In order for an officer to be certified as a DRE a police officer must undergo a certification process. It is usually a training sponsored by the International Association of Chiefs of Police. The officers are trained in drug detection, i.e. how to identify whether or not an offender is under the influence of a particular drug, or multiple drugs. If the drug is identified in the suspect, the officer is also trained on how much impairment the person is experiencing due to the presence of that drug. The training is only available to police officers. Once completed, these "experts" get wide latitude in testifying about their findings about a DUI suspect.
The officer is trained on the 12-step protocol used by DRE's to evaluate suspects. This process is a National Highway Transportation Safety Administration standardized 12 step evaluation involving physical and mental evaluations, the goal of which is to attempt to determine which drugs an individual may or may not be under the influence of. The extremely specialized nature of this testing demands that an officer conducting these examinations be precise in their testing methods as well as their reporting procedures. More and more police agencies are hiring DRE's and training DRE's, especially here in Colorado where marijuana and opioid related driving offenses are being investigated aggressively. Like any other officer, however, these cops can make mistakes and misidentifications.