A PPD test should be read within how many hours after injection?

A PPD test should be read within how many hours after injection?

• A. 24-48 hours
• B. 48-72 hours
• C. 72-96 hours
• D. 12-24 hours

Answer: (B)

Reading a PPD test at the right time matters because the tuberculin reaction is a delayed-type hypersensitivity that reaches its most reliable size around 48 to 72 hours after injection. At about 2 days, the induration becomes clearly measurable and less affected by transient skin changes. If you read too early, the induration may be small or absent, leading to a false negative. If you wait too long, the reaction can fade or become harder to distinguish from other skin changes, risking a false negative or less accurate measurement. So, the best window is 48 to 72 hours after administration. The reaction is assessed by measuring the induration (the firm, raised area) in millimeters, not just redness.

Reading a PPD test at the right time matters because the tuberculin reaction is a delayed-type hypersensitivity that reaches its most reliable size around 48 to 72 hours after injection. At about 2 days, the induration becomes clearly measurable and less affected by transient skin changes. If you read too early, the induration may be small or absent, leading to a false negative. If you wait too long, the reaction can fade or become harder to distinguish from other skin changes, risking a false negative or less accurate measurement. So, the best window is 48 to 72 hours after administration. The reaction is assessed by measuring the induration (the firm, raised area) in millimeters, not just redness.