Interferon gamma release assays for Diagnostic Evaluation of Active tuberculosis (IDEA): test accuracy study and economic evaluation.

BACKGROUND: Interferon gamma release assays (IGRAs) are blood tests recommended for the diagnosis of tuberculosis (TB) infection. There is currently uncertainty about the role and clinical utility of IGRAs in the diagnostic workup of suspected active TB in routine NHS clinical practice. OBJECTIVES: To compare the diagnostic accuracy and cost-effectiveness of T-SPOT.TB® (Oxford Immunotec, Abingdon, UK) and QuantiFERON® TB GOLD In-Tube (Cellestis, Carnegie, VIC, Australia) for diagnosis of suspected active TB and to estimate the diagnostic accuracy of second-generation IGRAs. DESIGN: Prospective within-patient comparative diagnostic accuracy study. SETTING: Secondary care. PARTICIPANTS: Adults (aged ≥ 16 years) presenting as inpatients or outpatients at 12 NHS hospital trusts in London, Slough, Oxford, Leicester and Birmingham with suspected active TB. INTERVENTIONS: The index tests [T-SPOT.TB and QuantiFERON GOLD In-Tube (QFT-GIT)] and new enzyme-linked immunospot assays utilising novel Mycobacterium tuberculosis antigens (Rv3615c, Rv2654, Rv3879c and Rv3873) were verified against a composite reference standard applied by a panel of clinical experts blinded to IGRA results. MAIN OUTCOME MEASURES: Sensitivity, specificity, predictive values and likelihood ratios were calculated to determine diagnostic accuracy. A decision tree model was developed to calculate the incremental costs and incremental health utilities [quality-adjusted life-years (QALYs)] of changing from current practice to using an IGRA as an initial rule-out test. RESULTS: A total of 363 patients had active TB (culture-confirmed and highly probable TB cases), 439 had no active TB and 43 had an indeterminate final diagnosis. Comparing T-SPOT.TB and QFT-GIT, the sensitivities [95% confidence interval (CI)] were 82.3% (95% CI 77.7% to 85.9%) and 67.3% (95% CI 62.1% to 72.2%), respectively, whereas specificities were 82.6% (95% CI 78.6% to 86.1%) and 80.4% (95% CI 76.1% to 84.1%), respectively. T-SPOT.TB was more sensitive than QFT-GIT (relative sensitivity 1.22, 95% CI 1.14 to 1.31; p < 0.001), but the specificities were similar (relative specificity 1.02, 95% CI 0.97 to 1.08; p = 0.3). For both IGRAs the sensitivity was lower and the specificity was higher for human immunodeficiency virus (HIV)-positive than for HIV-negative patients. The most promising novel antigen was Rv3615c. The added value of Rv3615c to T-SPOT.TB was a 9% (95% CI 5% to 12%) relative increase in sensitivity at the expense of specificity, which had a relative decrease of 7% (95% CI 4% to 10%). The use of current IGRA tests for ruling out active TB is unlikely to be considered cost-effective if a QALY was valued at £20,000 or £30,000. For T-SPOT.TB, the probability of being cost-effective for a willingness to pay of £20,000/QALY was 26% and 21%, when patients with indeterminate test results were excluded or included, respectively. In comparison, the QFT-GIT probabilities were 8% and 6%. Although the use of IGRAs is cost saving, the health detriment is large owing to delay in diagnosing active TB, leading to prolonged illness. There was substantial between-patient variation in the tests used in the diagnostic pathway. LIMITATIONS: The recruitment target for the HIV co-infected population was not achieved. CONCLUSIONS: Although T-SPOT.TB was more sensitive than QFT-GIT for the diagnosis of active TB, the tests are insufficiently sensitive for ruling out active TB in routine clinical practice in the UK. Novel assays offer some promise. FUTURE WORK: The novel assays require evaluation in distinct clinical settings and in immunosuppressed patient groups. FUNDING: This project was funded by the National Institute for Health Research (NIHR) Health Technology Assessment programme and the NIHR Health Protection Research Unit in Respiratory Infections, Imperial College London, London, UK.

Tuberculosis (TB) is one of the world's most important infectious diseases. In 2014, 1.5 million deaths were caused by the disease ­ about one death every 25 seconds. Traditional diagnosis of TB is based partly on the tuberculin skin test. Blood tests such as QuantiFERON GOLD In-Tube (QFT-GIT; Cellestis, Carnegie, VIC, Australia) and T-SPOT.TB^® (Oxford Immunotec, Abingdon, UK) are now available. However, these two tests are not used as part of current NHS practice because of the lack of evidence about how well the tests perform when diagnosing symptomatic (active) TB in routine clinical practice. The purpose of our study was to compare the ability of QFT-GIT and T-SPOT.TB to differentiate people with active TB from those without active TB in a population suspected of the disease. We also assessed new blood tests that are currently being developed for diagnosis of active TB. We recruited 1074 patients with suspected TB from 14 NHS hospitals in London, Slough, Oxford, Leicester and Birmingham into our study. We found that T-SPOT.TB correctly detected more people with active TB than QFT-GIT; T-SPOT.TB would miss about 18 people out of every 100, whereas QFT-GIT would miss about 33 people out of every 100 with active TB. For this reason, neither test is good enough for routine clinical use because the number of people with active TB who are incorrectly diagnosed as not having active TB is unacceptably high. In addition, neither test is good value for money. However, we did find that some of the newer blood tests performed better than T-SPOT.TB and their usefulness should be further investigated.

Clinical utility of existing and second-generation interferon-γ release assays for diagnostic evaluation of tuberculosis: an observational cohort study.

BACKGROUND: The clinical utility of interferon-γ release assays (IGRAs) for diagnosis of active tuberculosis is unclear, although they are commonly used in countries with a low incidence of tuberculosis. We aimed to resolve this clinical uncertainty by determining the accuracy and utility of commercially available and second-generation IGRAs in the diagnostic assessment of suspected tuberculosis in a low-incidence setting. METHODS: We did a prospective cohort study of adults with suspected tuberculosis in routine secondary care in England. Patients were tested for Mycobacterium tuberculosis infection at baseline with commercially available (T-SPOT.TB and QuantiFERON-TB Gold In-Tube [QFT-GIT]) and second-generation (incorporating novel M tuberculosis antigens) IGRAs and followed up for 6-12 months to establish definitive diagnoses. Sensitivity, specificity, positive and negative likelihood ratios, and predictive values of the tests were determined. FINDINGS: Of the 1060 adults enrolled in the study, 845 were included in the analyses and 363 were diagnosed with tuberculosis. Sensitivity of T-SPOT.TB for all tuberculosis diagnosis, including culture-confirmed and highly probable cases, was 81·4% (95% CI 76·6-85·3), which was higher than QFT-GIT (67·3% [62·0-72·1]). Second-generation IGRAs had a sensitivity of 94·0% (90·0-96·4) for culture-confirmed tuberculosis and 89·2% (85·2-92·2) when including highly probable tuberculosis, giving a negative likelihood ratio for all tuberculosis cases of 0·13 (95% CI 0·10-0·19). Specificity ranged from 86·2% (95% CI 82·3-89·4) for T-SPOT.TB to 80·0% (75·6-83·8) for second-generation IGRAs. INTERPRETATION: Commercially available IGRAs do not have sufficient accuracy for diagnostic evaluation of suspected tuberculosis. Second-generation tests, however, might have sufficiently high sensitivity, low negative likelihood ratio, and correspondingly high negative predictive value in low-incidence settings to facilitate prompt rule-out of tuberculosis. FUNDING: National Institute for Health Research.