When the required procedure is not followed, fermentation can take place, and that leads to a higher blood alcohol concentration reading. Fermentation is the process by which alcohol is naturally produced. I will provide a bit more detail.
Assuming a qualified phlebotomist has extracted blood from a vein, that blood sample will be collected in a vial. In order to slow down the natural fermentation process, that vial must contain sufficient amounts of a common preservative called sodium fluoride (along with anti-coagulant potassium oxalate).
If the preservative is missing or is insufficient, or had not been properly mixed with the blood, that fermentation process will not be slowed.
Furthermore, if the blood is not going to be tested within a day, then it must be properly refrigerated or frozen in order to slow down the natural alcohol producing fermentation process. If it is not refrigerated or frozen, then the fermentation process will continue to increase the blood’s alcohol content. The results are no longer reliable.
The failure to follow the scientifically sound and self-imposed procedures will produce inaccurately higher BAC results, and those inaccurately high results must not be imputed to the driver. Otherwise, the driver, who may be innocent of criminal charges, can be wrongly convicted of DWI/drunk driving or other alcohol-related offenses.