The PPD test does not require dilution. The tuberculin currently recommended by the World Health Organization is a pure protein derivative that is injected intradermally into the body, and if the subject is susceptible to Mycobacterium tuberculosis infection, the tuberculin can have a localized inflammatory reaction with the body, resulting in localized redness, swelling, or nodules. If the subject has not been exposed to Mycobacterium tuberculosis, the infection does not have a local metaplasia, which is usually reflected in the PPD test within 48-72 hours. There is also the possibility of false positives.