Which test checks for an immune reaction to TB exposure by injecting antigen under the skin?

• A. Chest X-ray
• B. Sputum culture
• C. TB Skin Test (Mantoux)
• D. NAAT

Answer: (C)

This item is about measuring the body's delayed immune reaction to TB exposure by introducing antigen under the skin. The TB Skin Test, also known as the Mantoux test, does this by injecting purified protein derivative intradermally. If someone has been exposed to TB and has memory T cells, a local immune reaction develops, producing an induration at the injection site that is read after about 48–72 hours. This indicates exposure or infection, not necessarily active disease.

This differs from other tests: a chest X-ray looks for lung changes of active TB, a sputum culture grows live bacteria from the sputum, and a NAAT detects TB genetic material. Because a positive skin test reflects immune memory rather than the bacteria itself, interpretation depends on the induration size and risk factors, and prior vaccination with BCG can complicate results in some settings.