To the Editor.
—Dr Lefrak1 says that close contacts of individuals with newly diagnosed active tuberculosis should be started on preventive therapy with isoniazid when the contact's tuberculin skin test is greater than 5 mm induration. (Active tuberculosis in the contact has been excluded by thorough history and physical examination and chest roentgenogram.) But withholding isoniazid in a tuberculin-negative contact appears contrary to basic principles and is contrary to the guidelines of the International Union Against Tuberculosis and Lung Disease.2After infection, the skin test takes at least 4 weeks to convert and may take as long as 12. (It may never convert: tests in 25% to 50% of very young children with proven tuberculosis remain negative.2) Withholding isoniazid leaves the unprotected contact at risk of developing active tuberculosis, which is especially dangerous in children, who may get disseminated (miliary) disease or meningitis.The International Union Against