We have now seen how continuous monitoring of organism activity assists in determining MICs. Continuous monitoring is also crucial in determining the incubation time required for an organism in the VITEK® 2 system. As stated earlier, the length of the lag phase is unique for each organism. Because the VITEK 2 can determine when an organism transitions from the lag phase to the exponential phase, the analysis software will determine the appropriate incubation time based on the growth displayed by each organism. After reaching the end of the lag phase in the growth control well, the VITEK 2 is programmed to wait for a predetermined amount of time in order to see if growth has occurred in the lowest concentration of antibiotic. If growth is still occurring during this interval, the VITEK 2 will again extend incubation for a predetermined amount of time to assess the growth occurring in the remaining concentrations. This extension process continues until 1) growth is seen in the highest concentration, or 2) no further significant growth has taken place. Using this process, the VITEK 2 has the capability to use a relatively short incubation time for very sensitive organisms. At the same time, the VITEK 2 analysis is able to detect delayed resistance. In the end, it is the organism that determines the length of incubation. The instrument simply adjusts according to growth as it is detected.
In summary, the VITEK 2 approach to determine AST results does break from tradition. However, it is important to remember that continuing tradition is not the goal in science. The ability of the VITEK 2 to continuously monitor organism activity in order to determine the length of incubation and determine the MIC is an efficient and effective use of data. During clinical trial and market evaluations, it has been demonstrated, repeatedly, that this process provides accurate and reproducible results. If other methods (e.g. disk diffusion and Etest®) established that their MICs are “real” through similar evaluations, how can VITEK 2 results be considered anything other than “real” MICs?
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