The PPD test is performed by injecting tuberculin into the forearm skin for 72 h. If an erythema of less than 5 mm appears at the injection site, it is a negative result, suggesting no tuberculosis infection or BCG vaccination. If the erythema is 5-10 mm, it is a weakly positive result, and if the erythema is 10-20 mm in diameter, it is a positive result, both of which are indicative of TB infection or BCG vaccination. A strong positive result is indicated if the erythema of the skin after the PPD test is greater than 2 cm, or if blisters, ruptures, or hard nodules are present. A strong positive result indicates that the patient may be infected with TB or due to active TB, and the interpretation of these results needs to be determined at the vaccination clinic.