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Caution Using IGRAs for Latent TB in U.S. Healthcare Workers

January 21, 2014

Caution Using IGRAs for Latent TB in U.S. Healthcare Workers

  1. Neil M. Ampel, MD

In a large study of U.S. HCWs, two interferon-γ release assays had higher baseline positive rates and higher conversion rates than the tuberculin skin test.

  1. Neil M. Ampel, MD

Healthcare workers (HCWs) are an important target group for latent tuberculosis infection (LTBI) testing. Until recently, the only assay available was the tuberculin skin test (TST), which is limited by low sensitivity, a subjective end-point requiring two clinical visits, and cross-reactivity in individuals who have been vaccinated with bacille Calmette-Guérin or have a nontuberculous mycobacterial infection. Interferon-γ release assays (IGRAs) do not share these problems and have the potential to improve LTBI testing.

In a cross-sectional study with longitudinal follow-up between February 2008 and March 2010, researchers examined the performance of IGRAS among 2418 HCWs in the U.S. who were undergoing serial testing for LTBI. Results were compared between the TST and two commercially available IGRAs — the QuantiFERON-TB Gold In-Tube (QFT-GIT) and the T-SPOT.TB (T-SPOT).

Among HCWs with no prior positive test or TB treatment, a baseline positive test was significantly less likely with the TST (1.8%) than with the QFT-GIT (3.8%) or the T-SPOT (5.0%). Test conversion was also significantly less likely with the TST (0.9%) than with the QFT-GIT (6.1%) or the T-SPOT (8.3%). No association was seen between reported TB exposure and conversion by any test. Finally, among HCWs who had repeat IGRA testing 7 to 21 days after a TST, 9.1% showed a boosted response on the QFT-GIT and 11.3% did so on the T-SPOT.

Comment

The authors of this large prospective comparison of the tuberculin skin test, the QuantiFERON-TB Gold In-Tube, and the T-SPOT.TB among U.S. healthcare workers conclude that the majority of newly positive interferon-γ release assays at baseline and follow-up were falsely positive, and that the false-positive rate was much higher for the IGRAs than for the TST. As they note, the findings suggest that the IGRA specificity in HCWs at low risk for new TB infection is less than previously reported. IGRAs should be used with caution in this group.

  • Disclosures for Neil M. Ampel, MD at time of publication Editorial boards Medical Mycology

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