Final answer:
A PPD test should be read 48 to 72 hours after administration by a healthcare professional, who will measure the induration to check for a tuberculosis infection.
Step-by-step explanation:
The PPD test, which stands for Purified Protein Derivative, is used to detect tuberculosis (TB) infection. After a PPD is administered, the test site should be checked for a reaction after 48 to 72 hours. This time frame allows for a detectable immune response if TB bacteria are present. It's important that a healthcare professional performs the reading, as they are trained to measure the induration (the raised, hardened area) rather than redness, which is crucial in determining a positive result.