Blood evidence is all the rage in DWI cases. Like most DWI defense lawyers I need an expert to help analyze blood results. Let’s talk to one.
Today’s expert is Dwain Fuller, Technical Director of the Toxicology Laboratories at a leading North Texas medical center. Mr. Fuller holds a Bachelor of Science degree in Chemistry from the University of Oklahoma and has worked in toxicology since 1984.
His impressive bio would devour this entire post, so click here for his complete resume.
Are blood tests more accurate than breath tests?
This could be discussed in all of its nuances for several pages and still not completely exhaust the subject, but to be somewhat brief: It depends on what you mean by your question. If you mean “Which test most accurately measures the concentration of alcohol in a person’s blood?”, then the answer is, “a blood test is more accurate than breath.” A breath test can only estimate the alcohol concentration in the blood. To estimate the blood alcohol concentration (BAC) from a breath test it must be assumed that a 2100:1 concentration ratio exists between blood and breath. In fact this is an average ratio; some people have a higher ratio, some lower. In fact, the actual average ratio is probably a bit higher at around 2300:1. Therefore, calculating a BAC from a breath alcohol at the 2100:1 ratio would likely underestimate the BAC to some degree.
However, Texas law recognizes the problem of the 2100:1 ratio being an average and defines the per se concentration separately for each allowed specimen. In other words, Texas law defines the per se concentration in whole blood as 0.08 g/dL, in breath as 0.08 g/210 liters (2100 dL) of breath, and for similar reasons, urine per se concentration is defined as 0.08 g/67 mL of urine. Since there is no need to convert to a BAC the problem of what ratio to use is eliminated.
Probably what you are really asking is “Which test is analytically more accurate (and/or precise) in measuring alcohol in its respective matrix? The short answer is: “Both techniques are based on sound analytical principles and are capable of highly accurate results.” However, there are several other factors that must be considered.
Breath tests are typically performed by specially trained law enforcement officers. While I have the greatest respect for most law enforcement officers, analytical chemistry is usually not the course of study one pursues to work in this field. Likewise, I would put more confidence in a law enforcement officer protecting me and my loved ones from bad guys than I would in most of my forensic toxicologist colleagues. It’s all about doing best those things we understand the best.
Since a breath test is conducted directly on a living human subject, there exist certain conditions that must be controlled to ensure accuracy and precision. For example the subject must be observed for a period of at least 15 minutes to ensure that there is no remaining mouth alcohol or that the subject hasn’t place anything into his or her mouth or belched. During the test the subject must blow for a sufficient period of time to ensure that the air sample is of deep lung or alveolar air. The concentration of alcohol in the air first expired from the lungs is typically lower in alcohol concentration than that of deep lung air. In other words, the concentration of alcohol in the expired air being blown into the breath testing device changes from the beginning of the blowing until the end.
Furthermore, the breath instrument works on the assumption that the breath temperature is 34C which may or may not be the case, depending on the subject. This is important because this is the temperature at which the device is calibrated and the temperature at which the reference sample, used to validate the results, is obtained from the breath simulator. This is based on a physical chemistry principle known as Henry’s law. According to Henry’s Law, at a given temperature, the amount of alcohol in the air above a solution of alcohol (reference sample) is proportional to the amount of alcohol in the solution (reference solution). If the solution temperature is low, the reference results will be low. If the solution temperature is high, the reference results will be high. Likewise, if the subjects body temperature is increased, his or her breath alcohol concentration will be increased and vice versa.
There exist many other factors that must be controlled or otherwise dealt with, such as electromagnetic interference and possible interfering substances such as acetone or other volatile compounds that may have been produced, ingested or absorbed by the subject, or are perhaps in the ambient air of the room housing the testing device.
To be fair, most testing devices enlist “fail safe” mechanisms in an attempt prevent errant readings from these sources, but there exists some dispute at the effectiveness of these mechanisms. Perhaps you might want to envision a juggler; the more balls one must keep in the air, the more chances are that you will drop one.
More from Dwain after the jump… including what problems can occur with blood testing, discovery advise, and even a pot question
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