New test to predict patients at risk of active TB
21 October 2008
Currently, the tuberculin skin test is used to detect latent tuberculosis (TB) infection. However, this 100-year old test commonly gives falsely positive results if a patient has previously been vaccinated against tuberculosis.
The authors of a new study, published in the ‘Annals of Internal Medicine’, believe that the new ELISpot blood test will allow doctors to identify and treat better those who need preventative treatment, while reducing the numbers of people treated unnecessarily, thus avoiding the risk of drug side-effects. This is especially important in the developing world where there are limited resources for both testing and treatment.
The test is the result of ten years of Trust-funded research by Professor Ajit Lalvani, a Wellcome Trust Senior Clinical Research Fellow, and colleagues at the Centre for Respiratory Infection at Imperial College London.
Professor Lalvani says: “We now know that [ELISpot] really helps to target treatment to those who most need it in order to prevent them from developing active TB. Building on this work, we are now validating a next generation of tests that have been developed by our TB Task Force at Imperial.”
The study looked at 908 children in Istanbul, Turkey, who had recently been exposed to TB. Of these, 594 tested positive for latent TB using ELISpot, 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 ELISpot, but it still picked up 11 of the 12 children who went on to develop active TB.
The study identified a significant risk of developing active TB for children with a positive ELISpot blood test result. The majority of participants received prophylactic TB treatment as a precautionary measure, and the researchers say the risk is would be even higher in untreated children.
The ELISpot test detects interferon gamma, a protein signal released by the immune system in response to TB infection. It has been recommended for use alongside the skin test in around 20 countries worldwide, including some in the European Union and North America.
The World Health Organization estimates that around a third of the world’s population is infected with the TB bacteria, with about nine million new cases of active TB diagnosed each year.
Image: Professor Ajit Lalvani
References
Lalvani A et al. Prognostic Value of a T-Cell-Based Interferon-Gamma Biomarker in Child Tuberculosis. Annals of Internal Medicine 2008.