Breathalyser Vs. Blood Test for B.A.C.

Updated February 21, 2017

While there are several methods for testing the level of alcohol in a person's bloodstream, the two most common are measuring breath alcohol content and blood alcohol content. Police departments throughout the nation primarily use these two testing methods when investigating possible impaired drivers. Each method has its own strengths and weaknesses, and learning about the differences can help you determine the best testing method for your needs.

Testing Methods

Both breath testing and blood testing measure the same thing: the amount of alcohol found in your bloodstream. The two methods arrive at the measurement in different ways. Blood testing directly measures the amount of alcohol in the bloodstream. Breath testing estimates the amount in the bloodstream by measuring the amount of alcohol particles in your breath.


Neither method is particularly cost-effective, although there are definitely cost differences between the two. Breath testing has a fairly high initial investment, as a single breath testing device, such as a Breathalyzer or an Inotxilyzer 8000, can cost as much as £6,500. Add to that the cost of properly training operators and technicians, and the cost of outfitting an entire police department with the instruments can be prohibitively expensive.

Once the initial start-up costs have been absorbed, breath testing becomes much less expensive. Maintenance costs are relatively minor, as periodic repairs and calibrations can be done for a fraction of the initial cost.

Blood tests are much more expensive on a test-by-test basis. Each blood sample must be taken by a proper individual and sent to the state crime, or an independent private lab, for analysis. Blood testing is the most expensive alcohol testing method.


Blood alcohol tests are the most invasive method of testing for alcohol. A qualified technician will actually withdraw a blood sample from your arm. The technician must be able to find a vein, and bruising from the needle is not uncommon.

Breath testing is completely non-invasive. You merely exhale in to the device and it measures you breath alcohol content.

Consequently, many people are far more leery of consenting to a blood sample than they are a breath sample. Police will typically not ask for a blood sample unless they have reason to believe there may be something in your system, like drugs, which cannot be measured by a breath test.


Both testing methods are highly accurate, though blood testing is the most accurate method for measuring alcohol content. As long as the sample was properly taken and stored, and not contaminated in anyway, the test result is positively accurate.

Breath testing is highly accurate, but can present some problems. The test subject must provide an adequate breath sample in order for the instrument to get a proper measurement. Further outside interference from radios or other electronic devices can interfere with instrument.

The device must also be properly calibrated and tested on a regular basis to ensure it is reporting accurate results. Finally, certain substances, such as mouth wash, can cause the instrument to render a false positive result.

Modern breath testing devices have internal safeguards to prevent a result from being given if everything isn't perfect, but there is still more room for error. A properly maintained and administered breath test will essentially be as accurate as a blood test, but it does provide more opportunity for user error or inaccurate results.


Both blood alcohol tests and breath alcohol tests are equally admissible in a court of law in most states. For a breath test, the prosecution is required to show that the test was properly administered, and that the equipment was functioning properly.

For a blood test, the prosecution must show the sample was properly taken, stored and could not have been tampered with. As long as those basic requirements are fulfilled, both tests are equally admissible and are presumed to be equally accurate.

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About the Author

Michael Scott is a freelance writer and professor of justice studies at Westminster College in Salt Lake City, Utah, and is a former prosecutor. Scott has a J.D. from Emory University and is a member of the Utah State Bar. He has been freelancing since June 2009, and his articles have been published on and