Fil: Kurbatova, E V. Centers for Disease Control and Prevention, Atlanta, Georgia; Estados Unidos.Fil: Cavanaugh, J S. Centers for Disease Control and Prevention, Atlanta, Georgia; Estados Unidos.Fil: Shah, N S. Division of General Internal Medicine, Albert Einstein College of Medicine, Bronx, New York; Estados Unidos.Fil: Wright, A. World Health Organization, Geneva; Suiza.Fil: Kim, HeeJin. Korean Institute of Tuberculosis, Seoul; Korea.Fil: Metchock, B. Centers for Disease Control and Prevention, Atlanta, Georgia; Estados Unidos.Fil: Van Deun, A. Institute of Tropical Medicine, Antwerp; Belgica.Fil: Barrera, Lucia. ANLIS Dr.C.G.Malbrán. Instituto Nacional de Enfermedades Infecciosas; Argentina.Fil: Boulahbal, F. Institut Pasteur d’Algérie, Alger; Argelia.Fil: Richter, E. National Reference Center for Mycobacteria, Borstel; Alemania.Fil: Martín-Casabona, N. Hospital Universitaris Vall d’Hebron, Barcelona; España.Fil: Arias, F. Institute of Public Health of Chile, Providencia Santiago, Santiago; Chile.Fil: Zemanova, I. National Institute of Public Health, Scrobarova; Chequia.Fil: Drobniewski, F. Health Protection Agency, London; Reino Unido.Fil: Santos Silva, A. National Institute of Health, Porto; Portugal.Fil: Coulter, C. Queensland Mycobacterium Reference Laboratory, Brisbane, Queensland; Australia.Fil: Lumb, R. Institute of Medical and Veterinary Science, Adelaide, South Australia; Australia.Fil: Cegielski, J P. Centers for Disease Control and Prevention, Atlanta, Georgia; Estados Unidos.Based on data from 14 Supranational Tuberculosis (TB) Reference Laboratories worldwide, the proportion of rifampicin (RMP) resistant isolates that were isoniazid (INH) susceptible by phenotypic drug susceptibility testing varied widely (0.5-11.6%). RMP-resistant isolates that were INH-susceptible had significantly lower rates of resistance to other first- and second-line anti-tuberculosis drugs (except rifabutin) compared to multidrug-resistant isolates. RMP resistance is not always a good proxy for a presumptive diagnosis of multidrug-resistant TB, which has implications for use of molecular assays that identify only RMP resistance-associated DNA mutations.