The result of a blood alcohol content or breathalyzer test is a snapshot of the blood alcohol concentration (BAC) at the time the test was administered. When a person begins drinking alcoholic beverages, the alcohol passes from the stomach and intestines into the blood, a process referred to as absorption. When the alcohol reaches the brain and nervous system, the characteristic signs of intoxication first appear. The period of time it takes for alcohol to be absorbed depends on many factors, including the presence of food in the stomach, the person's gender, the person's weight, and other factors including age, mental state, drinking pattern, the amount consumed and the time period of consumption. As these variables indicate, the time it takes for blood-alcohol concentration to peak after ingestion of alcohol varies greatly from one person to another. After it peaks, the BAC falls at a slow but steady rate. Because of this rise and fall (known as the blood-alcohol curve), a defendant's blood-alcohol concentration at the time of the breathalyzer test could either be higher or lower than at the time he or she was driving. Stated differently, it is entirely possible to have a breathalyzer reading at or above the legal limit when the driver’s BAC while driving was below the legal limit.

The crime of drunk driving, of course, requires that a person operate a motor vehicle, on a public way or right of access, while under the influence of alcohol. Thus, it is the person’s blood alcohol content (BAC) at the time of operation which is important.If a long time passes between the breath test and the driving, it can be claimed that the BAC found at the time of testing was irrelevant because it did not prove what the BAC was at the time of operation. Also, "breath alcohol" technically has no relevance in a Mass. DUI case, becasue only alcohol in the brain affects a person's ability to drive.

For someone's blood alcohol content (BAC) to be admissible in a Massachusetts DUI trial, the prosecution must prove that the breathalyzer used to obtain the BAC reading was working properly and can be trusted to produce accurate readings. The breathalyzer test must have been conducted by a certiifed Massachusetts breathalyzer operator using a certified and properly maintained breathalzyer.

With respect to alcohol readings derived from blood tests, certain admissibility conditons must be met and several legal rules apply. For example, absent a DWI defendant's consent, or a valid search warrant, police officers or other government agents cannot ask that a doctor, nurse, or other health care provider draw blood from a drunk driving suspect for use in a DUI prosecution. However, blood drawn by medical personnel, for medical treatment, not at the direction of the police, may be admissible in th patient's drunk driving trial.

Good Massachusetts DUI Attorneys such as Attorney Paul B. Watkins and Attorney George E. McCarthy, Jr. can win DUI cases even when there is BAC evidence.