For the tuberculosis skin test, also known as the tuberculin test, no significant changes are seen until about 48 hours. The tuberculin test is a preliminary test to screen patients for active tuberculosis, mainly by intradermal injection of tuberculin, which is observed for 72 hours. The presence of erythema <5mm suggests a negative result, indicating no previous tuberculosis infection. If the erythema is 5-10 mm, it is a weak positive result, and if the erythema is 10-20 mm, it is a positive result, indicating a previous TB infection or vaccination. Only erythema > 2 cm, or the presence of blisters, ruptures, or hard nodules are considered active TB infection in patients.