Which tests are commonly used to screen for tuberculosis?

• A. Purified protein derivative (PPD) or Mantoux test; QuantiFERON-TB Gold test
• B. Chest X-ray and CT scan
• C. Sputum culture and Gram stain
• D. Gram stain alone

Answer: (A)

Screening for tuberculosis looks for people who have been exposed or infected, often before symptoms appear. The most common tools are the tuberculin skin test (PPD/Mantoux), which is given as an intradermal injection and read after 48–72 hours, and interferon-gamma release assays (IGRAs) like QuantiFERON-TB Gold, which use a blood sample to detect immune response to TB antigens. The skin test can yield false positives if someone has received the BCG vaccine or been exposed to non-tuberculous mycobacteria, while IGRAs are more specific in vaccinated individuals and aren’t affected by prior BCG exposure.

Imaging tests such as chest X-ray or CT scan are used to evaluate potential active disease or complications when someone has symptoms or a positive screening result, not as primary screening tools for latent infection. Microbiologic tests like sputum culture and smear are used to diagnose active tuberculosis by identifying the bacteria itself, and Gram staining is not helpful for TB because the organism is acid-fast and requires special staining.