Determining Blood Alcohol Content in DC DUI Cases

There are three ways that police can determine a person’s blood alcohol content during a DUI investigation: breath, urine, and blood samples. If asked to take any of these, it is important you consult with a DUI attorney familiar with these types of tests and how they are used to determine your blood alcohol content. To learn more or discuss the impact of testing, call and schedule a consultation today.

Breath Testing

The first and most common method that police use to determine levels of alcohol consumption is to take a breath sample using a breathalyzer machine. If a person has been arrested on suspicion of driving under the influence, they would be taken to the nearest police district station for booking and processing. That person would be asked to submit to a breath test pursuant to D.C.’s implied consent rules. Implied consent means that by driving in the District of Columbia, a person has implicitly given consent to provide a sample of their breath or urine for the purpose of determining their level of alcohol consumption or impairment. Breath testing is the easiest and most common way to determine BAC.

Every police district should have a breathalyzer machine. The machine uses a sample of that person’s breath to calculate a ratio of the amount of alcohol that person has in their bloodstream. The amount of alcohol a person has in their breath has a direct correlation to the amount of alcohol that person has in their blood, and the amount of alcohol a person has in their blood determines that person’s level of impairment. A low percentage of blood alcohol would produce a┬árelatively low level of impairment, but the higher the blood alcohol computation, the higher the impairment.

Urine Samples

The other common way police determine a person’s impairment is by collecting a sample of that person’s urine. That would also be done at the police station whereby the police would provide a person with a cup and ask for a urine sample. Urine testing is more common in situations where a person either does not have the ability to provide a sufficient breath sample or the police suspect that the person might be under the influence of drugs.

Breath testing can only detect the amount of alcohol in a person’s system but cannot detect other types of intoxicants, such as marijuana, other drugs, or medication. If the police suspect that a person might be under the influence of something besides alcohol, it is likely that the police would ask a person to submit a urine sample.

Blood Samples

In rare situations, a person may be required to submit a blood sample. Blood samples cannot be drawn at police district stations. Only a qualified medical professional, such as a nurse, has the ability to be able to take a blood sample from a person.

In some situations, the police might be permitted to forcibly take a blood sample from a person, but that situation is only when the person gets into a traffic accident that results in some level of serious injury. In those situations, the person does not have the ability to refuse a blood sample. The police would have to take the person to a hospital and have a professional take a sample of their blood.


All of these types of alcohol measuring systems take their measurement as a proportion. They will not determine a total amount of alcohol in a person’s system; they will determine a proportion of the amount of alcohol per volume of blood to allow for a standardized system that could be used for many different types of people.

The D.C. laws do not differentiate between blood alcohol content depending on factors such as a person’s height, weight, age, or any other factors.

As an example, on average, a woman would have to consume less alcohol than a man to get to the same blood alcohol content level. However, the laws do not differentiate between the two, so it is not an argument at trial that a person is not legally impaired at a certain level based on certain physiological factors. Numbers are the same for everyone.

A person who is heavier may be able to drink more alcohol to get to 0.08 compared to a person who is a lower weight. If a person has eaten and has food in their stomach, that food could absorb a portion of the alcohol that a person drinks, resulting in a lower blood alcohol concentration compared to a person who has not eaten food.

All of these factors might affect the proportion of alcohol to blood volume. A DUI attorney is knowledgeable about the factors in determining levels of alcohol consumption and can help you navigate issues as they arise.