How is it determined that there is no reaction in a PPD skin test?

Patient's question:

No

Doctor's answer:

After the skin test, the reaction should be checked within 48 to 72 hours. Redness alone should not be used as the sole criterion. If there are pinpoint red spots or slight redness and swelling at the injection site, and the hard knot has a diameter of less than 0.5 centimeters, it is considered a negative reaction.