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The interpretation of purified-protein-derivative (PPD) skin tests for tuberculin sensitivity is difficult when the patient does not return within 48 to 72 hours. QuantiFERON-TB is a blood-based interferon-γ release assay (IGRA) that measures the response to Mycobacterium tuberculosis and other nontuberculous mycobacteria. To determine the validity of skin-test results at 7 days, Tat and colleagues compared readings on days 2 and 7 for each of the two available tuberculin skin tests and evaluated them against IGRA results.
They tested 116 adults at increased risk for tuberculosis, using one of the skin-test products on each arm, and obtained blood samples for IGRA. Fifty-two had positive skin-test readings on day 2. Agreement between the two…