A negative or weakly positive tuberculosis skin test is a normal condition. The tuberculosis skin test, also known as the tuberculin test, or PPD test, involves injecting tuberculin toxin into the skin and observing changes in the skin 48-72 hours later. The skin is negative if there is redness or small nodules with a diameter of <0.5 cm. If the diameter of the nodule is >0.5cm and less than 2cm, it is weakly positive. If the nodule is >2cm in diameter, it is strongly positive. A negative test indicates that the patient has not been infected with Mycobacterium tuberculosis and can be prevented from tuberculosis by BCG vaccination. A weak positive indicates a history of Mycobacterium tuberculosis infection, and a strong positive can be used as an auxiliary diagnosis of active tuberculosis.