Your level of impairment from alcohol depends not only on how much you have had to drink, but on a variety of other factors, including how much you have had to eat, your weight, your gender, how much rest you have had, and your mental state. While the legal limit is .08% BAC (blood alcohol content), impairment can begin at a level of .04% BAC or even lower. Everyone responds differently to alcohol; some people may be reasonably safe at a lower BAC level while others may not.
A precaution when using any breathalyzer is to consider what your personal limit may be as well. Intoxication can occur almost immediately and also progress over several hours of drinking.
For these reasons the BAC results from the BreathKey or from any breathalyzer cannot be used on their own to determine whether or not you are impaired or if you are safe to drive. Only you can make that determination.
In addition, no breathalyzer can assure that you will not get arrested for driving under the influence of alcohol. You can be arrested for DUI with a BAC that is less than the legal limit. Impairment can occur at any point during drinking depending on the individual.
Also note that you could have a higher BAC reading even a few minutes after using any breathalyzer, depending on how much you have had to drink and on when you stopped drinking. In addition, alcohol residue in your mouth can read at a higher concentration for up to twenty minutes after your last drink.
However, when used properly the BreathKey will give you an accurate measurement of your blood alcohol level. This can be very useful information when considered together with all of the other factors that can affect whether or not you are impaired.
Remember that it is always safest when you do not drink and drive!