NEW YORK (Reuters Health) – In populations with low prevalence of latent tuberculosis infection, the majority of positive results on the tuberculin skin test (TST) and the newer interferon gamma release assays (IGRAs) are false-positives, results of a study indicate.

The finding supports the current recommendation from the US Centers for Disease Control and Prevention (CDC) that people at low risk of latent TB infection (who are at greatest risk of a false positive result) not be targeted for testing, investigators s say.

In a report in the American Journal of Respiratory and Critical Care Medicine, Dr. James D. Mancuso from Walter Reed Army Institute of Research in Silver Spring, Maryland, and colleagues also point out that risk stratification, which is not currently used for the newer IGRAs, should be.

There continues to be uncertainty as to which diagnostic test for latent TB infection is most accurate in the US population, Dr. Mancuso and colleagues note in their report.

The aim of their study was to assess discordance between commercially-available tests for latent TB in a low prevalence population, including the impact of nontuberculous mycobacteria.  Subjects include 2,017 US military recruits at Fort Jackson, South Carolina.

The researchers obtained a TB risk factor history and performed the TST for latent TB as well as three other tests: the QuantiFERON-TB Gold In-Tube (QFT-GIT) test, the T-SPOT TB test, and the Battey skin test (BST) using purified protein derivative from the Battey bacillus.

Using the risk stratified interpretation for TST, 88 recruits (4.9%) had a positive result to at least one of the three tests. Of these, only 10 (11.4% were positive to all three tests; 20 recruits (22.7%) were positive to at least two of the tests.

“For the majority of positive results the three tests identified different people, suggesting that in low prevalence populations the majority of discordant results are due to false positives,” Dr. Mancuso and colleagues say.

They say false positive TST reactions associated with reactivity to nontuberculous mycobacteria and Bacille Calmette Guren (BCG) vaccination “may account for a proportion of test discordance observed.”

The authors also say they saw no differences between the tests in specificity. The IGRAs were designed to increase specificity and while both IGRAs are generally reported to have specificity higher than the TST, “there was surprisingly little difference in specificity between TST and either IGRA seen in this study,” they note.

“This study,” Dr. Mancuso and colleagues conclude, “highlights the need for better understanding of the significance of test discordance, particularly the need for longitudinal data on progression to active TB among those with discordant test results.”

The study was funded by the National Institutes of Health.

SOURCE:

Discordance Among Commercially-Available Diagnostics for Latent Tuberculosis Infection

Am J Respir Crit Care Med. 2011. Published online December 8, 2011.