A new screening process for tuberculosis (TB) infections in Canadian prisons could mean that more than 50% of those screened will not undergo unnecessary treatment due to false positives.

A test for TB using interferon-gamma release assays (IGRA) will detect a preexisting TB infection, or latent TB, that might not present itself for many years, or until the body becomes weakened by another source.

A study group led by infectious disease experts from Queen’s University (Kingston, ON, Canada), tested inmates for TB at a Canadian intake institution before moving on to different corrections facilities. The scientists used both the tuberculosis skin test (TST) for TB which has been used for over 100 years but comes with two main limitations, and the IGRA. The TST requires two visits to determine the results: one to perform the test and then another visit a couple of days later to read the results and depending on the patient's exposure to other mycobacteria or the Bacille Calmette Guerin (BCG) vaccine, accordingly the TST test can give many false positives.

The IGRA tests cannot prove that latent TB infections will progress into active TB until the patient begins to show symptoms. Better tools to predict who will go on to develop active and potentially infectious TB are being actively pursued. The investigators found that 96 inmates tested positive for TB via the TST test, but only 31 of these inmates were confirmed as true latent TB infection when using the IGRA test.

Wendy L.Wobeser, MD, the study’s lead author said, “It is fairly uncommon that latent TB will reactivate, with only about a 10% chance. That said, given the crowding in corrections facilities, the mass exposure of inmates to TB could be disastrous. It is such a slow disease progression that it is hard for us to say with certainty who will actually go on to develop TB. I hope that this test will eventually be used in corrections and is able to reduce people who might otherwise be treated unnecessarily for latent TB.”

Ilan Schwartz, MD, a coauthor of the study said, “What I found surprising was just how much discordance there was between the TST and IGRA tests. Historically, all of those who tested TB-positive by the TST test would have been subjected to 12 months of drug treatments that can have considerable side effects.”

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