A negative tuberculin test is normal and indicates no infection with Mycobacterium tuberculosis. The tuberculin test, also known as the PPD test, involves injecting the tuberculosis virus into the skin and observing the size of the nodule. If the nodule is less than 5mm in diameter, it is negative. If the nodule is larger than 5mm in diameter and smaller than 2cm, it is positive. If the nodule is larger than 2 cm in diameter it is strongly positive, and strong positivity helps in the diagnosis of active tuberculosis. However, there are a few TB-infected patients who have a negative PPD test, especially in immunocompromised patients. Tuberculosis is an acute respiratory infectious disease caused by Mycobacterium tuberculosis invading the lungs, and is managed as a Class B infectious disease under the law of China. The susceptible population is mainly immunocompromised people, AIDS patients, diabetic patients are high incidence of the population, the general prognosis is good, through standardized treatment, most of them can be cured.