The fastest response in the PPD test can occur between 24-48 hours. The PPD test is used to screen patients for tuberculosis infection and does not necessarily confirm the diagnosis of tuberculosis.The PPD test is performed by injecting tuberculin intradermally and observing the patient for 72 hours. If the skin shows a red spot <5mm, it is a negative result, suggesting that the patient is not infected with tuberculosis or has not received BCG vaccine. If the erythema is 10-20mm, it is positive. If the erythema is more than 2cm or there are ulcers and blisters, it is a strong positive result. Strongly positive PPD test is usually found in tuberculosis patients who are currently infected.