Which test is commonly used to screen TB infection, including latent TB?

• A. Mantoux test
• B. QuantiFERON-TB Gold test
• C. Purified Protein Derivative (PPD)
• D. Sputum smear

Answer: (C)

Screening for TB infection, including latent TB, is most commonly done with the tuberculin skin test that uses purified protein derivative. A small amount of PPD is injected under the skin, and if the person has been exposed to TB before, their immune system reacts to the antigen, producing a raised induration within 48–72 hours. The size of that induration is used to judge a positive result, which signals infection but cannot tell whether TB is active or latent on its own. This method is widely used because it is inexpensive and easy to administer to large groups.

A useful alternative you may encounter is a blood test that measures interferon-gamma response to TB antigens (an IGRA). It offers greater specificity in people who have received the BCG vaccine, but it requires more resources and lab work. Sputum smear, by contrast, is used to diagnose active pulmonary TB by detecting bacteria in the sputum, not for screening latent infection. The tuberculin skin test with PPD remains the standard screening approach for TB infection.