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.

If a police officer suspects that a driver is under the influence of alcohol or drugs (DUI or DWI), the officer will administer various test, starting with a field sobriety test, and then possibly moving on to a blood, breath, or urine test. 

In a "field sobriety test," the officer tests the driver's test balance, coordination, and cognitive abilities. The person may have to stand on one leg, walk in a straight line a certain number of steps, then turn and walk back. The person will also have to watch as the officer slowly moves a pen or small flashlight horizontally back and forth (the "horizontal gaze nystagmus test") so the officer can test for jerky eye movements, which indicate intoxication.

If the police officer continues to suspect that the person is under the influence, he or she may arrest the person, and/or administer more scientific tests. In fact, almost every state has a so-called "implied consent" law, which says that anyone lawfully arrested for driving under the influence (DUI) or driving while intoxicated (DWI) must give either a blood, breath, or urine sample (with a choice among the three) when taken to the jail or police station. Refusal to take a test will result in suspension of the person's driver's license for anywhere between three to 12 months, depending on the state even if the court decides the defendant is not guilty on the DUI or DWI charge.

The results of these tests may be vitally important in determining whether a driver is charged or convicted with DUI or DWI, so it's worth asking, how accurate are the various tests? We'll examine them one by one below.

Blood Tests

Other than directly measuring the drug or alcohol content of someone's brain cells (which would be dangerous), the most accurate available test of alcohol or drugs in the body is the blood sample test.

But a good defense lawyer may discover problems with the testing on cross-examination, based on the chemistry involved in analyzing the blood. 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.

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

These are only remote possibilities, however. More than likely, the analysis of a blood sample will be correct.

Breath Tests

An analysis of breath gas, using a "breathalyzer" (which the police can do immediately, on the road) gives only an indirectly determined value for blood alcohol level (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 content of blood alcohol from that of exhaled air, the content of alcohol in the air is normally multiplied by the number 2,100. This number, known as a "partition coefficient" or "partition ratio," is used because the lung air exhaled by an "average" person usually has 1/2100th the amount of alcohol of an equal volume of blood.

Using this average figure amounts to little more than scientific guesswork. In fact, 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 (printed or displayed on a readout on the machine) may be wrong.

Alcohol-containing substances in the mouth can also produce falsely high readings, since the amount of alcohol vapor they give off is much greater than any amount exhaled from the lungs. This includes stomach fluid vomited or regurgitated up within 20 minutes of taking the test, some toothache medicines, mouthwashes, and breath fresheners. Even a burp just before or while blowing into the breathalyzer tube may cause a falsely high reading. For this reason, the person administering the test is supposed to watch the subject for at least 20 minutes prior to taking the test to make sure he or she doesn't burp, belch, regurgitate, vomit, or put anything into the mouth.

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.

Finally, because breath gas analysis (the breathalyzer test) is often inaccurate, the driver may be asked to take the tests two, or even three, times to produce a consistent result. Failure to give the police all the breath samples they want will result in the driver's license being suspended.

Urine Tests

The urine test is less accurate than the blood or breath tests, which is why the trend is now to allow use of the test only when the blood and breath tests are unavailable.

One reason that result may not be accurate is because they have to be "correlated" to an equivalent blood alcohol level. An average 1.33:1 ratio of urine alcohol to blood alcohol is generally used. And studies have shown that some people have alcohol levels only 40% as high in their urine as in their blood, while others have twice the alcohol content in their urine as in their blood.

Also, a specimen of bladder urine represents only a composite of a continuously changing blood alcohol content. The pool of urine in the bladder at any given time is an accumulation of secreted urine since the last emptying of the bladder. It therefore tells much less about a person's blood alcohol at a particular moment than does a blood sample.

This can work for or against the DUI or DWI defendant. If the person had a lot to drink several hours beforehand and hasn't urinated since that time, the urine test result may be misleadingly high. If the drinking was relatively recent, though, say within an hour of giving the sample, and especially if the person had had any nonalcoholic liquids before that, the urine test would give a misleadingly low result.

Because of this, the only way to properly test a person's urine is to have him or her void the bladder and then produce a second sample. The police know this, and will insist that the driver produce a second sample 20 minutes after the first. If that's impossible, the driver will have to take the more accurate blood test or the breath test.

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.

In sum, most chemical analyses of a driver's breath, blood, or urine will give an accurate indication of actual blood alcohol level (BAC). However, the tests are not infallible. An experienced criminal defense attorney may be able to cast enough doubt on borderline test results to convince a jury that a driver might not be guilty. For more information, see Nolo's article Dealing with a DUI or DWI Charge. See Nolo's Lawyer Directory, which provides comprehensive attorney profiles and information to help you select the right attorney.

For More Information

To learn more about your state's current traffic laws and court procedures, see Beat Your Ticket, by David Brown (Nolo).