The three-day ppd skin test may show a negative result or a positive result. 1. Negative result: the negative result is usually the erythema at the injection site is less than 5 millimeters, which means that it has not been infected with tuberculosis bacillus before, but it may not have been injected with BCG vaccine. 2. Positive cases: the diameter of subcutaneous erythema is more than 5 millimeters, but less than 10 millimeters, indicating weak positivity; erythema with a diameter of 10-20 millimeters belongs to positive cases, suggesting that the patient has been vaccinated with BCG vaccine or infected with tuberculosis bacillus; the diameter of the erythema is more than 20 millimeters, with the appearance of hard nodules and blisters around the erythema, suggesting that the patient has strong positivity. For strong positive patients, it is recommended to consult regular infectious disease hospitals, need to be combined with chest CT examination, sputum to find tuberculosis bacilli examination, to determine the existence of tuberculosis, do not be negligent, so as not to delay the condition.