Drivers arrested for DUI / DWI in Florida must submit to a chemical test of their breath, blood, or urine to determine blood alcohol content (BAC). Many drivers fear that a chemical test result that shows a BAC of .08 percent or greater – now the legal limit in all 50 states – means an automatic conviction. However, breath tests and other chemical tests can be effectively challenged. Criminal defense lawyers who concentrate on DUI / DWI defense have developed proven strategies to attack results evidential breath tests. 

It’s important to note that breath tests in drinking and driving cases are an indirect measurement of BAC.  Breath testing machines estimate BAC on the basis of certain scientific assumptions, which are not always applicable to the individual being tested.  These assumptions include the driver’s blood/breath partition ratio, his or her breath temperature, and many other factors. 

Because breath testing in a DUI / DWI case always occurs sometime after the motorist was driving, the resulting BAC is calculated through a process called retrograde extrapolation.  Retrograde extrapolation is the process of looking backward to estimate the motorist’s alcohol level at the earlier time of driving.

Retrograde extrapolation is riddled with serious problems.  An individual’s BAC changes significantly over time, and the rate of change varies greatly from one person to the next.  Stomach contents, the length of the drinking period, body weight, gender, the amount of alcohol consumed, elimination or “burn-off” rate, and other personal metabolic factors all affect retrograde extrapolation in a DUI / DWI case. 

In addition, an expert calculating a driver’s BAC through retrograde extrapolation must assume that the motorist was “post-absorptive” – an assumption that is not always accurate, since the absorptive phase can last for two hours or more.

Breath testing in DUI / DWI cases can be challenged in two different ways.  Some defense strategies attack the validity of the breath test result itself. Another defense tactic involves acknowledging the accuracy of a breath test result but questioning whether the driver exceeded the legal limit at the time of driving. 

The most basic challenge in a breath-testing case is questioning the functioning of the machine itself.  Obviously, the machine must be working properly to give a reliable result.  This means ensuring that the accuracy checks, calibration records, usage logs, and maintenance history are all properly documented and don’t reveal any irregularities.