The tuberculosis skin test, also known as the tuberculin test, is called the PPD test. Usually 0.1 ml, or 5 units of the injection solution, is injected intradermally in the middle of the flexor side of the patient’s left forearm at the injection site. After 48-72 hours, signs suggestive of abnormalities can be observed, mainly the presence of skin congestion, edema, and hard nodules with a diameter range of more than 5 mm. If between 5-9mm, the skin test result can be judged as weakly positive; if the diameter is between 10-19mm, the result can be judged as positive; if the diameter is above 20mm or there is skin necrosis and blistering around the local area, the suggestive result is judged as strongly positive.