Thanks to the availability of two new blood tests called T-SPOT.TB and QuantiFERON-TB Gold, physicians around the world can better detect latent tuberculosis (TB) infection. The tests, which reduce the number of false positive and negative results inherent to the old tuberculin skin test, are just two examples of clinical advances in TB control that could potentially eliminate the disease during the 21st century.
This update on TB diagnosis and treatment appears in the first issue for October 2006 of the American Journal of Respiratory and Critical Care Medicine, published by the American Thoracic Society.
Luca Richeldi, M.D., Ph.D., of the Department of Oncology, Hematology and Respiratory Disease at the University Policlinco of Modena in Modena, Italy, highlighted the significant operational advantages of the new blood tests over the skin test: The new tests require no return visit, provide results the next day and are significantly more accurate than the tuberculin skin test.
The fact that the blood tests do not usually indicate disease in healthy people is especially important in populations that have had the bacillus Calmette-Guerin (BCG) vaccine. BCG is the most widely used vaccine in the world; to date, more than three billion people have received it.
"Targeted tuberculin testing for latent TB infection is a key component of TB control," said Dr. Richeldi. "It is based on identification and treatment of persons infected by Mycobacterium tuberculosis who are at high risk for progression to active disease. This strategy is powerful because preventive treatment of latently infected people diminishes the risk of subsequent development of active TB by about 90 percent."
The T-SPOT.TB and QuantiFERON-TB Gold, tests detect interferon gamma released in T cells in response to M. tuberculosis- specific antigens. (T cells, a type of white blood cell, produce a number of substances that regulate the body's immune response.)
The U.S. Food and Drug Administration (FDA) recently approved QuantiFERON-TB Gold for use in America and the Centers for Disease Control and Prevention has published guidelines for its use in diagnosing TB. The FDA is currently evaluating T-SPOT.TB.
"The classic diagnostic tool for latent TB infection is the tuberculin skin test, or TST," said Dr. Richeldi. "It is the oldest diagnostic test in modern medical practice, in use since 1910, and its limitations constitute the weakest element in the strategy of targeted testing of latent TB infection."
According to Dr. Richeldi, the higher-risk groups targeted for preventive therapy are those in which the skin test most often fails to detect latent TB infection.
For active TB infection, however, recent research shows that the T-SPOT.TB test is more likely to return positive results when disease is present than the QuantiFERON-TB Gold test.
In an editorial on the article in the same issue of the journal, Peter F. Barnes, M.D., of the Center for Pulmonary and Infectious Disease Control at the University of Texas Health Center, wrote: "Current published data indicate that the T-SPOT.TB test yields more evaluable results and is probably more sensitive for the diagnosis of latent TB infection in immunosuppressed patients than the QuantiFERON-TB Gold test."
"The Centers for Disease Control and Prevention have recommended that the QuantiFERON-TB Gold test can generally be substituted for the tuberculin skin test. I agree with this recommendation except that I favor continued use of the tuberculin skin test in populations with depressed cell-mediated immune responses. In Europe, when both gamma-based tests are available, I believe that the T-SPOT.TB test should be used for persons in whom cell-mediated immunity may be depressed. For immunocompetent persons, either gamma-based test can be used."
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