The ppd test is the tuberculin test, the appearance of redness is a general reaction, if the redness is combined with a hard nodule of >0.5cm within 48-72h, it indicates a positive reaction. Tuberculin test is a common test for diagnosing tuberculosis, which is used to evaluate and judge tuberculosis through the skin reaction after local subcutaneous injection of tuberculin-containing drugs. When the drug containing tuberculin is injected into the palmar side of the anterior 1/3 of the forearm, it will be sensitized with the body’s immune T-lymphocytes, showing redness, hard nodules, or even blisters and necrosis of the skin in the injection area. When observing no hard nodule within 48~72h or the diameter of hard nodule is within 0.5cm, it can be recognized as negative. Positive tuberculin test generally indicates tuberculosis infection or BCG vaccination; positive test in children who have not been vaccinated with BCG usually indicates tuberculosis.