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A woman in her 20s presents for follow-up of a tuberculin skin test (TST) placed 2 days earlier. The patient is from India and arrived in the United States 1 month ago for graduate studies. She reports no significant medical history and is unaware of any tuberculosis (TB) exposures. She has no symptoms. She received the BCG vaccine as a child. Laboratory testing showed normal chemistry and hematology values and a negative HIV test. The TST reveals an induration of 20 mm. Because of the TST result, the patient has a chest radiograph performed, which was unremarkable. Treatment for latent TB infection (LTBI) was recommended based on the positive TST, negative chest radiograph, and recent US arrival from a high-TB–incidence country. The patient asks whether the positive TST result could be due to her BCG vaccination and wants further testing before beginning therapy. Blood is drawn for an interferon-γ release assay (IGRA; see Figure for results).
Blumberg HM, Kempker RR. Interferon-γ Release Assays for the Evaluation of Tuberculosis Infection. JAMA. 2014;312(14):1460-1461. doi:10.1001/jama.2014.4928