In a cover story written for the peer reviewed
Medical Laboratory Observer Cara Weisbrod explains how the new
TB immigration guidelines will affect TB testing
The poor specificity of the TST in immigrants may be the reason that the CDC has recently recommended new TB diagnostics in its 2009 update for panel physicians. The most important change is that panel physicians may now choose an interferon-gamma release assay (IGRA) instead of the TST.
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IGRAs are cost-effective. Many health officials across the U.S. report that switching from TST to the ELISA-based strategy may harness major cost-savings through reductions in the number of chest X-rays, sputum smears, and the public-health resources required to evaluate and treat immigrants who are TST-positive but unlikely to have true TB infection. Oxlade, et al, found when BCG vaccination among immigrants was widespread, the ELISA test was significantly cheaper than TST.
IGRAs are significantly more sensitive in detecting people with active TB than the TST. A recent meta-analysis of published literature found the ELISA-based test had a sensitivity of 84.5% in studies of developed countries vs. 71.5% for the TST. The sensitivity of the enzyme-linked immunospot was reported at 88.5%, determined from studies that used a cut-off lower than that recently approved for use in the U.S.
High test specificity, the percentage of people without infection who test negative, is essential to avoid false-positive results and their consequences. The Diel, et al, meta-analysis found specificity of >99.2% for the ELISA-based test, and — again at different cutoffs than were recently approved for use in the U.S. — 86.3% for the enzyme-linked immunospot. A previous meta-analysis found that the specificity of the TST in BCG-vaccinated populations is 59%.
Confidence in the diagnostic capabilities of IGRAs is high. Data indicates acceptance of latent TB prophylaxis is higher in healthcare workers tested with one of the IGRA tests compared to TST.10 In addition, the 2008 American College Health Association TB testing guidelines now recommend the ELISA-based IGRA technology for international visitors studying in the United States.
