In vitro susceptibility testing of clinical isolates of Nocardia

Abstract

There is a strong rationale for determining antimicrobial susceptibility profiles of clinical isolates of Nocardia. Methods for documenting these profiles are seemingly reproducible, if not yet standardized. The NCCLS has recently organized a working group to evaluate and standardize susceptibility test methods for the aerobic actinomycetes, including the Nocardia spp. Once a standard method is recommended, inter- and intralaboratory reproducibility can be assessed and the role of susceptibility profiles as predictors of clinical outcome can be better studied. Currently, although retrospective reports of such studies do exist, most are based on individual vignettes or patient groups too small for appropriate interpretation or outcome assessment (1, 2). Frequently, the patients evaluated have underlying conditions too complex to provide a simplistic evaluation of bug/drug outcomes (1, 2). Inadequate length of follow-up for relapse to occur may also provide erroneous correlation between in vitro and in vivo results. Until standardization and clinical evaluations have been achieved, care must be exercised in susceptibility testing of the Nocardia spp. and interpretation of results (1, 5, 17). Individual laboratories should weigh their own experience (and volume of tests) in the performance of such testing before providing the service; more appropriately, isolates should be submitted to reference laboratories having greater experience (5). Results of in vitro studies should be interpreted conservatively and should be reported with the understanding and comment that they are “not standardized.”