Chest X-rays are not effective at detecting TB infections

Enlarged microscopic image of TB-causing bacteria, Mycobacterium tuberculosis

When I immigrated to the UK as a student, I had to do something that I wasn’t expecting. I had to carry with me a recent chest X-ray. I thought this was completely unnecessary. Why should I be exposed to X-rays for no good reason?

Turned out that there was a reason. It was to stop the spread of tuberculosis (TB) in the UK. Immigrants from sub-Saharan African and the Indian subcontinent are more likely than other immigrants to be infected by TB.  The UK has seen TB cases increase continuously over the past 30 years. Between 1998 and 2009 the numbers rose by 50% to 9040 cases. Most of those affected are foreign-born people. TB infection rates in the UK today are as high as they were in the 1930s, and they are among the highest in any developed country.

The rise in number of cases is despite chest X-ray screening that immigrants have to undergo. That’s because of TB’s quirky ability to show up many years after the TB-causing bacteria infected a person. Researchers find that early detection makes it easier to treat TB, because more severe infections are becoming resistant to current treatments.

With an aim to improve detection rates, Onn Min Kon, a physician at Imperial College London, and colleagues report, in a paper just published in Thorax, that chest X-rays are actually not as effective at detecting TB. Instead, they suggest, the UK government should use an advanced test called interferon-gamma release assays (IGRA).

IGRA works on the principle that when the TB-causing bacteria are exposed to a set of chemicals, which are harmless to humans, it causes them to release a protein called interferon-gamma. If a patient who has been given these chemicals breathes out interferon-gamma, then he is infected by TB (the latent variety or not).

Although a previous consultation with the National Institute for Health and Clinical Excellence had recommended the use of IGRA, it had made that recommendation in combination with chest X-rays. What Dr Kon’s work finds that a more cost-effective, and still efficient, method would be to just use IGRA. It would also spare the patient from being exposed to X-rays!

This research is one among many other papers published in Thorax’s special issue dedicated to TB which coincides with the upcoming World TB day.  

Reference: Pareek, Bond, Shorey, Seneviratne, Guy, White, Lalvani & Min Kon, Community-based evaluation of immigrant tuberculosis screening using interferon γ release assays and tuberculin skin testing: observational study and economic analysis, Thorax 2013

Image from here.