The PPD test is generally observed for 72 h. The rash or erythema produced around 48 h is not likely to shrink slowly, and the PPD test is a test for initial screening of patients with current TB infection, but is not a confirmatory test for TB, and is determined mainly by injecting tuberculin intradermally and then observing for 72 h. If the erythema is <5 mm, it is negative. If the erythema of the skin is <5 mm, it is a negative result, which does not require special treatment and only indicates that there is no tuberculosis infection or BCG vaccination. If the erythema is 5-20 mm, it is a weakly positive or positive result, indicating a possible previous TB infection or vaccination. If the erythema exceeds 2 cm, it indicates a current TB infection and requires treatment at the infectious disease department of a designated hospital.