Breath testing can not always be performed
As an attorney representing the victims of drunk drivers, I know that due to circumstances beyond the control of law enforcement, BrAC (breath alcohol concentration) measured with a machine commonly referred to as a Breathalyzer can not always be tested during the time required by law to determine the BAC (Blood Alcohol Concentrations).
Usually this is because the drunk driver has also been injured in the wreck they caused and must be rushed to a hospital for treatment.
Thankfully, as a matter of course, hospitals do test for alcohol (ETOH) concentrations in the blood.
For the lawyer attempting to convert serum alcohol to breath alcohol, the following formula will give you a rough conversion. I say it is rough because many physiological factors can affect the serum concentration.
SERUM to BAC FORMULA
Blood alcohol levels are coded in terms of percent by volume (serum %). Percent by volume equals the milligrams of alcohol found per deciliter of blood (mg/dl) divided by 1000. For example, a level of 30 mg/dl would be 0.03% alcohol. How labs report blood alcohol concentrations (BAC) varies. Many use the format used here (serum %), while others report BAC as milligrams of alcohol per deciliter of blood (mg/dl)—as in 30 mg/ dl. To convert mg/dl results to serum % results, divide by 1,000. Only BAC levels should be entered here; levels based on other body fluids such as vitreous fluid should not. Use caution when interpreting BAC levels because variation in the time elapsed between ingestion of substances, time of death, and time of drawing body specimens for toxico-logical analysis will affect the outcome.
General Effects of Alcohol
|Number of Drinks||Blood Alcohol Content||Typical Effect|
|1||0.02-.03||Noticeable cognitive changes|
|5||.10-.15||Person obviously intoxicated, delirium|
|24||.24-.36||Loss of consciousness|
|36||.48||Severe coma possibly resulting in death|
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-absorptive - that 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.
For a look at legal BAC's from around the US and penalties for DUI, check out this link.