The tuberculin test, also known as the PPD test, is a primary screening test for patients with active tuberculosis infection. It is performed by intradermal injection of tuberculin and then observed for 72 hours. If an erythema of less than 5 mm appears at the injection site after 72 hours, the result is negative, indicating that there is no tuberculosis infection or vaccination. If the erythema is 5-10 mm, it indicates a weak positive result. If the erythema is 10-20 mm, it is a positive result, indicating previous TB infection or vaccination. If the erythema exceeds 2 cm or is accompanied by ulcers, nodules, or blisters, this is a strong positive result and suggests that the patient may currently have active TB.