DUI and DWI testing is forensic evidence governed by state law. Alcohol and drugs are analyzed differently, and the presence of a substance does not always establish impairment at the time of driving.
Alcohol testing
Roadside breath screening, evidential breath testing, and blood analysis may be used. Alcohol has established per-se concentration laws, but instrument calibration, observation periods, sample timing, and retrograde interpretation can matter.
Drug testing
Blood is often preferred for relating a drug concentration to recent exposure, while urine mainly shows prior exposure. Oral-fluid roadside or laboratory testing is expanding. Screens should be followed by specific forensic confirmation.
Field observations
Officers may document driving, speech, coordination, standardized field sobriety tests, and a Drug Recognition Expert evaluation. Toxicology is interpreted alongside those observations. NHTSA notes that drug presence alone does not necessarily mean impairment.
Implied consent and warrants
States have different implied-consent laws, refusal penalties, warrant procedures, and rules for breath, blood, urine, or oral fluid. Constitutional decisions and local statutes affect whether a sample was lawfully obtained. Prompt legal advice is important.
Prescription medication
A valid prescription may affect the legal analysis but does not guarantee that driving was safe. Many prescribed and over-the-counter drugs can impair coordination, alertness, or judgment. The prosecution still must meet the elements required by state law.
Cannabis interpretation
THC pharmacology is complex, and concentration does not map to impairment as predictably as blood alcohol. States use impairment-based, zero-tolerance, or per-se approaches. Experts should explain specimen timing, active THC versus metabolites, tolerance, and limits.
Evidence to review
- stop and arrest reports;
- warrant or consent documentation;
- collection time and medical records;
- seal and chain-of-custody records;
- screening and confirmation data;
- calibration, quality-control, and analyst records; and
- video and field-observation evidence.
Sources
- NHTSA: Drug-Impaired Driving
- NHTSA: Enforcement of Drug-Impaired Driving
- NIST OSAC: Forensic Toxicology Standards
Legal and scientific review: October 29, 2026.