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New TB test may improve detection

| Friday, April 4, 2003

LONDON (AP) -- Scientists have developed a new diagnostic test for tuberculosis that experts say could help control the disease in the developed world by more accurately detecting infections before people get sick.

The World Health Organization estimates that 2 billion people -- one-third of the world's population -- are infected with the bacteria that cause TB. The disease kills 2 million people a year.

About 10 percent of people progress from a latent, or inactive, infection to the disease, but doctors cannot predict who or when. People can transmit the bug only when they get sick, so picking up infections early helps stop the spread.

A study published this week in The Lancet medical journal indicated that the new test detected latent infections more accurately than the standard skin-prick test used for a century.

However, experts said there was a long way to go before the blood test could be adopted routinely.

"In the developing world where there is lots of TB, we don't focus on latent infection because everyone is already infected," said Dr. Mark Perkins, a TB diagnostics specialist at the World Health Organization who was not involved in the research.

"In the developed world, where TB case rates are really low, the way we control TB is to try to find people who have latent infection and treat them so that they won't develop active TB and spread it."

The tuberculin skin-prick test is the cornerstone of tuberculosis control in developed countries, but it has many drawbacks. It involves injecting a substance just under the skin on the arm and technician "reading" the resulting bump a few days later.

The test can give false positive readings in people who have had the BCG tuberculosis vaccine because antibodies are made in both cases.

"A test that avoids some of that would be great," Perkins said.

The new test, developed by scientists at Oxford University in England, is a blood test using a different substance to stimulate a reaction. Instead of looking for antibodies, it detects the activation of immune cells called T-cells.

In the study, the Oxford researchers compared the new test with the skin-prick test on 535 children at a British school where a student was diagnosed with TB in 2001.

Children who were more exposed to the student with full-blown tuberculosis were significantly more likely to test positive with the new method, the study found.

While the skin test was more likely to be positive in BCG-vaccinated children than in nonvaccinated children, but there was no link with vaccination in the new test.

The two tests reached the same conclusion in 89 percent of the children.

When the results from the two tests did not match, it was impossible to know for certain which test was correct, the study found. However, when the new test was positive but the skin test was negative, that was a strong predictor of TB exposure in the children.

But when the results were reversed, it was not a strong indicator of a child's exposure to the TB germ.

That finding suggests that isolated positive results from the blood test were more likely to be true positives than isolated positive results from the skin test, the scientists said.

"While the data are compelling and very interesting, it doesn't necessarily constitute evidence for a complete overhaul of what we've been doing," Perkins said.

"Right now, it's not even a little bit quick and easy, but it's conceivable that this could be adapted into a more field-friendly format.

"This is useful for developed countries -- but that's not where most of the TB is. However, tests for latent infection would clearly be useful."

Perkins said about 95 percent of TB cases occur in the developing world, where a new diagnostic test for active TB is crucial because the current technology detects less than one-third of cases.

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