Chemical tests to determine blood alcohol content (BAC) are used in every Florida DUI / DWI case.  A BAC of .08 percent or greater can be the most damaging evidence against a motorist accused of driving under the influence. However, there are proven ways to challenge this evidence. A skilled Florida criminal defense lawyer will attack chemical test results as part of an aggressive defense strategy. 

One common challenge to a chemical test is to question whether the test itself was accurate. An experienced defense attorney will use an independent toxicologist to review whether the chemical test was conducted correctly and whether any sample was properly collected, stored and analyzed. Mistakes are more common than most drivers realize.

However, even if the defense concedes that the blood, breath or urine test accurately reflected the motorist’s BAC at the time of testing, that result doesn’t reveal the driver’s BAC at the time of driving. It is the condition at the time of driving that is critical in DUI / DWI cases, and there is typically a significant delay between when the driver was behind the wheel and when the test was conducted. A chemical test taken hours after a driver was last behind the wheel only allows a technician to estimate the BAC at the time of driving.

Every individual varies greatly in his or her metabolic rates, stomach contents, and drinking patterns, so it’s extremely challenging to look backward in time and accurately pinpoint BAC.  This difficulty increases with the passage of time, and becomes more and more speculative.  Obviously, for the DUI / DWI criminal defense attorney whose skill lies in highlighting reasonable doubt, this information is extraordinarily valuable.

To appreciate the challenges available to this type of evidence, it’s helpful to understand how the human body absorbs, distributes, and eliminates alcohol. When an individual ingests alcohol, it goes first into the stomach, and then into the small intestine, where most absorption takes place.  The type of alcohol consumed, as well as the stomach contents prior to and during drinking, will affect the speed at which the alcohol is absorbed. 

Absorption begins immediately; therefore the body cannot eliminate alcohol as quickly as it is absorbed.  This delay between absorption and elimination is what causes alcohol levels to rise. The process can be compared to a bathtub that has a slow drain –water flows into the tub and drains at the same time, but because it flows in faster than it drains, the water level rises.  Alcohol absorption in the human body works in roughly the same way.

Even after an individual stops drinking, alcohol absorption continues.  The driver’s BAC continues to increase, and finally levels off or plateaus.  This peak level represents perfect equilibrium – absorption and elimination occur at the same rate, so the alcohol level flattens out and is consistent for a period of time. The plateau typically lasts for 15 to 45 minutes, depending upon metabolic rate and stomach contents.

Once the plateau period ends, the individual enters the pure elimination phase.  If no more alcohol is consumed, the person’s BAC will decrease at approximately .02 percent per hour. However, this rate can vary widely from person to person and from situation to situation. 

If this process were shown as a graph, it would resemble a bell curve, where the individual’s BAC continued to rise even after consuming that last drink. Therefore, it’s easy to see why a chemical test above the legal limit taken hours after the driver was behind the wheel doesn’t accurately show what part of the curve the motorist was at the time of driving. 

Breath testing is particularly questionable when performed during the absorptive phase – the first half of the bell curve.  Many scientific studies have shown that breath tests conducted during the absorptive phase overestimate true alcohol levels by at least 40 percent, and by as much as 100 percent.