Many people who are pulled over for the suspicion of drunk driving falsely assume that the test they take on the handheld machine on the side of the road is going to be what is used to establish that they were intoxicated at the time they were stopped. In reality, the reading that will be used in court will come from either a larger machine at the police station or the blood or urine sample taken at a hospital or medical center.
The handheld machine that police officers carry in a squad car is actually a preliminary alcohol screening device or PAS. These aren't as accurate as the larger machines, or Breathalyzers, that are in the departments. Because of this, the PAS can only establish probable cause to conduct an arrest. Still, it is possible for the police officer to arrest someone based on other points even if they don't have the results from the PAS.
One of the primary differences between PAS and Breathalyzers is that the Breathalyzer is subject to calibration standards and procedures. These larger devices are less likely to produce false positives due to safeguards that are standard in the machines. The Breathalyzer test results are the ones that can be included in the court case for the drunk driving.
It is imperative that anyone who is being accused of driving while drunk fully understands the differences between these tests. This might help you as you set the strategy for your defense against the charges. Make sure that you consider all avenues for calling the prosecution's case into question.