The truth of the matter is we don’t really know. Most labs in the Phoenix Arizona area claim to be accurate within 5%. That means if your blood result came back at .08, then the true result can be anywhere from 5% lower or 5% higher.
Other scientific organizations claim 5% is not a realistic range of accuracy. For example, the American Academy of Forensic Sciences claims that the accepted range of accuracy is 10% higher or lower.
After interviewing toxicologists over 100 times, doing a substantial number of DUI trials with blood results at issue, I am convinced that the accuracy is totally dependent on the procedures used by the lab, and most labs overstate their accuracy.
To support my conclusion, I need to explain how blood testing with a gas chromatograph works. At its most basic level, gas chromatography simply compares known alcohol concentrations to unknown blood samples. A blood tester does not inherently know what a blood alcohol concentration (BAC) is. You must calibrate it every time you do a test. You teach the machine what a .08 is by putting known alcohol concentrations into it, and essentially build a ruler.
Most labs in the Phoenix area put four known alcohol concentrations into the blood tester to build their ruler. These known concentrations are called calibrators. It is important to remember these calibrators are water based. That is, they are known alcohol concentrations in water. See the graphic below for an illustration.
As you can see in the example, there are four points on the ruler. The blood tester simply connects the dots on the ruler. If the four places on the ruler are accurate, then you should have a fairly accurate ruler. However, many labs make their own calibrators, and there is no way to know how accurate the ruler really is. There is no outside agency auditing their work. All we have is their word that they are accurate.
In addition, while it is a good first step to be able to build a ruler using water and alcohol, we are not testing alcohol in water in DUI cases. We are testing alcohol in blood. In science, we need to take into account what is known as the matrix effect. Simply put, water and blood are not the same substance. Water does not have red blood cells, white blood cells, plasma, virus, and bacteria. In order to measure alcohol in blood, we need a blood-based ruler. However, law enforcement labs do not actually use a blood-based ruler. This is where the procedures of the lab really make a difference.
Labs will use a known concentration of alcohol in blood and compare it to their water-based ruler. This is known as a calibrator. This procedure may be acceptable if done enough times with an accurate blood based sample.
Here is the problem. There are very few companies that make the blood based alcohol concentrations, they are not accurate, and some labs use only one calibrator (not four like the water-based.) When the blood-based alcohol sample comes from the manufacturer, there is an insert. The insert tells you that the stated blood alcohol concentration is just a target value. It states that the known concentration it is really just a range. For example, I recently had a case with a blood-based control with a target value of .182. However, upon reviewing the insert that came with the sample, according to the manufacturer, .182 could be anywhere between a .166 and a .198. Thus, the ruler used is not as accurate as we would like it to be. That is a tremendous range when we are trying to determine someone’s true blood alcohol concentration. The picture below illustrates what the blood-based ruler looks like with only one this one known value.
As you can see, you can’t build a ruler with only one point on a line. Thus, with using only one known value, your ruler just is not very accurate – unlike the water-based ruler. The less accurate your ruler is, the less accurate your test result will be. Consequently, the true range of accuracy could be significantly greater than even 10%.