The preferred test for tuberculosis is the primary screening test, also known as the PPD test, in which tuberculin is injected intradermally and then observed for 72 hours, as follows: 1. If the erythema under the skin is <5mm, the result is negative, suggesting no previous tuberculosis infection or vaccination. 2. 3. If the erythema exceeds 2 cm, accompanied by blisters and rupture, it is a strong positive result, and the present TB infection should be suspected and further consultation with the Department of Infectious Diseases is needed.