From the JournaL of New England Medicine July 2006
The tuberculin skin test has low specificity, but there are newer tests that are more specific for M. tuberculosis.
These tests include whole-blood assays that detect the release of interferon gamma by cells incubated ex vivo with M. tuberculosis peptides or proteins.
In May 2005, one such assay, the QuantiFeron-TB Gold (QFT-G) test, was approved by the Food and Drug Administration for the diagnosis of latent M. tuberculosis infection. This assay uses, as the stimulating antigens, synthetic peptides based on two proteins that are secreted by all strains of M. tuberculosis and pathogenic strains of M. bovis (early secretory antigenic target 6 and culture filtrate protein 10) but that are absent from BCG vaccine strains and most nontuberculous mycobacteria.
The CDC recently established guidelines for use of the new assay.5 Previous BCG vaccination is less likely to lead to a false positive result on this test than it is on the tuberculin skin test. The new test may have lower sensitivity than the older test, but it is difficult to make comparisons because there is no gold standard for the detection of latent infection.
There are limited data regarding the use of the QFT-G test in persons who have recently been exposed to tuberculosis, immunocompromised persons (including those infected with the human immunodeficiency virus), children younger than 17 years of age, populations with a high likelihood of harboring latent tuberculosis infection, and persons undergoing periodic screening. Nonetheless, the CDC indicates that the QFT-G test can be used in place of the tuberculin skin test in any circumstances, including contact investigations, evaluation of recent immigrants who previously received BCG vaccination, and screening of health care workers.5 Additional studies of the new assay are needed to assess its value for predicting the development of active tuberculosis.
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