides the immunological framework for the development of a specific, peptide-based immunodiagnostic test for leprosy.

Immune responses to defined antigens of Mycobacterium bovis in cattle experimentally infected with Mycobacterium kansasii

Cross-reactive responses elicited by exposure to nontuberculous mycobacteria often confound the interpretation of antemortem tests for Mycobacterium bovis infection of cattle. The use of specific proteins (e.g., ESAT-6, CFP-10, and MPB83), however, generally enhances the specificity of bovine tuberculosis tests. While genes for these proteins are absent from many nontuberculous mycobacteria, they are present in M. kansasii. Instillation of M. kansasii into the tonsillar crypts of calves elicited delayed-type hypersensitivity and in vitro gamma interferon and nitrite concentration responses of leukocytes to M. avium and M. bovis purified protein derivatives (PPDs). While the responses of M. kansasii-inoculated calves to M. avium and M. bovis PPDs were approximately equivalent, the responses of M. bovis-inoculated calves to M. bovis PPD exceeded their respective responses to M. avium PPD. The gamma interferon and nitrite responses of M. kansasii-inoculated calves to recombinant ESAT-6-CFP-10 (rESAT-6-CFP-10) exceeded corresponding responses of noninoculated calves as early as 15 and 30 days after inoculation, respectively, and persisted throughout the study. The gamma interferon and nitrite responses of M. bovis-inoculated calves to rESAT-6-CFP-10 exceeded the corresponding responses of M. kansasii-inoculated calves beginning 30 days after inoculation. By using a lipoarabinomannan-based enzyme-linked immunosorbent assay, specific serum antibodies were detected as early as 50 days after challenge with M. kansasii. By a multiantigen print immunoassay and immunoblotting, serum antibodies to MPB83, but not ESAT-6 or CFP-10, were detected in M. kansasii-inoculated calves; however, responses to MPB83 were notably weaker than those elicited by M. bovis infection. These findings indicate that M. kansasii infection of calves elicits specific responses that may confound the interpretation of bovine tuberculosis tests.

Latent tuberculosis in HIV positive, diagnosed by the M. tuberculosis specific interferon-gamma test

Background

Although tuberculosis (TB) is a minor problem in Denmark, severe and complicated cases occur in HIV positive. Since the new M. tuberculosis specific test for latent TB, the QuantiFERON-TB In-Tube test (QFT-IT) became available the patients in our clinic have been screened for the presence of latent TB using the QFT-IT test. We here report the results from the first patients screened.

Methods

On a routine basis the QFT-IT test was performed and the results from 590 HIV positive individuals consecutively tested are presented here. CD4 cell count and TB risk-factors were recorded from patient files.

Main findings

27/590(4.6%) of the individuals were QFT-IT test positive, indicating the presence of latent TB infection. Among QFT-IT positive patients, 78% had risk factors such as long-term residency in a TB high endemic area (OR:5.7), known TB exposure (OR:4.9) or previous TB disease (OR:4.9). The prevalence of latent TB in these groups were 13%, 16% and 19% respectively. There was a strong correlation between low CD4 T-cell count and a low mitogen response (P < 0.001;Spearman) and more patients with low CD4 cell count had indeterminate results.

Conclusion

We found an overall prevalence of latent TB infection of 4.6% among the HIV positive individuals and a much higher prevalence of latent infection among those with a history of exposure (16%) and long term residency in a high endemic country (13%). The QFT-IT test may indeed be a useful test for HIV positive individuals, but in severely immunocompromised, the test may be impaired by T-cell anergy.

Routine Hospital Use of a New Commercial Whole Blood Interferon-γ Assay for the Diagnosis of Tuberculosis Infection

RATIONALE

Interferon (IFN)-gamma blood tests may improve the current level of diagnostic accuracy for tuberculosis infection. The QuantiFERON-TB Gold (QFT-Gold) has been used in selected populations and shows higher specificity than the tuberculin skin test (TST).

OBJECTIVE

To evaluate the QFT-Gold test in unselected patients and assess the level of agreement with the TST.

METHODS

The test has been routinely performed on whole blood samples in our microbiology laboratory for 8 months. Demographic, clinical, and microbiological data have been collected and correlated to the QFT-Gold results.

MEASUREMENTS AND MAIN RESULTS

Of 318 patients tested, 68 (21.4%) gave an indeterminate (low positive mitogen control) QFT-Gold result. Indeterminate results were significantly overrepresented in patients with a negative TST (28.9% vs. 6.6% in TST-positive patients; p < 0.0001, chi2 test) and were more frequent in patients receiving immunosuppressive therapies than in those who were not receiving such treatments (odds ratio, 3.35; 95% confidence interval, 1.84-6.08; p < 0.0001). After excluding indeterminate results, the concordance between QFT-Gold and TST was significantly lower in Bacille Calmette-Guérin-vaccinated individuals (41.5%) than in nonvaccinated individuals (80.3%) (p < 0.0001). In 11 patients with active tuberculosis (5 culture-confirmed), QFT-Gold provided more positive results than the TST (66.7% vs. 33.3%; p = 0.165).

CONCLUSIONS

The QFT-Gold test is feasible in routine hospital use for the diagnosis of tuberculosis infection. As with the TST, immunosuppression may negatively affect the test's performance, with a significant rate of indeterminate results in the most vulnerable population.