Antimycobacterial susceptibility testing: present practices and future trends.

Along with the worldwide renewed interest in tuberculosis, the high incidence of non-tuberculous disease in HIV-infected patients and the continuing problem of disease caused by rapidly growing mycobacteria, there has been a renewed interest in mycobacterial susceptibility testing. This renewed interest stems from the needs both to provide accurate, reliable and timely susceptibility test information for the management of patients infected with these mycobacteria and to identify new and more effective antimycobacterial agents. Indeed, many new agents have already been identified and are currently used to treat these infections, but the conventional susceptibility testing methods, in many instances, have not been adequately evaluated for these new agents. Perhaps the time has come to give consideration to abandoning such time-honored practices and criteria as the proportion method, the 99% threshold, and "critical concentrations". New methods of susceptibility testing are in various stages of development and many of these methods have advantages and improvements over the existing methods. There is an increased understanding of the pathobiology of mycobacterial infections and an increased understanding of the pharmacokinetics of antimycobacterial agents and the mechanisms of action and resistance. This article gives an overview of the present practices and an assessment of the current needs and potential for the near future of antimycobacterial susceptibility testing.