Per se vs. impairment: how Nevada DUI charges work

Per se (automatic, by the numbers) liability means the number is the offense. Under NRS 484C.110, driving with a blood alcohol concentration of 0.08% or higher, measured within two hours of driving, is unlawful regardless of whether the driver appeared or felt impaired. The statute treats the number as conclusive on the question of fitness to drive. The prosecution does not need a witness who observed erratic driving, slurred speech, or any behavioral sign of impairment. The test result, if properly obtained, is the case.

The two-hour window is a meaningful limitation. A test administered outside that window cannot establish a per se violation on its own. The state may still attempt retrograde extrapolation (working backward from the late result to estimate the BAC at the time of driving), but that calculation introduces assumptions about absorption rate, the timing of the last drink, food consumption, and individual metabolic variation. Each assumption is a point of attack.

The defense of a per se charge focuses on the reliability of the test itself: calibration records, the required observation period before breath testing, chain of custody for blood draws, and whether proper procedures were followed at every step. A number produced by a flawed process is not reliable evidence of anything.

The impairment theory requires no number at all. Under this theory, the state must show that the driver was actually under the influence of alcohol, a controlled substance, or both, to a degree that affected their ability to drive safely. This is an observation-based theory, and it depends on witness testimony, officer notes, body camera footage, field sobriety test performance, and the totality of the circumstances at the scene.

A driver can be convicted under the impairment theory with a BAC well below 0.08%, if the state can demonstrate that person was nonetheless impaired. Conversely, a driver acquitted on a per se theory (perhaps because the test was suppressed or the two-hour window was missed) may still face an impairment charge based on the arresting officer's observations.

Defending an impairment charge means contesting the quality and interpretation of the evidence: whether the field sobriety tests were properly administered, whether the officer's observations were accurate, whether there were alternative explanations for the driver's appearance or performance. It is more subjective than a per se case, and that subjectivity gives the defense room to work.

Nevada's per se theory extends beyond alcohol. The statute sets specific nanogram thresholds for a range of controlled substances in blood and urine. Exceeding a threshold is a per se violation, with the same logic as BAC. The state need not prove impairment; the level in the blood or urine is the offense.

Marijuana is treated differently. Since AB 400 took effect in 2021, Nevada has no per se marijuana or marijuana-metabolite limit for a misdemeanor DUI. In those cases the state has to prove actual impairment through driving behavior, officer observations, and field sobriety evidence. A blood result showing THC or its metabolites is not, by itself, the offense. The nanogram thresholds for marijuana survive only in certain felony DUI contexts, such as DUI causing death or substantial bodily harm. Because metabolites can remain in the blood long after any effect has passed, a cannabis blood result is often weak evidence of impairment at the time of driving.

The defense of a drug DUI per se charge often turns on the same procedural questions as alcohol testing (collection, handling, and chain of custody) and, where relevant, the scientific validity of using metabolite levels as a proxy for impairment.