PPD is tested by observing the hard nodes 72 hours after injection and is positive if the average diameter of the hard nodes is ≥5mm. Of these, 5-9mm is a fair positive, 10-19mm is a moderate positive, and ≥20mm is a strong positive. A general positive or moderate positive PPD test only means that the body is infected with tuberculosis and does not confirm the diagnosis of tuberculosis, but must be combined with other imaging tests and clinical manifestations. If the test is strongly positive, it means that there is an active TB lesion in the body.