If the PPD test is negative, it means that the patient has not been infected with Mycobacterium tuberculosis and needs to be injected with BCG to prevent tuberculosis.PPD test is also called tuberculin test, or tuberculosis skin test.PPD test is to inject tuberculin into the skin and observe the skin changes after 48-72 hours. If the skin only appears red, no nodules appear, or if the nodules are small and the diameter of the nodules is <5mm, it is considered negative; if the diameter is >5mm and less than 2cm, it is considered positive; if the diameter is >2cm, or if blisters appear around the nodules, it is a strong positive, and a strong positive helps to diagnose active tuberculosis. A positive test indicates that the patient has been infected with Mycobacterium tuberculosis and does not need BCG vaccination. A negative test indicates no previous infection with Mycobacterium tuberculosis and requires BCG vaccination.