There are three different types of chemical tests that officers use to determine the driver's blood alcohol content: breath, blood, and urine. The urine test can be considered the most unreliable form of chemical test because there are many ways the results can be faulty.
Urine tests, just like most other sobriety tests, are dependent on the person giving the test's ability. If the officer isn't trained properly to administer the test correctly, or does not follow the right procedures, the test results areoften unreliable.
The concentration of alcohol in urine is 1.33 times the concentration of alcohol in the blood, so the results are inflated. Because this is a known fact, urine tests are typically used just to determine if the driver had taken drugs at all.
According to a study done by the National Institute of Drug Abuse, 20% of the labs that were surveyed had wrongly reported that illegal substances were discovered in urine samples that were actually completely free of illegal substances. This is because many illegal substances have similar chemical compounds to other substances which are legal. For example, if you are sick and are taking a cold medicine that's not supposed to make you drowsy, amphetamines could be found in your results. If you are taking Ibuprofen (Advil, etc), marijuana may be found in your results. It's not an old wives tale: it is even a possibility to fail a drug test from eating a poppy seed bagel!
Another issue with the urine test is that it is not able to determine when the illegal substance was used. A urine test will find any traces of the substances even if it was used a week ago and is no longer affecting the user. Therefore, if a driver is pulled over on a Wednesday and had used illegal substances last Sunday, even though the drugs have no affect on their driving ability anymore, they can still test positive for using them.
The good news: Because the urine test has countless flaws, test results can be easily challenged in court.