The tuberculin (PPD) skin reaction is a delayed cellular hypersensitivity reaction. It is the entry of an antigen (Mycobacterium tuberculosis or BCG) into the body that sensitizes the body’s immune T lymphocytes and creates a memory. When the same antigen invades again, the sensitized lymphocytes recognize and initiate a specific immune response and can cause a metabolic inflammation. The manifestation is the formation of hard nodules or even blisters and necrosis at the site of tuberculin injection. 1. A positive PPD skin test indicates that the organism is susceptible to Mycobacterium tuberculosis, indicating that the organism has been infected with tuberculosis or has been vaccinated with BCG. In general, it is considered that a nodule diameter of 5-9 mm after 3 months of BCG vaccination is considered positive, and a nodule diameter of 20 mm or more within 30 months is considered to be naturally infected with Mycobacterium tuberculosis. It is also suggested in the literature that children with positive PPD skin test >15mm in diameter or with blisters are newly infected with M. tuberculosis. 2, PPD skin test positive also indicates that the body has a certain immunity to tuberculosis bacteria. Since the implementation of BCG vaccination policy for newborns in China, the probability of blood-borne tuberculosis and tuberculous meningitis among children, especially pediatric patients, has been significantly reduced, which has greatly improved the health level and safety of children. 3. A positive PPD skin test does not mean that a person is a TB patient. Because there is a high tendency to recover naturally from initial TB infection, clinical morbidity accounts for only 5 to 10 percent of initial TB infections. It does not mean that a positive PPD skin test is infectious. 4.A positive PPD skin test after infection with M. tuberculosis or BCG vaccination will diminish with time. However, this process takes several years. 5. However, there are a few immunocompromised individuals (about 5%) who are negative or have false negatives due to technical reasons. A negative PPD skin test after BCG vaccination is usually an indication of vaccination failure.