The PPD test, also known as the tuberculin test, is a test in which tuberculin is injected into the skin of the flexor side of the upper arm of the person being tested, and whether or not there is a hard lump in the area, as well as the diameter of the hard lump, is observed to determine whether or not the person is suffering from tuberculosis. The PPD test, also known as the tuberculin test, is one of the tests for tuberculosis infection, in which tuberculin is injected into the skin of the flexor side of the arm, and after 24 to 48 hours, it is observed to see whether the injected area becomes red, swollen, hardened, and other perverted reactions in order to identify tuberculosis. If the human body is infected by tubercle bacillus, it will stimulate the body to produce corresponding antibodies. During the PPD test, the injected tuberculin will combine with the antibodies to form an immune complex, which will be adsorbed inside the skin, and this may lead to redness, swelling and hardness at the injection site, and if the diameter of the hardness is larger than 5mm, it indicates that the patient may suffer from tuberculosis and other diseases. Tuberculosis patients are recommended to go to a specialized hospital to clarify the cause of the disease and then under the guidance of the doctor for further examination and treatment.