Scroll Top

How Do You Calculate BAC?

Criminal Defense Attorney Explains DUI Blood Test Results

Driving under the influence is the number one most litigated criminal law trial in America. For over 20 years, more than 1.5 million drunk driving arrests (on average) were made in the United States. Therefore, a DUI sobriety test exists in most of these driving while intoxicated cases.

How do I calculate BAC? Most accused citizens with a criminal case that has DUI blood test results (or breath test results) never ask their DWI defense attorney how to measure blood alcohol level, or how do you calculate BAC?

Although most DWI-DUI-OWI cases involve a breathalyzer test, many of the more serious criminal law prosecutions have blood tests for the DUI sobriety test. Several reasons exist for the use of a blood alcohol test. The 5 most common fact patterns for having a blood test in an impaired driving criminal case are:


1. In states like California that allow a DWI – DUI suspect to opt for a voluntary blood test, and not take the breath alcohol test mandated by the implied consent law, an arrested drunk driver may elect a blood level analysis over a breathalyzer test, so that independent testing can be obtained. Several states, like South Carolina and California, have DUI laws that default to a breath test when only DUI alcohol is suspected.

2. Where drugs are the only impairing substance suspected (no smell of alcohol or admission of consuming alcohol), a breathalyzer cannot detect or quantify any drugs, except ethanol (alcohol). Thus, a blood test or urine test would be the police officer’s choice of sobriety test for people changed with drugged driving. After the case of Missouri vs. McNeely in 2013, many arresting officers are seeking a search warrant from a judge before drawing blood for a DUI sobriety test for drugs. Your criminal defense attorney may challenge the legitimacy of the DUI blood test results.

3. Where a DUI sobriety test of breath is sought by an arresting officer, and this type of implied consent test is refused by the drunk driving suspect, most states have statutory provisions to enable the arresting officer to seek an electronic warrant or a regular, in-person warrant from a magistrate judge or other judge, and upon a proper showing (such as an affidavit of the facts supporting the drunken driving arrest) will authorize issuance of a blood test for alcohol, drugs or both.

4. In accident cases, where the DWI suspect is unconscious or otherwise incapable of answering the implied consent questions of whether he or she will agree to a blood sobriety test, almost all states have passed drunk driving laws that allow “consent” to be given, when the suspected intoxicated driver is incapable of consenting. So, consent is “implied”, under the provisions of informed consent or implied consent law in the state.

5. In serious vehicle crashes involving a death of one or more people in a drunk driving wreck (with other vehicles or with a pedestrian, or a serious boding injury to another person), the existence of reasonable suspicion of one or more drivers being impaired can be the basis of a law enforcement officer utilizing a statutory provision or (more likely) a judicial search warrant to forcibly draw blood and/or collect urine, to determine the BAC level or the drug concentration within a DUI driver’s bloodstream.

With any felony DUI or other situation that involves a blood test, for alcohol or for drugs, you need to have a DUI defense attorney at your side. If you cannot afford to hire a private criminal defense lawyer, you should seek to qualify for an appointed criminal attorney who is skilled in impaired driving cases, and the scientific evidence associated with drunken driving cases or drugged driving cases. Your criminal lawyer must know how to calculate blood alcohol level as done by the state crime laboratory. Furthermore, your impaired driving lawyer must know all of the rules about DUI drugs cases or DUI felony charges involving prescription drugs, relating to therapeutic dose of any lawfully prescribed medications. This type of driving while impaired case typically involves the DUI defense attorney locating an expert witness in pharmacology and the pharmacokinetics of drugs.

If you need an appointed lawyer, see your court administrator to apply for a public defender. If you need a private criminal defense lawyer, call 1-888-839-4384 to be connected to a private criminal attorney who can review your case and assist you in your criminal law case.