A negative TB skin test means that the patient is not infected with Mycobacterium tuberculosis and needs to be vaccinated to prevent infection with Mycobacterium tuberculosis. If the TB skin test is still negative after the vaccination, it means that the vaccination has failed. Therefore, for patients with a negative TB skin test, the TB skin test should be repeated after the injection of Mycobacterium tuberculosis to determine whether the vaccination was successful. The tuberculosis skin test, also known as the tuberculosis bacillus test or PPD test, involves injecting the conjugated toxin into the skin and observing the changes in the skin 48-72 hours later. The skin is negative if there are no nodules, or if the nodules are very small, less than 5 mm in diameter, with only redness. If the nodules are larger than 5 mm, the test is positive, and a positive test indicates that the skin is infected with Mycobacterium tuberculosis.