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Which test is better to take -- the breath test or the blood test?

Preparing for Blood Test

When arrested, most people end up submitting to a breath or blood test. The question often asked is which test provides you the best chance to prevail in your DUI case. The answer depends on the circumstances of your case. Are you 250 lbs or 100 lbs? Did you drink 15 minutes before getting on the road or has it been five hours since your last drink? Did you smoke marijuana, ingest prescription meds, or take any other narcotics? Do you want the ability to retest the sample later on? These are the kinds of factors that weigh into which test is more strategic to take. The bottom line from a defense perspective is there are potential advantages to either test. As an introduction, I’ll discuss some bigger picture factors and considerations.


First, with breath tests, there’s generally more susceptibility to inaccurate readings. Blood tests are more accurate are thus typically harder to attack later at trial. The agencies administering the breath devices must comply with strict regulations requiring routine maintenance, calibration and accuracy checks. These accuracy checks must be all be written down in logs that you will have access to. Any competent defense attorney can spot potential errors in the device, which could help out your case. Also, there are many rules that must be followed with regard to the manner in which these devices are administered. For example, the officer must wait 15 minutes before you submit to the test. The more rules for law enforcement to follow, the more opportunities there are for officers to make a mistake, which is good for your potential defense.


Second, the timing of the tests can have a significant effect on your blood alcohol level. That’s because alcohol takes time to absorb into your bloodstream as well as leave your bloodstream. Roughly speaking, your blood alcohol will rise over the course of the hour you had a drink and then drop off the following hour. The exact rates depend on your weight and sex. Most of the alcohol from a drink is absorbed within 15-20 minutes; the rest of the hour, it’s still slightly rising. But after approximately an hour, the alcohol will start to drop off. Of course, the more drinks you had, the longer it will take for alcohol to leave your bloodstream. As an example, for most people, a single drink (e.g., a small shot, a can of beer, a glass of wine, etc.) will translate roughly into .02% blood alcohol, but lighter skinnier folks, that amount is higher. Many people mistake a pint of beer (which is 16 oz. and probably the most common serving size at bars and restaurants) as equaling one drink when in reality it’s 1.3 drinks. In other words, two back-to-back pints of beer is closer to three drinks than two. Within an hour of consuming those pints, most men, again depending on their weight, are either above or getting close to the legal limit of .08% blood alcohol at the end of the hour they consumed the pints. Thus, they’ll have to wait at least an hour for the alcohol to start tapering off, which generally occurs at a rate of .018% per hour.


The main point about timing in helping you choose between a breath test or blood test is determining, during your DUI arrest, whether the alcohol in you is still absorbing (thus increasing) or whether you’re fully absorbed (thus the alcohol is tapering off). With that in mind, understand that if you choose a blood test, it will generally take the officers at least an hour to get the phlebotomist (the blood technician or nurse) to a specific location to draw blood. This is a general observation, not a rule. Blood draws must occur under sterile conditions. That is, the officer isn’t going to bust out a needle right there in the street after arresting you. These things take time. By contrast, the breath test is typically more immediate—it can be administered on the road after placing you under arrest. However, most agencies will conduct the official breath test at the booking station. Even if an officer takes you back to the station to perform the test, it is still often a faster process than the blood test. So...if you suspect you’re still absorbing because you drank very shortly before driving, then the FASTER you take a test, the LOWER the blood alcohol content will likely show. Thus, choosing a breath test in that scenario would generally be your best bet. 


On the other hand, if you stopped drinking a few hours before driving, that means you’re probably fully absorbed and the alcohol is in the downward process. Therefore, the LONGER you wait to take the test, the LOWER your blood alcohol will most likely be, in which case a blood test may make more sense. Remember, however, that there’s no guarantee that a blood test will take longer. But, generally, they’re slower.


Finally, there are other considerations that may be important to you. For example, with a breath test, unlike blood, there’s no sample that can later be retested. Another example is if you were a drunk and high, a breath test won’t detect anything other than alcohol. A blood sample, however, can be tested for almost any kind of drug. And, lastly, of course a breath test doesn’t involve being injected with a needle, which isn’t easy for many folks.




There are many decades’ worth of complex scientific studies and data addressing the accuracy and reliability of blood alcohol reading instruments and correlation studies involving FSTs. A skilled defense attorney will be able to reference certain studies that call into question the accuracy and reliability of these tests, and thus effectively cross-examine the DA’s toxicology expert.

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