Mycobacterium tuberculosis is a biological product made from the culture filtrate of Mycobacterium tuberculosis that contains only the protein of Mycobacterium tuberculosis but not the body. At present, the tuberculin commonly used in China is a pure protein derivative of Mycobacterium tuberculosis, referred to as PPD. The result is judged as follows: within 24-72 hours after PPD injection, measure the straight warp of skin hard nodes. 5mm or less is negative (-), 5~9mm is weakly positive, 10~19mm is positive, more than 20Mmm or less than 20mm but blistering and necrosis is strongly positive. If you are positive for tuberculin, it only means that you have been infected with tuberculosis bacillus. Whether you have tuberculosis disease or not, your doctor should analyze your clinical manifestations, sputum bacteriological test and x-ray examination to make a final judgment. If it is negative, it does not negate the tuberculosis infection, because the initial infection immune response has not yet been established, the body’s immune function is suppressed, such as tumors, severe tuberculosis, severe malnutrition and the application of immunosuppressive drugs can be negative.