American Board of Family Medicine (ABFM) Practice Exam

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What is the preferred TB test for Mexicans or immigrants vaccinated with BCG?

  1. Skin test

  2. Interferon-gamma release assays

  3. Sputum culture

  4. Chest X-ray

The correct answer is: Interferon-gamma release assays

The preferred test for detecting tuberculosis (TB) in individuals who have received the Bacillus Calmette-Guérin (BCG) vaccine, such as many Mexicans or immigrants from regions where BCG is commonly administered, is the interferon-gamma release assay (IGRA). This choice is appropriate because the BCG vaccine can lead to false-positive results on the traditional skin test (also known as the tuberculin skin test or TST). Since the skin test measures the immune response to a protein found in the TB bacterium, individuals who have been vaccinated with BCG may test positive regardless of whether they have an actual TB infection. In contrast, IGRA is a blood test that detects the immune response to specific TB antigens that are not present in the BCG vaccine. Therefore, a positive IGRA result is more likely to indicate active or latent TB infection rather than a response due to the vaccination. This attribute makes IGRA a more reliable and accurate screening tool for those who have received BCG. While other options might be valid in certain contexts, they do not serve as the initial screening method in this specific scenario. The skin test is less reliable because of the influence of BCG, sputum culture is