How long after a TB skin test injection should the arm be checked for induration?

How long after a TB skin test injection should the arm be checked for induration?

• A. 24-48 hours
• B. 2-3 weeks
• C. 48-72 hours
• D. 1 month

Answer: (C)

The test relies on a delayed-type hypersensitivity reaction, so the induration develops over a few days after the injection. After introducing the antigen under the skin, T cells respond and create a palpable, raised area that peaks around 48 to 72 hours. Reading it earlier (like within 24 hours) may miss the full reaction, and waiting past 72 hours can underestimate or alter measurements as the reaction may start to subside. So, checking for induration about two to three days after the injection best captures the true response.

The test relies on a delayed-type hypersensitivity reaction, so the induration develops over a few days after the injection. After introducing the antigen under the skin, T cells respond and create a palpable, raised area that peaks around 48 to 72 hours. Reading it earlier (like within 24 hours) may miss the full reaction, and waiting past 72 hours can underestimate or alter measurements as the reaction may start to subside. So, checking for induration about two to three days after the injection best captures the true response.