The breath test is by far the primary item of evidence in most prosecutions for drunk driving in Texas. Blood tests are considered more accurate, but they are also more invasive and therefore used less often. It is also possible for prosecutors to charge a DWI case without any evidence of blood alcohol content, as long as there is other evidence that the driver's faculties were impaired by alcohol. However, these cases are the exception, and blood alcohol test results remain the basis of most drunk driving prosecutions.
Most law enforcement agencies in Texas use a breath test machine known as the Intoxilyzer 5000. This machine, like others in the current generation of breath test machines, uses a process called infrared spectroscopic analysis to measure the alcohol present in exhaled air.
Courts in Texas generally consider breath test results to be reliable evidence of intoxication. Studies have shown, however, that BAC readings from breath tests can differ by as much as 15 percent from the actual levels shown by a blood test. A court in New Jersey found that design flaws in breath test machines resulted in incorrect high readings for 14 percent of people tested. The same court noted that varying body temperatures, temperature variations in the machine itself, and improper calibration could also lead to incorrect test results.
A good criminal defense approach will typically include a review of the maintenance records of the machine in question to determine whether it was properly calibrated and in good working order at the time of the test. It may also be important to examine the test record for any evidence that the officer conducting the test without following proper testing procedures.