All states have "per se" DUI (driving under the influence) laws that prohibit driving with a blood alcohol content (BAC) of .08% or more (.05% or more in Utah). One method of testing a driver's blood alcohol level is by testing how much alcohol is present in his or her urine. While easy to obtain from a compliant suspect, police can but don't normally use urine tests in DUI investigations. This article will explain when police can request a driver take a urine test, how urine test results can be used in a DUI investigation and prosecution, and why police don't typically use urine testing in DUI cases.
When a person drinks a beer or some other alcoholic beverage, his or her body begins a process of metabolizing the alcohol. Essentially, the body first absorbs and then gets rid of it through various bodily processes. The primary way alcohol leaves the body is via urination.
Laboratories can test a urine sample and determine the amount of alcohol present. This measurement can then be used to calculate (it's actually an estimate) blood alcohol concentration. Generally, there's a proportional relationship between the amount of alcohol in a person's urine and blood. So a higher alcohol urine concentration correlates to a higher blood alcohol concentration.
Providing a urine sample is normally a simple enough task for someone who's consumed alcohol. However, for various reasons, urine testing isn't law enforcement's preferred method for drug and alcohol testing in DUI investigations.
Privacy issues and practical considerations. Privacy concerns can make urinalysis impractical. Generally, drivers can't provide a urine sample at the roadside because there's no appropriate place to do it. So police must transport the suspect to a police station or another facility to get a urine sample. Also, when the suspect gives a urine sample, someone should be observing to ensure the sample isn't tainted. Observation is not only awkward but requires the availability of an officer who's the same gender as the suspect.
Delays in the urine testing process. Whereas breath test results are instantaneous, it takes time to get urine test results. As noted above, police need to send the urine test sample to a lab for testing. This process can take several weeks or more. Prosecutors typically can't decide whether to file charges in a DUI case until they receive the lab results.
Urine test accuracy. Urinalysis is generally considered the least reliable BAC test for multiple reasons. As with breath tests, urine results must be converted (multiplied by 1.33) to obtain blood alcohol concentration. The multiplier used to convert urine to blood alcohol concentration is just an estimate, so the results aren't perfect. Studies have shown that this multiplier can range from .8 to 2 for different people. Many also argue that urinalysis doesn't give an accurate measurement of current impairment because it's measuring alcohol leaving the bladder. Alcohol in the bladder can be there for quite a while, meaning it might not give a good representation of how much alcohol is in the driver's blood or the driver's current level of impairment. For these reasons, many experts believe the accuracy of urine testing for alcohol is questionable at best.
Urine tests are most commonly used when the driver is suspected of using drugs. While still not as precise as blood tests, a urinalysis can show what drugs have been recently used and an approximate concentration.
Prosecutors can use urine test results, along with expert testimony, to prove driver impairment. And, in many states, drivers can be charged with driving under the influence if a urine test shows a certain concentration of alcohol. Some states also have "per se" DUI limits for drug usage. In Nevada, for example, a driver with a concentration of 500 nanograms of amphetamines per liter of urine can be charged with a DUI.