In most states when you operate a motor vehicle on the highways of that state or accept a driver’s license in that state, you impliedly consent to a test of your blood, breath or urine for the presence of alcohol or drugs. Although the Fourth Amendment to the U.S. Constitution prohibits the taking of samples of your bodily fluids or breath without a search warrant, drunk driving is an exception. By operating on a state’s highways or roads, you agree to these tests. States differ on which of these tests you must take and what happens if you refuse to take the test.

In most states you have the right to talk to a lawyer about these choices, and you should avail yourself of that right if you can. Be aware, however, that in some states the refusal to submit to a blood, breath or urine test may create an inference of guilt.

The prohibited levels of alcohol in the blood or breath vary depending on the jurisdiction, ranging from 0.08 percent to 0.10 percent. For those who possess a commercial driver’s license and drive a vehicle over 20,000 gross vehicle weight it is 0.04 percent. For those under 21 years of age it is 0.00 percent.

Blood tests
The adverse effect of alcohol on a person’s ability to function takes place not when the alcohol is ingested, but when the alcohol enters the blood system and reaches the brain. It takes alcohol between 30 and 90 minutes to completely enter the blood system after consumption. The alcohol in your bloodstream is your blood alcohol concentration (BAC).

Blood tests are the most reliable type of alcohol tests. The U.S. Supreme Court has determined that blood tests to determine the level of blood alcohol are an exception to the Fourth Amendment search warrant requirement because time is of the essence–alcohol rapidly leaves the blood stream. Current cases are challenging this exception, however.

Breath tests
Though not as accurate as blood testing, breath testing is the most common method of testing alcohol level, and a number of breath testing devices exist. All breath test devices test breath alcohol (BrAC), the alcohol that is contained in the breath. This can lead to misleading results. Recently consumed alcohol and even alcohol used as a mouthwash will give an inordinately and falsely high reading. For that reason, an arresting officer is required, in most jurisdictions, to wait 20 minutes before administering a breath test. The officer must keep the driver under observation during that time to ensure that nothing is ingested that could interfere with the test result reliability.

BrAC is much different from BAC, in that BrAC is the alcohol that is present in the mouth, whereas BAC is the alcohol in the blood affecting a driver’s ability to function. Some states prohibit operating a motor vehicle or vessel with a BAC above a stated level. Other states prohibit operating a motor vehicle with a BrAC above a stated level.

Types of breath tests
It is important that your lawyer be familiar with the scientific basis of the particular breath test unit used in your case. Some breath test devices have inherent errors. Others are subject to outside influences that may adversely affect your reading, such as “radio frequency interference.” In addition, breath test devices using chemicals (wet machines) may be subject to error due to improper chemical concentrations.

Other breath tests include preliminary breath tests, hand-held units that police officers may or may not ask you to take. They are much less reliable than the standard breathalyzer-type tests. State laws differ on whether or not they may be admitted into evidence. In some states they may only be used to establish probable cause to arrest you.

Urine tests
Urine tests are the least reliable of all BAC tests but are still used and available in some states at the option of the arrested person. A recent U.S. Supreme Court decision may impact the ability of the prosecution to use urine tests. Discuss this issue with your lawyer.

Other tests
Balance tests, referred to as psychomotor tests or “Romberg” tests, are routinely used by police officers. They include “walk the line,” “pick up the coin,” “finger to nose” and “alphabet” tests.

A controversial test is the horizontal gaze nystagmus test (HGN). A police officer will hold a fixed object such as a pencil in front of a subject and direct the subject to follow it with his or her eyes without moving his or her head. The officer will then slowly move the object to the right and left and observe the point and rate at which the eyeballs “flutter.” It is said that this test can estimate accurately blood alcohol levels. Factors other than alcohol, however, can affect the test. State laws differ on reliability of HGN.

Expert witnesses
Generally, a witness in court must be a percipient witness, that is, someone who has perceived–seen, touched, smelled, heard or tasted–something. There is an exception to this rule. When something is beyond the ability of the ordinary person to be able to understand, an expert witness may be presented to explain these complicated things. Alcohol and its effect on the body can be the subject of expert testimony. Expert testimony is often very useful, albeit costly, in alcohol-related offenses. Medical doctors, pharmacologists, toxicologists, criminologists and others are useful witnesses.