Major study involving UTHCT researchers shows new blood test better at diagnosing tuberculosis than current skin test

Thursday, December 15, 2005

Results from a major study of a new blood test for latent tuberculosis infection show it may be more sensitive than the TB skin test, said Dr. Peter Barnes, director of the Center for Pulmonary and Infectious Disease Control and professor of microbiology at The University of Texas Health Science Center at Tyler.

“This new blood test is better than the skin test, because it doesn’t require a follow-up visit for interpretation. And it isn’t subjective, as the skin test is. With the blood test, you test either positive or negative for tuberculosis infection,” Dr. Barnes said. With the skin test, medical professionals must examine any reaction to the vaccine and decide if it is enough to indicate the person has latent tuberculosis.

“This is the first large-scale use of the new blood test, called ELISPOT. The blood test had never been evaluated in a real world study in the United States,” he said. The U.S. Food and Drug Administration has not yet approved ELISPOT, though the test currently is under review.

UTHSCT researchers designed the ELISPOT study, acquired funding from the U.S. Centers for Disease Control and Prevention, and carried out the laboratory analysis of study results, Dr. Barnes said. The study involved more than 400 contacts of tuberculosis patients in Fort Worth. It was done in close collaboration with Dr. Steven Weis and his colleagues at the University of North Texas Health Science Center, as well as with Dr. Ajit Lalvani of Oxford University, who first developed the ELISPOT test. Dr. Homayoun Shams, assistant professor of microbiology and immunology at UTHSCT, and Dr. Weis were joint first authors of the paper describing the study results, which were published in the November issue of the American Journal of Respiratory and Critical Care Medicine. Peter Klucar, research associate at UTHSCT, was also a coauthor.

Tuberculosis is a chronic bacterial infection that usually targets the lungs, though other organs sometimes are involved. Most people infected with TB do not have symptoms of the disease; they have a latent tuberculosis infection. Tuberculosis symptoms include a low-grade fever, night sweats, fatigue, weight loss, and a persistent cough.

Many people with latent TB develop active tuberculosis each year. Once the disease becomes active, it can spread to other people. The person with TB coughs or sneezes and the airborne TB bacteria are inhaled by another person who then may develop tuberculosis. That’s why it’s important to know who has latent TB.

Until recently, the skin test was the only test that could detect latent tuberculosis, Dr. Barnes said. It has been used for the past 100 years, though the test was never approved by the U.S. Food and Drug Administration because it was in use long before the FDA was created.

There is no “gold standard” to determine the accuracy of any test for latent TB. So, Dr. Barnes and his colleagues measured factors such as how long each participant had contact with a TB patient, how close that contact was, and how infectious the person with tuberculosis was. This information was used to produce a numerical score for each study participant - the higher the score, the more likely that person was to be infected with tuberculosis. “This method fits all that we know about tuberculosis,” Dr. Barnes said.

The majority of tuberculosis in the United States occurs in foreign-born people. Many of them have been immunized against TB with the BCG vaccine, which is not generally used in the United States, Dr. Barnes said.

“Most TB infection comes from people with latent TB. The BCG vaccine causes false positives for tuberculosis when the skin test is used. People who have been vaccinated against tuberculosis may test positive for the disease when they really don’t have it,” he said.

“That problem is eliminated by this blood test. This test measures a response to proteins that are specific to tuberculosis and are not present in the BCG vaccine,” Dr. Barnes said. ELISPOT only identifies people with latent TB, not everyone who has been vaccinated.

Dr. Barnes, Dr. Shams, and Dr. Weis now are planning to compare ELISPOT with another blood test for tuberculosis called QuantiFeron-TB Gold. This test recently won approval from the FDA.

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