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Task Order 18: Evaluation of new interferon-gamma release assays in the diagnosis of LTBI in healthcare workers

Task Order 18 seeks to evaluate the test characteristics, feasibility, and cost-effectiveness of interferon-gamma release assays (IGRAs) and the tuberculin skin test (TST) when testing for latent tuberculosis infection (LTBI) in healthcare workers (HCWs).


Charles P. Felton National TB Center at Harlem, Denver Public Health and Hospitals Authority, Maryland Department of Health and Mental Hygiene, and Texas Department of State Health Services.

Study Objectives

  1. Determine and compare the test characteristics (estimated sensitivity and specificity) of two IGRAs (QuantiFERON TB Gold [QFT-G] and T-Spot) compared with each other and to the TST at initial testing.
  2. Determine and compare the repeatability and reproducibility of the results.
  3. Determine and compare the test characteristics of the two IGRAs compared with the TST in detecting the phenomena of “conversion and “reversion”.
  4. Determine and compare the costs, cost differences, and cost-effectiveness of the three methods.
  5. Determine and compare the feasibility of the three methods for serial testing of HCWs.
  6. Determine and compare the acceptability and usability of the IGRAs compared with the TST among patients and providers.
  7. Determine how much, if any, the TST causes boosting of results from the IGRAs.
  8. Observe how treating LTBI influences test results longitudinally.

Study Design

This study will compare three testing methods for latent tuberculosis infection (LTBI) in Healthcare workers who are tested serially. The three methods are: TST with an initial two-step assessment, QFT-G test (in-tube) and T-Spot TB test. After providing written consent, the Healthcare worker will undergo a standardized interview, have a TST placed, and have at least 30ml of blood drawn. A TST will be placed (Tubersol, 5 TU PPD), read, and interpreted by trained individuals. Two-step testing will be performed on healthcare workers who have initial negative TST result. The IGRAs will be repeated with the TST so three tubes of blood will be obtained at each visit. For healthcare workers who have two-step testing, two IGRAs will be repeated with the TST to observe whether the initial TST influences results from the IGRAs. The influence of treating LTBI will be studied by comparing longitudinal test results in healthcare workers who do and do not receive treatment. Direct and indirect costs will be calculated for each test method for comparing costs for each strategy and determining cost-effectiveness. The acceptability and usability of the IGRAs by providers and the acceptability of the testing procedures by the patients will be assessed using qualitative methodology such as standardized questionnaires and key informant interviews with providers and patients recruited from the participating sites.

Study Progress

TO 18’s data collection began on February 12, 2008.  Study subjects were enrolled for approximately 18 months, serially tested over a period of 18 months, and follow-up was completed in March 2011.  All data have been entered into the online data management system and are currently being cleaned and checked for quality assurance.  Preliminary analyses and manuscript preparation are underway with the nearly finalized dataset.

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