DUI and DWI Tests for Alcohol or Drugs: Are They Accurate?

Learn about the reliability of breathalyzer, blood, and urine tests for DUIs and DWIs.

By , Attorney · University of Santa Clara Law School

Police officers who suspect a driver is under the influence of alcohol or drugs will typically administer various tests designed to gauge the impairment and the amount of alcohol or drugs in the driver's body. The officer might start with field sobriety tests (FSTs) and then ask the driver to submit to a blood, breath, or urine test.

Field Sobriety Tests

With FSTs, the officer tests the driver's balance, coordination, and cognitive abilities. FSTs typically involve tasks like balancing on one leg, walking a straight line, or following a pen or other object with your eyes.

FSTs are optional—meaning, there aren't any legal consequences for refusing to participate.

Implied Consent Laws and DUI Chemical Tests

If the police officer continues to suspect the person is under the influence, he or she might arrest the person and move on to more scientific tests. Every state has "implied consent" laws, which say that anyone lawfully arrested for driving under the influence must agree to take a chemical test to determine the amount of alcohol and drugs in his or her system. In most cases, the test will be of the driver's blood or breath. However, occasionally, the officer will ask the driver to give a urine sample.

The results of these tests can be vitally important in determining whether a driver is charged or convicted of DUI. (A "per se" DUI charge is based on the concentration of drugs or alcohol the driver has in his or her system.) Prosecutors often rely heavily on chemical test results in proving a DUI charge at trial.

DUI Blood Tests

Generally, blood tests are the most accurate means of measuring the amount of alcohol or drugs in a person's body. But still, a good defense lawyer can often find ways of attacking blood test results.

In some cases, blood samples that aren't properly preserved and sit around a long time before being analyzed will coagulate or decompose, which can lead to a false high reading. In other cases, the defense might be able to argue that while the person's blood alcohol concentration (BAC) was high when measured, it was below the legal limit while the person was actually driving—the "rising-blood-alcohol defense."

Also, most laboratories that analyze blood (or urine) samples run numerous samples every day. The different parts of the analyses are carried out in different bottles and beakers. This can lead to errors in some samples, particularly if the laboratory doesn't follow proper record-keeping and organizational procedures.

Breathalyzer Testing

An analysis of breath gas, using a "breathalyzer" (which the police can do immediately, on the road) gives only an indirectly determined value for BAC. It's based on how much alcohol is in some portion of exhaled air, not how much alcohol is in the blood.

To calculate the content of blood alcohol from that of exhaled air, the alcohol content in the air is multiplied by a "partition coefficient" or "partition ratio." Breathalyzers use the same partition ratio for everyone. But in reality, the value varies for the same person over time and depends on body temperature and even respiration rate. For these reasons, the calculated blood alcohol level isn't always very accurate. However, many states—including California, Colorado, and Kansas—have side-stepped this problem by making it illegal to drive with a certain concentration of breath alcohol. In other words, these states have eliminated the need to do the partition ratio calculation.

Alcohol-containing substances in the mouth can also produce falsely high breathalyzer readings since the amount of alcohol vapor they give off is typically much greater than any amount exhaled from the lungs. Stomach fluid vomited or regurgitated within 20 minutes of taking the test, toothache medicines, mouthwashes, and breath fresheners can also affect breath test results. Even a burp just before or while blowing into the breathalyzer tube may cause a falsely high reading. To guard against these types of contamination, the person administering the test is supposed to watch the subject for at least 20 minutes prior to taking the test.

There's also the possibility of a malfunction in the breath-testing devices. To assure accuracy, the device must be frequently calibrated with air containing known amounts of alcohol. The police department's records should indicate how often the device has been calibrated, serviced, and used.

DUI Urine Tests

Urine tests are typically less accurate than the blood or breath tests, which is one of the reasons they aren't often used. Urine samples are analyzed for alcohol in almost the same way as blood samples. The results are therefore also subject to some of the same laboratory errors. And in states that don't directly make it illegal to drive with a certain urine alcohol concentration, conversion from urine to blood alcohol concentration can lead to substantial inaccuracies (similar to the partition ratio issue with breath tests).

Talk to an Attorney

DUI chemical tests are not infallible. So, even if there are test results showing you were over the limit, it's best to talk to an experienced DUI attorney. A qualified DUI lawyer can look at the facts of your case and determine if you have any viable defenses.

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