By Michael Kahn

LONDON, Oct 20 (Reuters) - A new blood test will allow doctors more accurately to pinpoint patients likely to develop the symptoms of tuberculosis, researchers said on Monday.

Traditional testing for the disease involves injecting the subject with components of the TB bacterium; a resultant swelling of the skin can signal dormant tuberculosis.

Such skin tests are prone to false positives -- people wrongly identified as needing treatment -- and, conversely, can sometimes wrongly show TB carriers to be free of the infection.

A new blood test known as ELISpot is 1.5 times better at spotting tuberculosis carriers, said Ajit Lalvani, a researcher at Imperial College London.

"On a global level, when you stack up those numbers, that is going to make a huge difference," Lalvani, whose findings were published in the Annals of Internal Medicine, said in a telephone interview.

Tuberculosis is an infectious bacterial disease which typically attacks the lungs and affects about 9.2 million more people each year, killing an estimated 1.7 million. Many of its victims are in developing countries whose cash-strapped heath systems have limited means of screening for the disease.

The emergence and spread of drug-resistant germs makes treating tuberculosis more difficult and could make the disease even deadlier in the future.

Lalvani and his team studied 908 healthy children in Turkey exposed to tuberculosis in their homes. A little over half tested positive for latent TB using the two tests.

The skin test suggested 580 children required drugs to ward off active TB, but the blood test pointed to just 380. Twelve children developed active TB even with treatment.

"If they had not had preventative treatment you would have expected about 50 or 60 cases," Lalvani said. "By using the blood test you only need to treat 380 children instead of 550 children to prevent the same number of active cases."

The next step is to make the new test even more accurate and establish its use in the devloping world. It is recommended in much of the developed world including the European Union and North America, he added. (Reporting by Michael Kahn; Editing by Maggie Fox and Dan Williams)

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