The tuberculin test is performed on patients with current tuberculosis infection and is usually performed by intracutaneous injection of tuberculin into the forearm and observation of the size of the erythema. If the erythema is less than 5 mm, it indicates a negative result, indicating that there is no tuberculosis and no BCG vaccination. A nodule of 5-10 mm is a weakly positive result, while an erythema of 10-20 mm is a positive result. Both of these results suggest the possibility of previous TB infection or TB vaccination. If the erythema is >2 cm, or if there are hard nodules, blisters, necrosis, or even ulcers, this is a strong positive result, and it is possible that the patient has a current TB infection.