The tuberculin test, also known as the PPD test, is a primary screening test to screen patients with current TB infection by observing them for 72 hours after an intradermal injection of tuberculin. If there is no erythema or erythema less than 5mm at the injection site in about 72 hours, it means that the patient was not originally infected with tuberculosis and has not been given the BCG vaccine, which is then recommended to prevent potential future tuberculosis infections. If the skin test shows erythema greater than 5 mm and less than 2 cm, this is a benign result, suggesting possible TB infection or BCG vaccination. Only the presence of erythema greater than 2 cm or with blisters, ulcers, or hard nodules is a strong positive result and may be in the midst of a current TB infection.