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No. 3, 2006
TUBERCULOSIS EPIDEMIOLOGIC STUDIES CONSORTIUM
New TBESC Study to Be Launched: Evaluation of New
Interferon-gamma Release Assays in the Diagnosis of Latent TB
Infection in Health Care Workers
Principal Investigator: Chuck Daley
Co-Principal Investigators: Nick Deluca, Jerry Mazurek, Susan
Dorman
The TBESC is planning a new study titled “Evaluation of New
Interferon-gamma Release Assays in the Diagnosis of Latent
Tuberculosis Infection in Health Care Workers.” The study will
evaluate the test characteristics, feasibility, and
cost-effectiveness of interferon-gamma release assays (IGRAs) and
the tuberculin skin test (TST) when testing for LTBI in health care
workers (HCWs) at 4 to 6 TBESC sites.
HCWs make up a critical study population for IGRAs. According to
the Labor Department, there are approximately 14 million HCWs in the
United States, most of whom undergo serial skin testing. HCWs
represent a diverse yet stable and accessible population with a wide
range of rates of LTBI and risks of TB exposure. Testing them is
costly and time consuming, but detection of new infection is
necessary for discovering M. tuberculosis transmission and for
preventing occupation-related TB. The consequences of missing TB in
a HCW are potentially disastrous (CDC. MMWR 2004: MMWR 2005).
Until recently the only method for detecting infection was the
TST, which requires two-step testing for the initial evaluation. The
TST has a number of important deficiencies: it lacks sensitivity and
specificity, its results are influenced by BCG vaccination, and it
requires two health care visits to obtain results. IGRAs have
excellent sensitivity and specificity and provide some advantages
over the TST. IGRAs require one health care visit, the result can be
available within one day, and interpreting the test results is less
subjective than for the TST. Many HCWs in the United States were
born in countries where the incidence of TB is high and where BCG
vaccination is routine. IGRAs using region-of-difference 1 antigens
are not influenced by BCG vaccination, which may increase HCW
confidence in the result.
IGRAs must be evaluated longitudinally in HCWs to determine the
frequency of reversion and conversion, to determine the optimal
definition of conversion, and to study the repeatability and
reproducibility of these new tests. Because a switch from skin
testing to IGRAs will represent a major change in our approach to
testing HCWs, it is important to assess the utilization and
acceptability of these new tests among patients and providers. Given
the large number of HCWs in the United States, it is important to
evaluate the cost-effectiveness of the different testing methods in
health care settings.
The specific
aims of the study are as follows:
- To determine and compare the test characteristics (estimated
sensitivity and specificity) of two IGRAs (QuantiFERON TB Gold [QFT-G]
and T-Spot.TB) compared with each other and to the TST at
initial testing.
- To determine and compare the repeatability and
reproducibility of the results.
- To determine and compare the test characteristics of the two
IGRAs compared with the TST in detecting the phenomena of
“conversion” and “reversion.”
- To determine and compare the costs, cost differences, and
cost-effectiveness of the three methods.
- To determine and compare the feasibility of the three
methods for serial testing of HCWs.
- To determine and compare the acceptability and usability of
the IGRAs compared with the TST among patients and providers.
- To determine the extent to which, if any, the TST causes
boosting of results from the IGRAs.
- To observe how treating LTBI influences test results
longitudinally.
After providing written consent, approximately
3000 HCWs from four to six TBESC sites will undergo a standardized
interview and will be tested serially with all three tests, except
for subjects who have 15 mm or greater of induration; they will be
tested with the two IGRAs. Test results will be recorded as
currently recommended by manufacturers’ and national guidelines (MMWR,
December 2005) and numerical output (i.e., the number of
spot-forming cells and amount of interferon gamma produced) from the
IGRAs will be recorded in order to determine the optimal definition
for conversion. For HCWs who have two-step testing, two IGRAs will
be repeated with the TST to observe whether the initial TST
influences results from the IGRAs. A sample of HCWs will have each
test performed twice at the initial testing in order to assess the
repeatability of the assays. HCWs will be retested every 6 months
after the initial testing until they have been retested at least
three times (a total of four testings). The influence of treating
LTBI will be studied by comparing longitudinal test results in HCWs
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.
Submitted by Chuck Daley and Rachel Albalak TBESC
Last Reviewed: 05/18/2008 Content Source: Division of Tuberculosis Elimination
National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention
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