Which test is used to screen for TB infection by causing a delayed-type hypersensitivity reaction?

Which test is used to screen for TB infection by causing a delayed-type hypersensitivity reaction?

• A. Tuberculin skin test (TST)
• B. Chest X-ray
• C. Sputum culture
• D. PCR test for TB DNA

Answer: (A)

Screening for TB infection relies on a delayed-type hypersensitivity reaction, a T cell–mediated immune response. The tuberculin skin test inserts purified protein derivative just under the skin. If the person has been infected before, memory T cells recognize the protein and release cytokines, causing inflammatory cells to accumulate and form an induration at the injection site within 48–72 hours. The size of the induration indicates prior exposure and immune sensitization to TB, not whether the bacteria are actively present. This is why the test is used to screen for infection rather than to diagnose active disease. Other tests serve different purposes: chest X-ray looks for lung changes associated with active TB; sputum culture grows the TB bacteria to confirm infection; PCR detects TB DNA. Remember that prior Bacillus Calmette-Guérin vaccination or certain mycobacterial infections can cause false positives, and immunocompromised status or very recent infection can yield a false negative.

Screening for TB infection relies on a delayed-type hypersensitivity reaction, a T cell–mediated immune response. The tuberculin skin test inserts purified protein derivative just under the skin. If the person has been infected before, memory T cells recognize the protein and release cytokines, causing inflammatory cells to accumulate and form an induration at the injection site within 48–72 hours. The size of the induration indicates prior exposure and immune sensitization to TB, not whether the bacteria are actively present. This is why the test is used to screen for infection rather than to diagnose active disease. Other tests serve different purposes: chest X-ray looks for lung changes associated with active TB; sputum culture grows the TB bacteria to confirm infection; PCR detects TB DNA. Remember that prior Bacillus Calmette-Guérin vaccination or certain mycobacterial infections can cause false positives, and immunocompromised status or very recent infection can yield a false negative.