There are multiple factors that can influence the result of the PBT, including how it was stored, when it was last calibrated and what the base level is. It is necessary to establish a base level before asking someone to blow into the machine. If a base level has not been established, then there’s no way to know what is really being measured or how to interpret the results. An officer also has to be certified in order to use the PBT. A lawyer should be aware of all of these factors when handling these cases.