A tuberculosis intradermal skin test to detect tuberculosis infection is given to a high-risk adolescent. How long after the test is administered should the result be evaluated?
A. Immediately
B. Within 24 hours
C. In 48 to 72 hours
D. After 5 days
Correct Answer: C. In 48 to 72 hours
Tuberculin skin tests of delayed hypersensitivity. If the test results are positive, a reaction should appear in 48 to 72 hours. Two visits are required in this test. First visit to get the test administered, and the second visit to get the reading of the test after 48 to 72 hours of test placement.
Option A: Immediately after the test is too soon to observe a reaction. The results of this test are interpreted by measuring the hypersensitivity reaction (delayed-type hypersensitivity) to tuberculin purified protein derivative, derived from Mycobacterium tuberculosis.
Option B: Within 24 hours is too soon to observe a reaction. The peak of the induration reaction occurs after 24 hours of the test injection. Induration of the skin at the injection site occurs secondary to cell infiltration.
Option D: Waiting more than 5 days to evaluate the test is too long because any reaction may no longer be visible. It takes about 6 to 8 weeks after exposure to the bacteria for the PPD test to be positive.