Blood Alcohol Testing in Drunk and Impaired Driving Cases


Drunk driving prosecutions often rely upon the measurement of a driver's blood alcohol level. Most drivers undergo breath tests, through which their blood alcohol levels are estimated based upon the analysis of their breath. In some states and circumstances, testing may also be performed upon a blood or urine sample obtained from the driver.

What Are Your Chemical Test Rights

If you are arrested for impaired driving and will be subject to blood alcohol testing, you must be informed of your chemical test rights before an officer conducts the test. The explanation of your rights will normally include:

  • A description of the consequences you will face if you refuse to take a chemical test; and

  • An explanation of your right to obtain a blood or urine sample so you can conduct your own test.

If the police fail to inform you of the consequences of your refusal to take a chemical test, you may be able to keep the test result from being admitted into evidence during the prosecution of your case, and you may be able to challenge a suspension of your driver's license that would otherwise result from your refusal.

How Is Your Blood Alcohol Level Measured By The Police

In most states you have some degree of choice as to how your blood alcohol will be measured.

  • For impaired driving involving the use of alcohol, you may be allowed to choose between a breath test and a blood test, and in some states you may be allowed to choose a urine test. Some states require the breath test, but allow you to demand an additional blood or urine test so that you can verify the breath test results.

  • If the police suspect intoxication from the use of medications or other drugs, you may be able to choose between a breath test and a blood test.

Breath tests are normally performed using a machine such as a Intoxilyzer or Datamaster that estimates blood alcohol through the infrared spectroscopy, electromechanical oxidation, or both.

  • Infrared Spectroscopy - As a suspect's breath passes through the machine, a beam of infrared light passes through the breath, and ethanol levels are calculated from the absorption of specific wavelengths of the infrared light by the suspect's breath.

  • Fuel Cell / Electromechanical Oxidation - Breath is drawn through a fuel cell that causes the oxidation of alcohol within the breath sample, creating an electrical current that may be used to measure alcohol levels.

Normally two tests are conducted, with a short interval between the tests, to help ensure accuracy. Once the suspect's breath has been analyzed, the machine prints out a ticket with the suspect's mechanically determined breath test results.

Can Chemical Tests Be Inaccurate?

The most accurate form of testing is normally the testing of the suspect's whole blood, followed by breath testing with a properly calibrated machine, with urine testing being the least accurate means of calculating blood alcohol levels at a specific moment in time.

For impaired driving charges that involve allegations of intoxication from the use of medications or other drugs, breath tests are not useful to measure intoxication. Urine and blood testing may show levels of medications and controlled substances, or the presence of chemicals that indicate their consumption, but may not be good evidence of the driver's level of intoxication.

Some people are tolerant of medications and drugs at levels that would leave others significantly impaired. Further, a test may show evidence of the use of a medication or controlled substance from days or even weeks before a traffic stop, but that evidence may nonetheless potentially be offered as evidence of intoxication at the time of the traffic stop.

Blood Testing Accuracy

A properly performed blood test is the most accurate means of calculating blood alcohol levels at a given point in time, as it involves no estimation -- it involves the direct calculation of ethanol levels from a sample of blood. Although lab testing errors can occur, they are rare. The most common reasons for inaccurate blood test results are:

  • Testing of Only the Blood Serum - When your blood is drawn, the sample that is produced is whole blood. Although police crime labs test whole blood, in a clinical setting the blood sample is likely to be processed before further testing. Clinical testing is often conducted on only the blood serum, the portion of the sample that is neither a blood cell nor a clotting factor, as opposed to whole blood. A test of only the blood serum may result in a blood alcohol test that is 25% - 33% higher than the result that would have been obtained from testing the whole blood.

  • Improper Procedure While Taking a Blood Sample - The manner in which a blood sample is taken may affect the level of alcohol in the sample. For example, the use of an alcohol swab to prepare the skin before a sample is drawn may introduce alcohol into the blood sample.

Breath Test Accuracy

Although breath testing machines are designed to give reasonably accurate estimates of a suspect's blood alcohol level, they do not directly test the suspect's blood. Some defense attorneys argue that certain breath testing machines are less consistent than others, or may give different readings based upon the force used by the suspect while breathing into the machine, even though the manufacturers claim that exhalation at a rate that will affect the test result will result in the machine declaring the result invalid.

