Everything You Ever Wanted to Know about Tests for DUI

Breath, blood and urine–law enforcement officials use a variety of tests to determine if they should charge a driver with DUI. Here’s a look at the science behind the various types of tests and what you should know about the pros and cons of each in the event that police arrest you for driving under the influence.


School Testing

Measuring your breath

Although the word “breathalyzer” is actually a trade name for a specific type of breath testing device, people now use it generically to describe all types of alcohol breath tests. There are differences in the way each of these tests work, however.

No matter what the test, it’s important to remember that breathalyzers don’t actually measure your blood alcohol content; instead, they provide an estimate of your BAC based on the amount of alcohol that’s in your exhaled breath sample.

Here’s a look at the three different types of breath analyzers used today:

Fuel cell breathalyzers contain your exhaled breath in a chamber where an electrochemical process oxidizes the alcohol. The oxidation creates an electric current; the greater the strength of the current, the more alcohol you have in your blood. Fuel cell breathalyzers are the most sensitive and the most accurate type of test, but they are also the most expensive. Police generally used them at their stations rather than at roadside checkpoints or when they initially pull you over.

Infrared breathalyzers use infrared spectroscopy technology to measure the BAC in your breath. The piece of equipment used—the spectrophotometer–can identify ethanol molecules by the way that they absorb infrared light. The level of ethanol in the breath sample helps police determine whether or not you should face DUI charges.

Semiconductor breathalyzers work by measuring the resistance of a semiconductor to an electric current passing through it. Since alcohol changes the resistance of a semiconductor, the equipment measures the change from a baseline. The more alcohol, the greater the change. These types of test are usually the most inexpensive to administer, but also the least accurate. Police often use them as preliminary screening tests.

No matter what type of breathalyzers police use, there are several factors that can affect their reliability:

• Mouthwashes, breath freshers and similar products can raise the BAC reading, since they contain alcohol.
• All these devices need to be calibrated regularly by qualified personnel. Lack of calibration can invalidate test results.
• The presence of fumes from paint, varnish and other chemicals in the area where the test is administered can skew the results.
• The temperature of your breath can impact BAC readings with breathalyzers.
• Diabetics and people with hypoglycemia may have more acetone in their breath, while people on low-carb diets may produce more ketone. Both of these substances can lead to false positive readings on a breathalyzer test.
• You’re supposed to take a deep breath before blowing into a breathalyzer to test the air from the lungs. If you take a shallow breath, the results may be invalid.

Law enforcement officials are supposed to follow certain procedures when administering a breathalyzer:

• The police officer must have taken the proper training in administering the breathalyzer test using that specific breathalyzer model.
• The officer must observe the person for 15 minutes before administering the test. During that time, the officer must ensure that the person does not eat, drink, vomit or regurgitate.
• Properly trained technicians must calibrate the breathalyzer every 10 days or 150 uses, whichever comes first.

Failure to follow these procedures can invalidate the breathalyzer test results.

Urine tests

A urine test is usually the last choice of law enforcement officials when they are trying to determine whether or not to arrest a person for DUI. That’s because it is (in general) less accurate than blood tests or breathalyzer tests in detecting the presence of alcohol.

The California legislature passed a law in 1999 that requires police officers to use either a breathalyzer test or a blood test when trying to determine if someone is driving under the influence. Police officers may use urine tests only when they don’t have the ability to administer a breathalyzer test or a blood test. (That could happen if they don’t have a properly calibrated breathalyzer and/or if there’s no trained technician available to take a blood sample.)

Urine tests do have one advantage over breathalyzers; they can indicate the presence of other drugs besides alcohol. A urine test can detect amphetamines, methamphetamines, barbiturates, marijuana, PCP, cocaine, methadone and narcotics (opioids).

Urine tests measure the amount of alcohol (or other substance) in your urine. But that’s not the same thing as measuring the amount of alcohol you have in your bloodstream. There are formulas that correlate the amount of alcohol in your urine to the amount of alcohol in your blood stream, but those results aren’t always accurate. The readings of a urine test can vary according to how long it’s been since you emptied your bladder, the amount of non-alcoholic drinks you consumed and how long ago you drank. Alcohol and other drugs can remain in your urine long after they’ve disappeared from your bloodstream.

Mistakes at the lab in collecting or processing your urine sample could also result in a false positive for alcohol or some other substance.

Drivers who must take a urine test for DUI:

• Are entitled to some privacy while giving the sample under California law. (But technicians may take steps to ensure the accuracy of the sample.)
• Should completely empty their bladder first and then wait 15-20 minutes before giving a new sample for analysis.

Blood tests

Blood tests measure the amount of alcohol (in the form of ethanol) in your body. They’re generally regarded as the most accurate type of tests for measuring BAC. But their results are not always right, because there are several factors that can influence the blood test results.

• When drawing a blood sample, a technician will generally swab the area from which he/she is going to draw blood with alcohol to ensure that it is clean. But alcohol from that swab can (and sometimes does) contaminate the sample and result in a higher BAC reading.
• Faulty seals on blood sample storage bottles can also result in contamination of the samples and inaccurate readings.
• Blood samples that sit too long or are stored at the wrong temperature before processing can provide inaccurate BAC readings. The combination of yeast, sugar and bacteria that are normally found in your blood can result in the formation of more alcohol when samples aren’t stored under the right conditions.
• As with breathalyzers, some substances like cough syrup and herbal medicines can get into your blood stream, boosting your blood alcohol content so that the reading isn’t accurate.
• Alcohol content in the blood continues to rise for 30 to 60 minutes after you’ve stopped drinking. It usually takes at least that long for technicians to administer a blood alcohol test. That means you could have been driving with a BAC of less than 0.08 –under the legal limit—but the BAC rose beyond that limit by the time they actually took your blood sample.

In California, failure to comply with a police request to submit to a breath test or a blood test can result in you losing your license for a year (for a first offense) and for two years for a second offense or with a previous DUI conviction.

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