The work on this European Standard has been prepared on an international level at ISO/TC 212 and CEN/TC 140. On a national level NA 063-05-10 AA "Chemotherapeutische Untersuchungsmethoden" ("Chemotherapeutic diagnostic methods") is responsible for the work. In vitro susceptibility tests are performed on microorganisms suspected of causing disease, particularly if the organism is thought to belong to a species that may exhibit acquired resistance to frequently used antimicrobial agents. The tests are also important in resistance surveillance, epidemiological studies of susceptibility and in comparisons of new and existing agents. Dilution procedures are used to determine the minimum inhibitory concentrations (MICs) of antimicrobial agents and represent the reference method for antifungal susceptibility testing. MIC methods are used in resistance surveillance, comparative testing of new agents for research or registration purposes, to establish the susceptibility of organisms that give equivocal results in routine tests, for tests with organisms where routine tests may be unreliable and when a quantitative result is needed for clinical management. In dilution tests, microorganisms are tested for their ability to produce discernible growth on a series of agar plates (agar dilution) or in broth (broth dilution) containing serial dilutions of the antimicrobial agent. The lowest concentration of an antimicrobial agent (in mg/l) that, under defined in vitro test conditions, reduces visible or optically measurable growth of a microorganism within a defined period of time is known as the MIC. The MIC is a guide for the clinician to the susceptibility of the organism to the antimicrobial agent and aids treatment decisions. The results are significantly influenced by the test conditions. For this reason careful control and standardization are required for corresponding intra- and inter-laboratory reproducibility, as results may be influenced by the method used. It is generally accepted that broth MIC tests are reproducible to within one doubling dilution of the true end point (that is, ±1 well or tube in a doubling dilution series). The reference methods described in DIN EN ISO 16256 are intended for the testing of pure cultures of yeast fungi. The broth microdilution methods described in DIN EN ISO 16256 are essentially the same as those described by the Clinical and Laboratory Standards Institute (CLSI) and by the European Committee on Antimicrobial Susceptibility Testing (EUCAST). These methods have been shown to provide MICs of fluconazole that are essentially the same, if not identical up to 2 mg/l. Studies with various other antifungal agents are planned or under way. The laboratory that wishes to use this standard for conducting studies of newer antifungal agents, or as a reference method for comparison to MICs generated by a diagnostic device, should select which of the procedure options to use based upon the choice of MIC reading determined by visual inspection (CLSI method) or by use of a spectrophotometer (EUCAST method). In either case, the procedural details for that option are to be followed explicitly.