PDD skin test refers to tuberculin test, and its test result is mainly to observe whether there are hard nodules, redness and swelling at the test site, as well as the specific size, and the result will be categorized as negative, suspected positive, and positive according to the different manifestations. 1. Negative (-): There is no redness or swelling of the skin at the test site, and there are no hard nodules. 2. Suspected positive (±): There are red spots the size of a pinhole, slightly some redness and swelling, and the diameter of the hard nodules is less than 0.5cm. 3. Positive: the diameter of redness and hard nodules in the test site is 0.5-0.9cm (+); if the diameter is 1.0-1.9cm (++); if the diameter is greater than or equal to 2cm, it will be (++++); in addition to redness and hard nodules, there are localized blisters and necrosis appearing then (++++). Positive tuberculin test indicates that the patient is infected with tuberculosis bacillus or has been vaccinated with BCG vaccine. This indicator alone cannot diagnose tuberculosis, and it needs to be combined with the patient’s clinical symptoms as well as other auxiliary examinations to confirm the diagnosis.