TECH TUESDAY: How blood alcohol is tested


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Photo by Susmita Paruchuri |

Various tests exist to determine blood alcohol content, such as blood tests, urine tests and breath tests, but these tests are not always accurate. 


With many celebratory events like St. Patrick’s Day and spring break approaching, police forces are likely to increase the number of officers on duty and patrols to increase safety. Included in this is an increase in traffic checkpoints, where people are tested for their blood alcohol content.

Police testing for blood alcohol may be conducted after a field sobriety test, which includes tasks like walking in a straight line, among other coordination and cognition related tests, according to nolo.com, a legal advice website.

If a police officer still suspects that the suspect is under the influence of alcohol, the suspect may be arrested for further, more scientific, testing. Such testing can use blood, breath or urine samples, and consent is not needed for the test to be done, according to the site.

Blood tests require the suspect’s blood to be sent to a laboratory to be tested. Although this process is extremely accurate, it is very time-consuming and has a lot of room for error, according to the site.

Law requires the blood specimen to be tested in the same condition as it was in the body, but this is not always the case. Errors can arise from many different sources along the testing process and provide false positive results, according to drivinglaws.org.

There may be errors from poorly drawn samples, poor storage of the sample causing fermentation of the sample and subsequently giving a false positive, improper testing equipment and errors not being reported by the testing agency, according to the site.

Improper storage of blood samples can result in error rates well over 100 percent, and errors in blood withdrawal can lead to up to 50 percent more false positive results, according to the site. 

Urine tests are another method of alcohol testing, but are extremely unreliable because alcohol lost through urine is alcohol no longer in the blood, so it would give no bearing on a suspect’s sobriety. Further, the amount of urine alcohol can range between 50 and 100 percent of blood alcohol, according to nolo.com.

One of the more accurate tests is the breath test, which calculates the amount of blood alcohol based on the alcohol content from a breath sample. This value is heavily dependent on device accuracy and an accurate equation, which is not perfect for everyone, according to the site.

Although they are more accurate than other tests, breath alcohol tests can have up to 50 percent error which is only overcome well above the legal blood alcohol limit. This means that even suspects below the legal limit may give false positive results, according to motorists.org.

Breath tests can be used because some alcohol in the blood permeates into the cell lining of the lungs, and is removed every time an individual exhales. By measuring this level and dividing it by the ratio of breath alcohol to blood alcohol, the level of blood alcohol can be found, according to howitworks.com.

Testing breath alcohol can be done through a variety of tests, which can rely on chemical reactions with the sample, light absorption by the sample and even fuel cells, according to the site.

Devices that use chemical reactions, commonly known as “Breathalyzers,” have vials inside them containing various compounds such as sulfuric acid, potassium dichromate and water, according to the site.

First, the breath sample moves through one vial, allowing any alcohol to react with the sulfuric acid and fall down into a liquid. The alcohol in the liquid then reacts with potassium dichromate to produce new compounds with a different color, according to the site.

The second vial contains the same compounds before any reactions. The vial colors are compared through the use of an electric current, which moves a needle, according to the site.

The needle is moved back, and the energy used to move do so is measured. Higher amounts of energy equal higher amounts of blood alcohol, according to the site.

Devices known as “Intoxilyzers” measure the light absorption of the compounds in the sample, which can help identify the compounds that were exhaled by the suspect, according to the site.

Molecules are composed of different atoms with varying types and lengths of bonds. These bonds can be identified by applying infrared radiation to the compound, as different bonds absorb different intensities and frequencies of light, according to the University of California, Los Angeles Department of Chemistry website.

Matching the frequencies absorbed by the compound’s bonds to frequencies already known shows the atoms present and their arrangement. This makes it possible to identify characteristics of the compound, such as if it is an alcohol, according to the site.

Intoxilyzers pass an infrared beam through the breath sample and is focused onto a filter wheel. The filter wheel filters through wavelengths specific to ethanol alcohol, the type of alcohol commonly consumed, according to howitworks.com.

The light filtered through the wheel is detected by a cell, which produces an electrical pulse. All of the pulses sent by the cell are interpreted by a processor, which calculates the blood alcohol content from the breath sample, according to the site.

Fuel cells have the potential to power things like cars and homes, but also can be used to measure alcohol content in a sample. These fuel cells are composed of two platinum electrodes with an acid electrolyte in between, according to the site.

The sample flows into one platinum electrode, reacts to form a new acid, protons and electrons. The electrons flow through a wire and into the other electrode to produce water, according to the site.

A processor measures the current passing through the wire and uses that to calculate the blood alcohol content, according to the site.

The breath tests may be used in the field by trained officers, but there are also larger devices that provide more accurate data for court cases, according to the site.

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Harshel Patel is a School of Arts and Sciences sophomore majoring in molecular biology and biochemistry. He is the digital editor at The Daily Targum. He can be found on Twitter @harshel_p.


Harshel Patel

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