Antimicrobial susceptibility testing and blood cultures.

Susceptibility testing of microorganisms isolated from blood cultures should follow the guidelines of the NCCLS in the selection of antimicrobial agents and the method for testing. Standardized testing can be performed on isolated colonies from pure subcultures of the blood/broth mixture. Direct susceptibility testing can provide results up to 24 hours sooner, but the techniques for testing have not been standardized. The disk diffusion and agar dilution techniques do not utilize automated testing techniques and seem to provide reliable results. Limited studies of direct susceptibility testing using automated procedures have been published, but the results are not conclusive. The number of discrepancies with direct automated procedures in comparison with standardized techniques is low, and most investigators agree that direct testing is appropriate given the rapidity with which the results are available. New resistance patterns for microorganisms are appearing regularly, and the technology for automated procedures is changing rapidly. New clinical evaluations of direct susceptibility testing with larger numbers of microorganisms that include some of the recently described resistant microorganisms would help to clarify the accuracy and reliability of current procedures. Standardized direct susceptibility testing guidelines should be established so that reliability and accuracy can be improved. There is general consensus that repeat testing by standardized techniques is advisable, although the decision as to whether to repeat the test using a standardized method rests with the individual laboratory director based on his or her expertise and experience with the procedures. Each laboratory director should compare the advantages and limitations of direct antimicrobial susceptibility testing with the methodology currently in use and determine the need for repeat testing by standardized techniques.