The tuberculin skin test (TST) for tuberculosis (TB) infection is over a century old. Is there a better alternative? Research co-ordinated by the University of Oxford found that the ELISPOT test is more accurate than TST. The new test could improve targeting of TB control in countries that treat latent infection.

Only ten per cent of people infected with tuberculosis develop active disease. People without symptoms are said to have ‘latent infection’. TB control programmes in many developed countries use the TST to identify people with latent infection. These people then receive drugs to clear the infection, prevent the development of active disease and stop the spread of infection to others.

The TST has some problems. Previous BCG vaccination or exposure to similar bacteria in the environment can lead to false-positive results. The test can be difficult to administer and interpret and patients have to return to the clinic to have the result checked. But is the ELISPOT test any better?

In 2001, there was a large TB outbreak in a UK school, which came from just one infectious pupil. The likelihood of transmission depends on the duration and proximity of contact with an infectious person. If the ELISPOT assay is a more accurate test, it should show a closer relationship than the TST with the degree of exposure to TB and should be independent of BCG vaccination status.

Researchers tested 535 students for TB infection with both tests. They found that:

• Agreement between TST and ELISPOT results is high. Findings are different in 11 per cent of cases.
• Compared with TST, ELISPOT results are more closely related to the length and proximity of exposure to TB, and are less sensitive BCG vaccination.
• There is a strong link between positive ELISPOT results and the size of a positive TST skin reaction. Larger TST responses are associated with higher rates of progression to TB disease.
• All students with five or more school-weeks of exposure to the infectious case have positive results on the ELISPOT assay.

The ELISPOT test has several advantages over the TST. It is:

• more specific – reducing the unnecessary treatment of uninfected individuals
• quicker – giving a result the morning after taking a blood sample
• more convenient – a return visit is not required if the result is negative.

It is, however, more expensive than the TST. But the researchers point out that the TST incurs indirect costs through the need for return visits and staff training. In countries that treat latent TB infection, it could be cost-effective in the long term by improving diagnosis, reducing the number of cases of active TB and decreasing the use of unnecessary treatment.

Source(s):
‘Comparison of T-cell-based assay with tuberculin skin test for diagnosis of Mycobacterium tuberculosis infection in a school tuberculosis outbreak’, The Lancet 361: 1168-1173, by K. Ewer, et al., 2003 Full document.

Funded by: The Wellcome Trust

id21 Research Highlight: 2 June 2003

Further Information:
Ajit Lalvani
Nuffield Department of Clinical Medicine
University of Oxford
Level 7, John Radcliffe Hospital
Oxford OX3 9DU
UK

Contact the contributor: ajit.lalvani@ndm.ox.ac.uk

Nuffield Department of Clinical Medicine, University of Oxford

Other related links:
'What's the diagnosis? Testing for tuberculosis in Kenya'

'Coughing up for TB tests - cost-effective diagnosis in Zambia'

See id21's collection of links relevant to infectious diseases.