Are Blood, Breath, and Urine Tests Reliable?

By October 7, 2014Thomas Law Firm

When a police officer believes you have been drinking and driving they have a number of tests they may give to determine blood alcohol content (BAC). We have already discussed why it’s a good idea to politely refuse sobriety tests. The next step in the process may be a blood, urine, or breath test. These tests will determine the way the Department of Motor Vehicles and the State of Colorado will view your case.

Blood testing

Blood testing is perhaps the most reliable form of determining the alcohol or drug levels in your body but there can be issues with the chemistry used to analyze a blood sample. The way samples are handled and stored can also have an impact on the results of a test. For instance, if blood samples aren’t stored correctly before being analyzed the blood can coagulate or decompose, which may lead to an inaccurate reading. WebMD also notes a few issues that can alter blood-test results; using rubbing alcohol to clean skin before inserting a needle, herbal supplements like kava or ginseng, and having diabetic complications are a few. There are a number of non-prescription and prescription medicines that may also change the results of this test.

An experienced Denver DUI attorney can make sure the company that analyzed the blood has been following procedures to ensure accuracy. Many labs carry out the testing process in stages, handling many samples at one time, which can lead to errors. Most of the time these test will be accurate but knowing how to spot inconsistencies can make a world of difference in a DUI case.

Urine Testing 

Urine testing is the least accurate out of the three major test types (blood, breath, urine). The way urine is tested is by taking the urine-alcohol ratio and correlating it to the blood-alcohol content. Generally a ratio of 1.33:1 urine alcohol to blood alcohol is used, but this method is fairly subjective. Urine tests are the most time-sensitive test, meaning that the time shortly before taking the test can have a major impact on the results. This can be good or bad for the person being tested; if your last alcoholic beverage was right before the urine sample, it may read falsely high. If you drank water before the test, it may read falsely low. Urine-alcohol content is only a “snapshot” view of your constantly changing blood-alcohol content. Many jurisdictions will require you to produce more than one urine sample, for this reason.

The tests that are used to determine toxicology in urine are similar to those of blood tests. That means the results can also be affected by mishandling, improper storage, and other factors.

Breath Testing

Breath testing or using a “breathalyzer” is when officers analyze breath gas to indirectly determine blood alcohol content. BAC is calculated by taking the amount of alcohol in the exhaled air and multiplying the reading by 2,100, a number known as a “partition ratio” because the “average” amount of alcohol in a given breath is 1/2100th of the amount of alcohol in an equal amount of blood. Using this standard number means great variability in the test results. In the Supreme Court case State v. Braymana chemist testified that partition blood alcohol ratios can vary from 1600:1 to 3000:1. That means an attorney can argue a breath test gave inaccurate readings based on the assumptions for an “average person.”

This testing method is also susceptible to equipment and human error. The testing devices must be regularly calibrated to ensure accuracy and law enforcement should keep a record of how often the device has been serviced. Body temperature, ratio of red blood cells to blood plasma, and even breath fresheners can also affect the test results to show false readings.

The bottom line is that these tests are a generally an accurate way to determine the blood alcohol content of someone accused of driving under the influence. They are not, however, one hundred percent accurate. An experienced Denver DUI attorney may be able to show the tests are not accurate enough for a conviction.

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