Chemical Testing for Blood Alcohol Content

At the police station, the arrestee will be offered a chemical test of breath, blood or, much less frequently, urine. Breath test results are usually available immediately while urine and blood samples are sent to a lab for later analysis to determine the BAC or possible presence of drugs.

If the arrestee refuses to submit to chemical testing, they will usually be booked for driving under the influence; there will be no evidence for filing the second charge of driving with .08% or higher blood alcohol content. The refusal will typically carry increased penalties on the DUI charge; longer license suspension and/or an increased jail sentence, and the act of refusing may be admissible in court as evidence of "consciousness of guilt". In some states, refusal to submit to a chemical test can result in an automatic suspension of driving privileges, regardless of whether the suspect is convicted of DUI. In an increasing number of jurisdictions, if the suspect refuses to take a chemical test the police may restrain the individual and forcefully withdraw blood. This is particularly common in situations involving an accident with injury or death. This act may require obtaining a warrant from a judge.

While chemical tests are used to determine the driver's BAC, they do not determine the driver's level of impairment. However, state laws usually provide for a rebuttable legal presumption of intoxication at blood alcohol levels of .08 or higher. Also, breath and urine tests can only estimate the BAC at the time the test is taken, which can be different than when the vehicle was actually operated.

Chemical Testing Assumptions

Blood alcohol tests assume the individual being tested is average in various ways. For example, on average the ratio of BAC to breath alcohol content (the partition ratio) is 2100 to 1. In other words, there are 2100 parts of alcohol in the blood for every part in the breath. However, the actual ratio in any given individual can vary from 1300:1 to 3100:1, or even more widely. This ratio varies not only from person to person, but within one person from moment to moment. Thus a person with a true blood alcohol level of .08 but a partition ratio of 1700:1 at the time of testing would have a .10 reading on a Breathalyzer calibrated for the average 2100:1 ratio.

A similar assumption is made in urinalysis. When urine is analyzed for alcohol, the assumption is that there are 1.3 parts of alcohol in the urine for every 1 part in the blood, even though the actual ratio can vary greatly.

Breath alcohol testing further assumes that the test is post-absorptiveÑthat is, that the absorption of alcohol in the subject's body is complete. If the subject is still actively absorbing alcohol, his body has not reached a state of equilibrium where the concentration of alcohol is uniform throughout the body. Most forensic alcohol experts reject test results during this period as the amounts of alcohol in the breath will not accurately reflect a true concentration in the blood.

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