New TB test more accurate in predicting those most susceptible

Scientists have developed a new blood test which promises to more accurately predict those most likely to develop tuberculosis (TB).

The new blood test known as ELISpot which was developed by Professor Ajit Lalvani, a Wellcome Trust Senior Clinical Research Fellow at the Centre for Respiratory Infection at Imperial College London, will enable doctors to spot TB 1.5 times more accurately than traditional tests.

Standard skin tests for TB involve injecting the subject with components of the TB bacterium and a swelling of the skin as a result can signal dormant tuberculosis - but these skin tests produce false results - people can be wrongly identified as needing treatment and can also sometimes wrongly be shown to be free of the infection.

Professor Lalvani says on a global level the improved accuracy will make a huge difference.

According to World Health Organisation estimates around a third of the world's population is infected with the TB bacteria and approximately 9 million new cases of active TB are diagnosed each year; it usually attacks the lungs and kills an estimated 1.7 million.

Worse hit are those in developing countries where heath systems have limited means of screening for the disease or for treating it.

The symptoms of active TB include fever, persistent cough, and loss of appetite, whereas patients with the dormant, 'latent' form of TB do not have such symptoms. Treatment can prevent many patients with latent TB from progressing to active TB.

The emergence and spread of drug-resistant germs has exacerbated the problem by making the treatment of TB more difficult and more costly and could mean the disease becomes even deadlier in the future.

Dr. Lalvani and his team studied 908 healthy children in Istanbul, Turkey recently exposed to tuberculosis in their homes and found more than half tested positive for latent TB using the two tests.

Of the children 594 tested positive for latent TB using the ELISpot blood test, the skin test, or both.

Of 550 children who tested positive for TB with the skin test, 12 went on to develop active TB - fewer children tested positive for TB with the blood test (381), but the test still picked up 11 of the 12 children who went on to develop active TB.

Dr. Lalvani says the blood test established that only 380 children needed treatment instead of 550 - and if they had not had preventative treatment, about 50 or 60 cases could have been expected.

Children with a positive ELISpot blood test result had approximately a four-fold higher risk of developing TB disease than children with a negative result and a higher proportion of children with a positive ELISpot blood test result developed TB disease compared to children with a positive TB skin test.

Professor Lalvani says many people in developed countries regard TB as an old disease that we no longer need to worry about, but he says even in developed countries the numbers of cases have been rising for almost 20 years and beyond the developed world TB has reached pandemic proportions and continues to cause immense suffering and death.

The ELISpot test works by detecting a protein signal, known as interferon-gamma, released by white blood cells of the immune system in response to TB infection.

The test has been recommended for use alongside the skin test in around 20 countries worldwide, including the EU and North America.

Professor Lalvani says the next step is to make the new test even more accurate and establish its use in the developing world.

The study is published in the journal Annals of Internal Medicine.