The normal result of PPD skin test in children is usually negative, but if the baby has been vaccinated with BCG vaccine or has had previous infections, some of them may also show positive results, and further examination and treatment is required if they are strongly positive. PPD test usually refers to the tuberculin test, is through the intradermal injection of tuberculin, according to the injection site of the skin condition, so as to diagnose tuberculosis infection caused by hypersensitivity reaction, under normal circumstances, the diameter of the hard nodule should be less than 0.5 centimeters, is a negative manifestation of the absence of tuberculin infection. However, a positive PPD test skin test result only indicates the existence of tuberculosis infection or BCG vaccination, and does not necessarily mean that the disease is present; if the ppd test result is strongly positive, the diameter is relatively large or there are localized blisters, it is very likely that tuberculin infection exists, and it is often necessary to cooperate with the doctor to take further examination and make a clear diagnosis.