New In Vitro Blood Test for Tuberculosis* If not detected and treated, latent tuberculosis (TB) infection (LTBI) develops into active TB disease in approximately 10% of infected persons with normal immune systems. The risks are higher for young children and persons with recent infection, certain medical conditions, or chest radiograph findings suggestive of previous TB. Because treatment for LTBI can be up to 90% effective in preventing the progression of LTBI to active TB, the detection and treatment of LTBI is an important strategy in efforts to control and prevent TB. In addition, the risk of TB transmission in the community is decreased when persons with active TB disease are identified and treated in a timely manner. Although the tuberculin skin test (TST) is an aid to diagnosing active TB, up to 25% of persons with active TB disease have a negative TST at the time of diagnosis. New In Vitro Blood Test Clinicians and public health professionals have long recognized the need for improved methods for identifying persons infected with TB. Since its development in the 1930s, the TST has been the only available method for identifying latent TB infection. The test has significant limitations, including the need for two office visits to administer and read the test; biases and errors in administering, reading and interpreting the test; false positive results due to prior Bacille Calmette-Guerin (BCG) vaccination or infection with nontuberculous mycobacteria; and false negative results due to anergy, immune suppression, or overwhelming TB disease. In December 2005, the Centers for Disease Control and Prevention (CDC) published interim guidelines for the use of a new in vitro blood test, QuantiFERON-TB Gold (QFT-G), as an aid for diagnosing LTBI and TB disease. QFT-G has significant improvements over the original QuantiFERON (QFT) test approved in 2001, which was not widely used and is no longer commercially available. The new guidelines are intended to help public health officials, clinicians and laboratorians understand how to interpret the QFT-G test and assess its potential for use in TB control efforts. QFT-G is an enzyme-linked immunosorbent assay (ELISA) test that detects the release of interferon-gamma (IFN-g) in blood from sensitized persons when it is incubated for 16-24 hours with mixtures of synthetic peptides representing two proteins present in Mycobacterium tuberculosis. Test results are based on the amount of IFN-g that is released during this process. The new guidelines indicate that QFT-G can be used in all circumstances in which the TST is used, including contact investigations, evaluation of recent immigrants who have had BCG vaccination, and TB screening of healthcare workers and others undergoing serial evaluation for M. tuberculosis infection. QFT-G usually can be used in place of (and not in addition to) the TST. Advantages of the QFT-G Test The test’s advantages include the following. • •••• It can be used for diagnosing both LTBI and active TB disease, whereas the QFT was approved only for use in diagnosing LTBI. • •••• Because the antigens used in the QFT-G test are more specific for M. tuberculosis than is the tuberculin used in the TST, there is less chance of false-positive results due to previous BCG vaccination or nontuberculous mycobacteria such as M. avium. This could be a distinct advantage in Minnesota, where most TB occurs among for-eign-born individuals, many of whom have received BCG vaccination overseas. • •••• Results are available less than 24 hours after testing, eliminating the second office visit. • •••• It does not cause “boosting” because, unlike the TST, no antigen is injected into the body. • •••• As a blood test, it is not subject to the biases and errors of TST placement and reading. • •••• It might be a cost-effective alternative to the TST in testing programs that are part of the infection control program in institutions such as healthcare settings, correctional facilities, or homeless shelters. Limitations of the QFT-G Test The test’s limitations include the following. • •••• Blood samples must be processed within 12 hours of collection. A future generation of the test is expected to address this issue. • •••• Errors in collecting or transporting blood specimens or in running and interpreting the assay can decrease the test’s accuracy. • •••• As with the TST, it cannot differentiate between TB disease and LTBI. • •••• Collecting the required 5-ml sample of blood from young children may not be possible or acceptable. • •••• Similar to any other diagnostic test, the predictive value of QFT-G results depends on the prevalence of M. tuberculosis infection in the population being tested. • •••• The significance of indeterminate QFT-G results has not been determined. Issues Requiring Further Study In addition to the above limitations, several issues have not yet been studied extensively, including: • •••• Its sensitivity for particular groups of TB patients (e.g., young children and immunocompromised patients) has not been determined. • •••• Its sensitivity for LTBI might be less than that of the TST, although the lack of a confirmatory test makes this difficult to assess. • •••• The ability of QFT-G to predict risk for LTBI progressing to TB disease has not been determined. • •••• Published data are relatively limited concerning the use of QFT-G among persons recently exposed to TB (e.g., contacts) and other populations at high risk for LTBI. • •••• No published data document the performance of QFT-G in children under the age of 17 years. Post-test Evaluations The majority of healthy adults who have negative QFT-G results are unlikely to have M. tuberculosis infection and do not require further evaluation. A positive QFT-G result should prompt the same public health and medical interventions as a positive TST result. Whenever LTBI or TB disease is being diagnosed by any method, clinicians 18 DCN 34;2 March/April 2006
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