General: Blood alcohol content (BAC) can be measured at a hospital after a blood or urine test. However, BAC is more commonly measured with a Breathalyzer® test during law enforcement investigations. If it is suspected that an individual is under the influence of alcohol, a law enforcement official can indirectly measure BAC by measuring the alcohol concentration in the person's breath. A BAC test can only be performed if an individual consents to a test. However, refusing a BAC test often has legal consequences and may result in a suspended driver's license.
Many factors can influence an individual's BAC. For instance, cough medicines or herbal supplements that contain alcohol (e.g. kava or ginseng) may increase an individual's BAC. Also, babies, pregnant women, diabetics, and people who exercise, diet, or have physical trauma can have higher amounts of acetone in their bodies. Acetone may be falsely detected as alcohol by some Breathalyzer® tests.
Measuring BAC: There are three ways that BAC can be measured. It can be measured as a percentage of body mass per volume, by body mass per volume of blood, or a combination.
For instance, if an individual's BAC is 0.10%, it means that there is one gram of alcohol per 1,000 grams or 1,000 milliliters of blood.
Mathematical estimate: BAC can be mathematically estimated, although this method is not as accurate as a Breathalyzer® test. Therefore, this technique is rarely used to measure an individual's BAC. However, it may help predict how an individual's BAC will change as the body continues to absorb and metabolize more alcohol.
The U.S. Department of Transportation uses the following formula to calculate an individual's BAC. A person's BAC = [(The number of drinks consumed) x (ounces of alcohol consumed) x (23.36 grams of alcohol/oz.) x (0.806 ml water/ml blood)] / (body weight in pounds/2.2046 lb/kg) x (total body water volume) x (1,000g/kg)] x 100 - (time). It is important to note that males and females have different amounts of water in their bodies. In adult males, about 58% of the body weight is water. In females, about 49% of the body weight is water.
Breathalyzer® test: A Breathalyzer® test does not directly measure the BAC. Instead, these tests estimate the BAC by measuring the amount of alcohol in an individual's breath.
Once alcohol is absorbed into the bloodstream, the liver breaks down (metabolizes) the alcohol. Once alcohol has been completely metabolized, the concentration of alcohol is equal throughout the body, including in the breath. Breathalyzer® tests can only detect that alcohol that has already been metabolized by the body. In other words, if the body is still absorbing alcohol, a Breathalyzer® test may detect a lower BAC than the individual actually has.
During the test, the individual blows into a plastic tube that is connected to a BAC monitor. The Breathalyzer® then provides an estimate of the individual's BAC. Eating breath mints or gum will not affect the results of a Breathalyzer® test. This is because the test does not measure the smell of the breath but the alcohol content of the breath.
Law enforcement officials commonly use Breathalyzer® tests to determine if an individual is under the influence of alcohol. Individuals can also purchase disposable Breathalyzer® tests at their local pharmacies and drug stores.
Standardized field sobriety test: Not all police departments use Breathalyzer® tests in the field. In many cases, police officers will use the standardized field sobriety test if it is suspected that someone is driving under the influence. The standardized field sobriety test is a series of three tests that are used to evaluate an individual's level of impairment.
First, the police officer looks at the individual's eyes for an increase in the normal, involuntary jerking of the eye that occurs when the eyes move to the side, called horizontal gaze nystagmus. When an individual is impaired by alcohol, nystagmus is exaggerated.
The police officer will also evaluate the individual's ability to walk and turn. During this part of the test, the individual is asked to take nine steps, heel-to-toe, along a straight line. Then the individual must turn around and repeat the exercise in the opposite direction. If the individual loses his/her balance, starts before the instructions are finished, stops walking to regain balance, steps off the line, uses the arms to balance, does not touch heel-to-toe, makes an improper turn, or takes an incorrect number of steps, it indicates that the individual may be impaired. Evidence has shown that 79% of individuals who exhibit two or more of these indicators have a BAC of 0.8 or higher.
The next test is commonly called the one-leg stand test. The individual is asked to stand with one foot about six inches above the ground and count out loud until told to put the foot down. The police officer times the person for 30 seconds. During this exercise, the officer looks for four specific signs of impairment: using the arms to balance, hopping to maintain balance, swaying while trying to balance, and putting the foot down. Evidence suggests that 83% of individuals who exhibit two or more of these indictors have a BAC level of 0.08 or higher.
If the police officer concludes that the individual appears to be under the influence, he/she is arrested, and a Breathalyzer® test is performed at the police station. Some states allow the person to request a blood or urine test to confirm the results.
Blood test: A blood alcohol test is the most accurate way to measure an individual's BAC. Because alcohol is quickly absorbed into the bloodstream, this test can be performed just minutes after consuming an alcoholic beverage. However, this test is more expensive and invasive, and it cannot be performed on site. Therefore, it is performed less often than a Breathalyzer® test.
Urine test: A urine test may also be performed. This test assumes that there are 1.3 parts of alcohol in the urine for every one part of alcohol in the blood. However, this ratio can vary significantly among individuals. Also, it may take up to two hours after alcohol consumption for alcohol to be detectable in the blood.