In many states, law enforcement can use urine testing to determine whether drivers who are suspected of driving under the influence (DUI) have drugs or alcohol in their system. However, for various reasons, urine tests are generally disfavored and police will almost always opt for a breath or blood test instead.
In every state, a motorist can be convicted of a DUI for driving with a blood alcohol concentration (BAC) of .08% or more (Utah has a lower limit of .05%) or while actually impaired by drugs or alcohol. When a DUI is based on BAC, it's normally called a "per se" DUI. Some states also have per se drug DUI laws that prohibit driving with a certain concentration of specific drugs in your system.
Evidence of a driver's BAC or the specific amount of drugs in the driver's system can be, and often is, used by the prosecution to prove both types of DUI charges. However, as compared to breath and blood test results, urine test results are often more easily discredited by defense attorneys. And, from a law enforcement standpoint, urine testing poses a number of practical concerns that aren't issues with breath and blood tests.
In order to determine a person's blood alcohol concentration from the alcohol concentration in urine, a conversion is necessary. Generally, the amount of alcohol in a person's urine bears a proportional relationship to the amount of alcohol in the person's blood. On average, the level of alcohol in urine is about 1.33 times the amount of alcohol in the blood. So, to convert a urine test result into an equivalent blood alcohol level, it's necessary to divide the urine alcohol level by 1.33. However, this conversion number is a very rough average—so rough, given the many variables, that most experts agree it lacks general reliability. This lack of reliability gives defense attorneys a strong argument that the results should not be admitted at trial to prove BAC.
Some states have effectively side-stepped the conversion reliability issues by setting a per se urine alcohol concentration level. In other words, the laws of these states—in addition to having a per se BAC level of .08%—make it illegal to operate a vehicle with a certain concentration of alcohol in your urine. For instance, in Ohio, you can be convicted of driving under the influence for having a BAC of at least .08% or a urine alcohol concentration of at least .11%. Many states—including Illinois, Georgia, and Iowa—have done the same thing with drug DUIs by setting a per se urine concentration for certain drugs.
By defining a DUI in this way (using urine concentration), a conversion from urine to blood concentration is no longer necessary.
All states have implied consent laws that require drivers who are lawfully arrested for a DUI to submit to chemical testing. The purpose of the testing is, of course, to determine the amount of drugs and alcohol in the driver's system. Drivers who unlawfully refuse testing generally face penalties such as license suspension and fines that are more severe than those that would ordinarily be imposed for a DUI conviction.
In many states, law enforcement has three options for testing—blood, breath, or urine. In most cases, blood and breath tests are good alternatives. The officer can simply request that the suspect blow into a breathalyzer or sit still while a blood draw is conducted. In either situation, the driver won't normally have a valid excuse to evade the test.
With urine testing, on the other hand, there isn't much an officer can do if a person claims he or she is unable to urinate. Additionally, privacy can be an issue with urine testing. To ensure the sample isn't tainted, someone needs to observe the process. So, there's certainly awkwardness involved. But also, there might not be someone of the suspect's gender available to observe. In order to avoid these problems, law enforcement typically just chooses one of the other two tests.
If you've been arrested for a DUI, it's always best to get in contact with a DUI attorney as soon as possible. An experienced DUI lawyer can assess your case and help you decide whether to pursue certain defenses and how best to handle your situation.