How long it takes for the redness of the PPD skin test to disappear is related to the strength of the reaction. Generally, a negative result may disappear in about 1-3 days, a strong positive result takes about 2-3 weeks to disappear, and a weak positive result is somewhere in between. The PPD test is the tuberculin test, which is an intradermal injection of tuberculin into the forearm, primarily to determine whether there is an infection of Mycobacterium tuberculosis in the body to diagnose tuberculosis. If the PPD test is negative, the redness of the forearm skin may disappear in 1-3 days; if the PPD test is weakly positive or positive, the redness of the forearm skin may disappear in about 1 week; if the PPD test is strongly positive, the area of the redness of the forearm skin is larger and may be accompanied by blisters and necrosis, which may take 2-3 weeks to recover. In addition, after the PPD test, the patient should pay attention to the cleanliness of the redness of the forearm skin and avoid contact with dirty water to avoid infection, which may lead to long-lasting healing.