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McMaster Textbook of Internal Medicine

Testing for latent TB infection in vaccinated populations

2017-06-13
Lori Whitehead

In Poland, a significant proportion of cases of tuberculosis (TB) is latent TB infection, and most of the population has been vaccinated against TB. Is there still a place for tuberculin skin testing in this population or should it be replaced with interferon-gamma release assays (IGRAs)?

Lori Whitehead: Both the TB skin test and the IGRA provide useful diagnostic aids for the diagnosis, or detection, of latent TB infection. In Poland, which I guess is sort of a mid-burden country in terms of incidence of TB, the IGRA would probably have a definite advantage over the TB skin test. From my understanding, in Poland, most people, babies, will receive their first BCG vaccine when they are infants. And up until 1990 or so – somewhere in that area – they were also given a booster shot when they were about 7 years of age. That means there is a significant proportion of people in Poland who have had not just one BCG, but they have had a booster. The booster is done to ensure lifelong immunity against TB but unfortunately it also boosts the TB skin test reaction. So in other words, in Poland, for that population who has had BCG twice, there will be a significant proportion of people who have false-positive TB skin test reactions, rendering the TB skin test less useful.

I think the IGRA blood test would have an advantage for that group of patients, but I guess it will be up to the local authorities and the national health regulatory bodies to determine your social determinants of health, your economic position, and your resources to establish if the IGRA could be used as a feasible option instead of the TB skin test.

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