New TB Blood Test Shows Promise

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

The study, "Comparison of T-cell-based Assay with Tuberculin Skin Test for Diagnosis of Mycobacterium tuberculosis Infection in a School Tuberculosis Outbreak," is published in the Lancet (2003;361(9364):1168-1173) and indicates that the new tests detected latent infections more accurately than the standard skin-prick test used for a century.

The tuberculin skin-prick test is the cornerstone of TB control in developed countries, but it has many drawbacks. It involves injecting a substance under the skin on the arm and a technician reading the resulting bump a few days later. The test can give false positive readings in people who have had the BCG TB vaccine because antibodies are made in both cases.

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 system T-cells.

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 exposed to the student with TB 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 non-vaccinated children, 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 did not match, it was impossible to know for certain which test was correct. However, when the new test was positive and the skin test was negative, this was a strong predictor of TB exposure in the children. When the results were reversed, this was not a strong indicator of a child's exposure to TB -- suggesting 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.

Dr. Mark Perkins, a TB specialist at the World Health Organization, 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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