New test could rule out TB diagnosis within 48 hours

Enzyme test could prevent long wait for negative culture result.

A novel test for TB will allow doctors to rule out infection in 48 hours, claim UK scientists.

Although culture tests are able to confirm TB infection after several weeks, a highly sensitive method for ruling out TB has so far been missing.

The enzyme-linked immunospot (ELISpot)-Plus test works by detecting interferon- released from T-cells in the presence of the TB antigen.

Importantly, ELISpot-Plus can distinguish between TB antigens due to vaccination with BCG and those caused by TB infection, unlike the standard tuberculin skin test.

ELISpot-Plus was used alongside skin tests and culture tests in 389 adults with moderate to high clinical suspicion of TB.

Overall, 194 patients had a final diagnosis of TB. The research showed that sensitivity for culture-confirmed TB infection was 89 per cent with ELISpot-Plus, compared with 79 per cent for skin testing alone. In combination, the tests had a sensitivity of 99 per cent.

Using both tests could rule out TB infection quickly, allowing doctors to focus on other possible diagnoses.

But patients with positive results on both tests will still need to undergo further testing to confirm TB, as ELISpot-Plus cannot distinguish between active and latent infection.

Lead researcher Professor Ajit Lalvani, from the National Heart and Lung Institute at Imperial College, London, said: 'It completes the diagnostic armourment of infectious disease and respiratory physicians when they're testing for TB.

'Until now (doctors) have had no quick way to rule it out. That means that patients wait for weeks for confirmation.'

Ann Intern Med 2008 ; 148: 325-36

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