Blood alcohol concentration (BAC) is a standard measurement used to describe how drunk a person may be. Different percentages of alcohol in the blood lead to different levels of mental and physical changes and risks. In the United States, a BAC of 0.08% is the legal limit, meaning one can no longer safely drive after this point. The percent is related to how many parts alcohol are in the blood; a BAC of 0.1% represents one part alcohol per 1,000 parts blood.
As the liver metabolizes about one standard drink per hour, one to two drinks raises the BAC a small amount: ranging from 0.01% to 0.05%, depending on individual factors like weight, gender, age, and potential prescription medications. Two to three drinks raise BAC to between 0.06% and 0.1%. Memory, judgment, reaction time, perception, and fine motor skills are affected at this point. Higher BACs lead to inappropriate social behaviors, loss of balance, slurred speech, memory loss, blackouts, low body temperature, trouble breathing, vomiting, and even sudden death.
Because of the dangers from high levels of alcohol in the blood, there are several scientific measurements that can measure BAC. The entire body is affected by alcohol, especially in large quantities, so there are several tests that are applied in different situations to determine how intoxicated someone may be.
Tests for Blood Alcohol Concentration
Romberg balance test: Often given to determine whether one is intoxicated or not, the Romberg balance test is based on the concept that three functions are necessary to remain standing still: vision, proprioception (knowing where limbs are in space), and vestibular function (equilibrium as provided by the inner ear). Drinking alcohol changes how the inner ear senses the body’s orientation in space and changes how the brain processes visual images. Clues listed by the Romberg test that may indicate drunkenness include:
- Amount and direction of body swaying
- Eyelid or body tremors
- Estimation of time passing
- Muscle tone
- Sounds or phrases during the test, including whether speech is slurred
- Ability to follow directions and how quickly
A police officer who pulls a person over for a suspected DUI may administer this field sobriety test. The officer will ask the suspect to stand still with their feet together; tilt their head back and close their eyes; and estimate how long it takes for 30 seconds to pass.
The Romberg test is not the only balance test, and it is not standardized, but it is one of the more popular.
Breath tests to determine BAC rely on the body’s metabolism of alcohol to assess the level of alcohol in the blood. About 92-98 percent of an alcoholic beverage moves through the digestive system to be broken down by the liver, so 2-8 percent is lost through other processes, including breathing. Some alcohol will enter the bloodstream through the stomach walls before the liver breaks it down; this alcohol-tinged blood will then circulate throughout the body, including the lungs, where some alcohol is exhaled. The liver processes about one serving of alcohol per hour, so if a person drinks more than one standard drink in an hour, more of that alcohol will enter the bloodstream at this point, circulating through the heart and lungs so more alcohol will be exhaled. The amount of alcohol collected in the lungs correlates to the amount of undigested alcohol in the bloodstream at a ratio of about 2,100 breath to 1 blood. So, for example, 2,100 milliliters of alcohol in an exhale correlates to about 1 mL in the blood.
Breath tests can detect alcohol consumption about 24 hours after the person drinks, although they do not measure how high the individual’s BAC may have been in the past. Any remaining alcohol in the lungs will be measured in a chemical reaction in the device, which detects whether alcohol is present in the exhale by changing color from reddish-orange to green.
Alcohol does not remain in the blood for long, and a blood test can only detect alcohol in the blood stream for up to 12 hours after the last drink. It is the preferred test to determine whether a person is drunk at the specific time their blood was drawn. This is used during DUI arrests or in rehabilitation centers treating people with chronic alcohol use disorder. Other tests, especially the hair test, can show that the person drank alcohol but not specifically when or how drunk they got.
Because the liver is the main alcohol metabolizer in the body, the digested toxins will filter out of the body, in part, through the kidneys. Urine tests can detect metabolized alcohol between 12 and 48 hours after the person drinks, depending on how much alcohol they drink.
Very few occasions require a hair test to determine whether a person drank alcohol, but alcohol consumption can be detected for up to 90 days in hair tests.