Medical Definition of XDR
extensively drug-resistant No one knows the exact number of cases of the two types of drug-resistant TB, called MDR and XDR for multidrug-resistant and extensively drug-resistant.—Donald G. McNeil Jr., The New York Times, 23 Mar. 2010 XDR tuberculosis is caused by a strain of Mycobacterium tuberculosis resistant to isoniazid and rifampin (which defines MDR tuberculosis) in addition to any fluoroquinolone and at least one of the three following injectable drugs: capreomycin, kanamycin, and amikacin.—Mario C. Raviglione and Ian M. Smith, The New England Journal of Medicine, 15 Feb. 2007
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