The results are judged as negative for less than 5 mm, weakly positive for 5-9 mm, positive for 10-19 mm, and strongly positive for more than 20 mm or local blistering and necrosis. A strong positive reaction indicates that TB is active and requires active treatment. A positive reaction is only indicative of a previous Mycobacterium tuberculosis infection, not necessarily a current infection, but is of greater diagnostic value in infants and children. A positive reaction in a child under three years of age should be considered as active tuberculosis with recent infection and requires treatment if necessary. The diagnosis of pulmonary tuberculosis in the clinical setting is relatively complex and requires a combination of pulmonary CT, clinical manifestations, and exposure history.