The common perception is that you need to have a blood alcohol concentration of 0.08 to be charged with a DUI. This is not always true. You can be charged if you blow into a breathalyzer and your BAC is over the limit. However, just coming in under the limit does not mean that you'll avoid charges.
For example, there are zero tolerance laws for those who are under 21. You can get your driver's license at 16, but you can't drink legally for five more years. That's a huge window for potential underage drinking and driving.
If you are under 21, it no longer matters what your BAC is and the 0.08 limit doesn't make a difference. If police think a 17-year-old driver is under the influence and then he or she blows a 0.03, for example, that's still enough to net DUI charges.
The reasoning, of course, is that anyone under 21 should always have a BAC of 0.00. Driving even with a low level of alcohol means that the law has been broken.
This is very important for young drivers to remember. While they may assume that it's not a big deal to have a beer at a friend's house and then drive home, believing they won't be impaired by a single, low-alcohol drink, they still open the door to legal DUI charges.
If this happens to you, while you may feel outraged by such a high standard and feel that you weren't actually drunk, it's important to know how the zero tolerance laws apply. You also need to know your legal options, as a DUI can have a drastic impact on your future.
Source: FindLaw, "BAC Below 0.08%? It Can Still Be a DUI," Aditi Mukherji, accessed May 03, 2017