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Independents' Day

Will new blood test replace the skin test for tuberculosis?

QWill the new blood test for TB be widely available and useful?

ATesting to see if someone is infected with TB is performed either by the Heaf test, percutaneous injection of tuberculin, or the Mantoux test, intradermal injection of tuberculin. These provide results seven days and 48 hours later respectively.

BCG vaccination gives a weak response to these tests. False-positive tests due to exposure to environmental mycobacteria can occur and false-negatives due to illness or immunodeficiency due to, say, HIV co-infection.

On the plus side, these tests can be done in the community and are cheap as tuberculin is supplied free by the Department of Health.

New tests, of which the Elispot assay is one, detect the release of cytokines from T-cells activated to tuberculin. These appear to have greater sensitivity and specificity, reducing false-positive and false-negative effects, and

can separate BCG effect

from true tuberculin response.

However, they require blood to be taken, processed usually within 24 hours, and are more expensive requiring kits and technician time.

If their greater sensitivity and specificity are confirmed in larger routine prospective studies, for example new immigrant screening, testing close contacts of respiratory tuberculosis, and screening positive reactors in the schools BCG programme, then over the next few years they may supersede the skin test, as the costs may be offset by savings in reduced interventions.

Dr Peter Ormerod is professor of respiratory medicine at the chest clinic, Blackburn Royal Infirmary

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