Chapter 3Infectious Diseases Related To Travel
Perspectives: Tuberculin Skin Testing of Travelers
Screening for asymptomatic tuberculosis (TB) infections should only be carried out among travelers who will be at significant risk of acquiring TB (see the preceding section on Tuberculosis). Screening with a tuberculin skin test (TST) in a very low-risk population may result in a false-positive test, leading to unnecessary additional screening or unnecessary treatment. Using even highly sensitive and specific tests in very low-prevalence populations will produce more false positives than true positives.
Therefore, the TST should be considered only for travelers who are spending years in a country with a high risk of TB, or for those traveling for any length of time who will come in contact routinely with hospital, prison, or homeless shelter populations. The general recommendation is that people at low risk for TB, which includes most travelers, do not need to be screened before or after travel.
For travelers who anticipate a long stay or contact with a high-risk population, careful pre-travel screening should be carried out, with use of 2-step TST screening. For 2-step TSTs, people whose baseline TSTs yield a negative result are retested 1–3 weeks after the initial test; if the second test result is negative, they are considered not infected. If the second test result is positive, they are classified as having had previous TB infection. The 2-step TST is recommended in this population for the following reasons:
- The use of 2-step testing can reduce the number of positive TSTs that would otherwise be misclassified as recent skin test conversions during future periodic screenings.
- Certain people who were infected with Mycobacterium tuberculosis years earlier exhibit waning delayed-type hypersensitivity to tuberculin. When they are skin tested years after infection, they might have a false-negative TST result (even though they are truly infected). However, the first TST might stimulate the ability to react to subsequent tests, resulting in a “booster” reaction. When the test is repeated, the reaction might be misinterpreted as a new infection (recent conversion) rather than a boosted reaction.
Two-step testing is important for travelers who will have potential prolonged TB exposure. Two-step testing before travel will detect boosting and potentially prevent “false conversions”—positive TST results that appear to indicate infection acquired during travel, but which are really the result of previous TB infection. This distinction is particularly important if the traveler is going to a country where extensively drug-resistant TB (XDR TB) is rampant: it would be critical to know whether the person’s skin test had been positive before travel.
If the 2-step TST result is negative, the traveler should have a repeat TST 8–10 weeks after returning from the trip, or as part of a periodic screening examination for those who remain at high risk. Two-step testing should be considered for the baseline testing of people who report no history of a recent TST and who will receive repeated TSTs as part of ongoing monitoring.
People who have had repeat TSTs must be tested with the same commercial antigen, since switching antigens can also lead to false TST conversions. Two commercial tuberculin skin test antigens are approved by the Food and Drug Administration (FDA) and are commercially available in the United States: Aplisol (JHP Pharmaceuticals) and Tubersol (Sanofi Pasteur).
An alternative to 2-step TST is a single FDA-approved interferon-γ release assay (IGRA), either the QuantiFERON TB test (Gold or Gold In-Tube versions) or T-SPOT.TB test. IGRAs are approximately as specific as TST in people who are not vaccinated with Bacillus Calmette-Guérin and much more specific in vaccinated populations. For a traveler whose time before departure is short, a single-step TST would be an acceptable alternative if there were insufficient time for the 2-step TST and the IGRAs were not available.
In general, it is best not to mix tests. There is approximately 15% discordance between TST and IGRA, usually with the TST positive and the IGRA negative. There are multiple reasons for the discordance, and in any person it is often difficult to be confident about the reason for discordance. However, if the clinician decides to mix tests, it is better to go from TST to IGRA than the other way around, because the likelihood of having a discordant result with the TST negative and the IGRA positive is much lower. Such discordant results may become unavoidable as more medical establishments switch from TSTs to IGRAs.
The use of TST among those visiting friends and relatives in TB-endemic areas should take into account the high rate of TST positivity in this population. In a study among 53,000 adults in Tennessee, the prevalence of a positive TST among the foreign born was 11 times that of the US born (34% vs 3%). Confirming TST status before travel would prevent the conclusion that a positive TST after travel was due to recent conversion.
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