TB kills up to three million people a year

The test enables doctors to reliably identify people who are infected long before they develop symptoms and become infectious.

It could replace the current reliance on a crude "skin prick" test that has been in use for 100 years and can take up to a week to produce results, which can be misleading.

Tuberculosis (TB) causes more deaths worldwide than any other infectious disease, killing between two and three million people each year, and the number of cases has been on the increase.

In England and Wales alone, a total of 6,669 tuberculosis cases were reported in 2001, an increase of 6% on the previous year.

New drug resistant strains of the bacterium are emerging in the worst hit areas, such as south-east Asia and eastern Europe.

National control

A major obstacle to controlling the disease is the lack of a reliable diagnostic blood test.

The new Elispot test developed at Oxford University looks for cells in the blood, produced by the immune system, called T-cells.

Our test is set to supersede the skin test and improve the control and prevention of this resurgent disease Dr Ajit Lalvani Wellcome Trust

It is the first blood test to look for T-cells rather than antibodies to make a diagnosis.

The test is simple, quick, and not confounded by TB vaccination.

Doctors hope it will supersede the tuberculin skin test in the next few years and transform the way TB is controlled.

Ultimately, it is hoped, it will help health authorities and national TB control programmes to eliminate the disease.

A report on research to validate the test at a British school appeared on Friday in the Lancet medical journal.

The un-named school was one of several in the Leicester area that experienced Britain's worst outbreak of TB for 20 years in 2001.

School tests

Wellcome Trust researcher Dr Ajit Lalvani, who developed the test, studied 535 children to compare Elispot with the old skin-prick technique.

"We worked closely with the school and the local health authority in the midst of the outbreak to improve diagnosis for students at risk of TB infection," he said.

"Our results have confirmed that Elispot is more accurate than the skin test.

"This means that our test is set to supersede the skin test and improve the control and prevention of this resurgent disease."

During the study, electronic school timetables allowed researchers to record how many minutes each child spent sharing room air with the individual who was originally infected.

This enabled them, for the first time, to determine the amount of exposure time needed to be sure of contracting TB infection - 130 hours.