The full name of PPD is tuberculin test, and a strong positive test means that 72 hours after vaccination, the diameter of local hard nodes (not redness) is approximately equal to 20 mm, or although it is less than 20, local blisters, double circles, lymphadenitis, and rupture are also strong positive. The degree of positivity indicates the degree of infection and does not correspond to the degree of morbidity. It is influenced by many factors. The stronger the reaction, the more important the diagnosis of tuberculosis, especially in infants and children, and the greater the need to consider the need for treatment. In general, in pediatric patients, depending on the presence or absence of symptoms and local lymph node enlargement, a chest CT or chest radiograph and abdominal ultrasound are required, with positive results mostly considered tuberculosis-causing and requiring regular anti-tuberculosis treatment. If all the results are negative, prophylactic medication should be considered. Nowadays, microcal is generally recommended for 3 months, and isoniazid is less recommended for single-drug prophylaxis. In infants, most require anti-tuberculosis treatment. In adults, if there are no other symptoms, chest CT and abdominal ultrasound are negative, and sputum is negative for tuberculosis, TB infection is mostly considered, but it is not pathogenic, so it can be left untreated and continue to be observed, and then considered for examination and treatment when there are symptoms. In conclusion, PPD is an auxiliary test that is of great significance in the diagnosis of infants and young children, but is only suggestive in the examination of adults, and needs to be combined with other conditions and tests to make a final diagnosis.