Which describes the correct administration procedure for a tuberculin skin test (TST)?

Which describes the correct administration procedure for a tuberculin skin test (TST)?

• A. Subcutaneous injection of 0.5 mL
• B. Intramuscular injection in the deltoid
• C. Intradermal injection of 0.1 mL PPD into the forearm
• D. Oral administration

Answer: (C)

The test relies on placing a small amount of PPD right under the skin so the immune system can respond in the dermal layer. The correct approach is a tiny intradermal injection (about 0.1 mL of PPD) into the forearm, which creates a small, raised wheal. If the person has been exposed to TB and is sensitized, memory T cells in the skin react over 48–72 hours to produce a localized induration (not just redness). Intradermal placement is crucial because it keeps the antigen at the surface where skin immune cells can process it; injecting subcutaneously or intramuscularly places the antigen in different tissues and can alter the reaction, and oral administration would not produce the skin reaction at all.

The test relies on placing a small amount of PPD right under the skin so the immune system can respond in the dermal layer. The correct approach is a tiny intradermal injection (about 0.1 mL of PPD) into the forearm, which creates a small, raised wheal. If the person has been exposed to TB and is sensitized, memory T cells in the skin react over 48–72 hours to produce a localized induration (not just redness). Intradermal placement is crucial because it keeps the antigen at the surface where skin immune cells can process it; injecting subcutaneously or intramuscularly places the antigen in different tissues and can alter the reaction, and oral administration would not produce the skin reaction at all.