Urine Test Accuracy

Urine tests are the least accurate measurement of blood alcohol at a specific point in time. Urine tests look for the presence of the metabolytes of alcohol, chemicals produced by the body when alcohol is metabolized. The figure that results from a urine test may thus be higher or lower than the actual blood level at the time of the test.

Verifying Your Blood Alcohol Level

If you want to be able to verify your sobriety, the best form of verification is a blood test. However, if you believe that you are over the legal limit and your state gives you the option, you may want to gamble on urine testing, as the delay between the traffic stop and the performance of the test could potentially result in a lower blood alcohol measurement.

What toTell The Police Before You Take A Breath Test

Before administering a breath test to calculate your blood alcohol levels, the police will observe you for a period of time, usually at least fifteen minutes, to ensure that you have not vomited or put anything in your mouth that might affect the test result. Before you take the test. tell the police about:

  • Medical Conditions and Medications - Certain medical conditions and medications may interfere with the accuracy of your breath test, raising the result. If you are diabetic, asthmatic, on ulcer medications, or on a number of other medications that could potentially result in an inaccurate breath test, you need to tell the police what medications you are taking and make sure that they write down the list.

  • Mouth Sores or Injuries - You need to tell the police if you have any sores, cuts, wounds, or infections in your mouth. If you are bleeding into your mouth, your breath test result will be increased, and the result of the test may be much higher than your actual blood alcohol level.

  • Recent Vomiting - If you have vomited, and even you taste just a little bit of vomit in your mouth, you must inform the police. If you still have alcohol in your stomach at the time you vomit, even a tiny amount of vomit may substantially increase your breath test result. Even burping or hiccupping may bring alcohol up from the stomach.

  • Dental Appliances - You should inform the police if you have any dental bridges or caps, or have other dental appliances in your mouth that may be trapping alcohol inside your mouth.

What Happens If You Refuse To Be Tested?

Under implied consent doctrine, when you obtain a driver's license, you agree that you will submit to testing for blood alcohol based when the police have proper cause to administer a test.

What Is Implied Consent

Implied consent is the principle that, by driving your car on the public roadways, in the event that you are arrested as an impaired driver you agree to take chemical tests, such as a blood, breath or urine test. If you refuse to take a test after you are arrested, even if you are not charged with drunk driving, or are acquitted, the state may add points to your driving record and administratively suspend your driver's license based upon your refusal.

Consequences of Refusing Alcohol Testing

If you refuse to take a mandatory test consequences include:

  • Driver's License Sanctions - Most often you will face administrative sanctions such as the addition of points to your driving record, and the suspension of your license, under your state's implied consent law

  • Search Warrant - The police may seek a search warrant that will allow them to take you to a hospital or clinic to have a blood sample drawn. Some states have trained officers to draw blood from suspects, right at the police station.

  • Criminal Charges - In some states the refusal to take an alcohol test is a criminal offense, separate from the drunk driving offense. Under that type of law, you can potentially be convicted for refusing to take an alcohol test even if you aren't convicted of the drunk driving charge.

Are There Times When You Should Refuse To Be Tested

Test refusal is a controversial issue, and whether it might make sense to refuse a test depends upon factors including:

  • The facts of the traffic incident;

  • Your prior record of drunk driving offenses; and

  • The laws of your state.

In some states, some attorneys will caution drivers with histories of drunk driving offenses that they may be better off refusing to take an alcohol test than by cooperating. The rationale is that the driver will be better able to defend himself against drunk driving charges if the police don't know his blood alcohol, and that the severe consequences of being convicted as a habitual drunk driver outweigh the mandatory license suspension associated with refusing to take an alcohol test. It should be remembered that if a breath test is refused, the police may apply for a search warrant to obtain a blood sample for testing, so that strategy may not be effective.

Test refusal is not a decision to be made lightly. It is so dependent upon state law and an individual's driving record that it is a decision best made after consulting with an attorney. If you make an ill-advised decision to refuse testing, you can do a lot of damage to your driving record and you may needlessly lose your license, .

Copyright © 2000 Aaron Larson, All rights reserved. No portion of this article may be reproduced without the express written permission of the copyright holder. If you use a quotation, excerpt or paraphrase of this article, except as otherwise authorized in writing by the author of the article you must cite this article as a source for your work and include a link back to the original article from any online materials that incorporate or are derived from the content of this article.

This article was last reviewed or amended on Aug 23, 2016.