Accurate identification of early tuberculosis infection possible with new blood test

A new, rapid blood test for tuberculosis which can accurately identify infection at an early stage could enable doctors to reliably identify people who are infected before they have actually developed the disease, conclude authors of a study in this week's issue of THE LANCET.

Several recent major tuberculosis outbreaks in the UK have highlighted the fact that the disease is resurgent. Diagnosis and preventative treatment of people with early tuberculosis infection before they develop disease (active tuberculosis) is a crucial element of tuberculosis control. However, this approach is limited by the inaccuracy of the existing test for diagnosing early tuberculosis infection, the tuberculin skin test (TST), also known as the Heaf test, which can give false-positive results in people vaccinated with BCG or exposed to related environmental bacteria.

Ajit Lalvani and colleagues from the University of Oxford, UK, developed a rapid enzyme-linked immunospot (ELISPOT) assay to detect T cells responding to ESAT-6, a protein expressed in the tuberculosis bacterium, but absent from all strains of BCG vaccine. The investigators did a prospective, masked study of 50 healthy contacts with varying degrees of exposure to tuberculosis who attended a contact-tracing clinic in London, UK. They assessed and compared the efficacy of their new blood test with the TST for the detection of symptom-free infected individuals by correlation of test results with the degree of exposure to an infectious index case.

The ESAT-6 ELISPOT assay results were more strongly correlated with intensity of exposure to tuberculosis than were TST results. In contrast with the skin test, ELISPOT results were independent of BCG vaccination status.

Ajit Lalvani Comments: "Our assay has the potential to be used to identify recently infected contacts and other individuals at high risk of tuberculosis infection in low prevalence countries. This assay could enhance containment of tuberculosis outbreaks and improve targeting of preventative therapy to people with latent tuberculosis infection. Appropriate application of this assay might significantly contribute to tuberculosis control, but only after larger studies with a longer follow-up have confirmed our promising results." (Quote by e-mail; does not appear in published paper).

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