Blood Alcohol Testing Explained Simply

Blood Alcohol Testing Explained Simply

Your Attorney Must Understand how Blood Alcohol Testing Works

Blood alcohol testing is not that complex. A skilled DWI defense lawyer can always fight a blood sample lab test. Not all criminal defense attorneys know how to fight one, including some less experienced DWI attorneys. Your DWI lawyer must understand how the labs develop a blood alcohol concentration number. Knowing what matters and why tells the lawyer what to look for and what questions to ask. That knowledge is essential when evaluating evidence and communicating about it in court.

No Magic is Necessary, and No Degree is Required to Understand

Let’s demystify crime lab blood testing for alcohol content. I can already hear something along the lines of “Oh, no! Science! Math!” No worries. You don’t need much science or math to understand the main concepts. I’ll explain it mainly using non-technical terminology, but I’ll tell you a few proper terms. I’ll leave out many details and science jargon to make it more easily understood.

Please don’t be intimidated. It’s not magic. Labs have no mysterious power. Lab workers are not wizards. They don’t have any superpowers. You can probably understand everything they do. You can do much of what they do, even though you don’t meet the job qualifications.

Lab workers need a formal scientific education to qualify for the job.  A degree in science is proof to the lab bosses that the lab worker has enough understanding to do the job right. Lab workers need to understand why their actions are essential as they do it. You don’t need their formal education to get a good idea of how it works.

What You Need to Know to Understand

The name might sound intimidating but relax. It’s much easier to understand than you think. Gas Chromatography is the testing method they use. The abbreviation to remember is GC.

A GC Autosampler machine analyzes your sample. It’s called an autosampler because it does all the work after the lab worker puts the samples into the machine. Lab workers can leave work at the end of the day, and the GC machine crunches through the analysis overnight.  When finished, the GC spits out a report of the blood components. The lab workers review this data and create reports. That kind of work requires knowing what the data means and what to do with it.

What Happens to Samples

Blood labs work routinely with tubes that contain blood from someone’s arm, which are then put into smaller glass vials that fit into the machine. You could do this part with some practice. They use a unique device like a magic wand to suck up a measured portion of blood from a larger tube and place it into a smaller tray. The device is called a pipette. The tubes are sealed by a cap with a rubbery center called a septum.

Some space is left between the sample and the top of the liquid level, and the lab worker secures a cap onto the vial to seal it. The lab worker places the vial with other blood samples onto a tray with many other vials and then sticks them into the GC machine.

What The GC Machine Does with The Samples

The machine heats the sample tray. Heating causes tiny amounts of the blood’s contents to rise into the space between the blood and the cap. The gas in that space is called headspace gas.  After sufficient heating, the machine sticks a needle through the rubbery top of each tube and sucks out a small amount of the gas above the liquid. The GC does not test the blood sample; it tests the gas above the liquid sample instead.

The headspace gas sample enters a specially treated tube called a column. This tube is extremely long, often well over 100 feet, and coiled to fit into the machine.

Pressurized gas pushes the headspace gas sample through the tube. The type of gas used should not cause any chemical reaction.

The time it takes for a substance to come out of the tube tells the machine what each substance is. During their journey through the tube, they separate. Different substances exit the tube at different times because they separate as they travel. As sophisticated as it is, that’s the simple central concept.

How The Machine Knows What Something Is

GC machines leave the factory unconfigured. Lab decision-makers program them to recognize certain substances based on their time signatures.

Lab workers run a known substance through the GC machine. When that known substance emerges from the tube at a specific time, lab personnel program the machine to identify it as the known substance. The GC machine will later identify a substance coming out of it at that time as the known substance programmed into it. Alcohol is one of the substances it identifies, of course.

How The Lab Knows the Training Samples Were Accurate

Labs get their training samples from NIST (National Institute of Standards and Technology), an entity of the federal government, or from a NIST-approved vendor. NIST produces the highest quality training samples known, called standard reference materials. These materials are used in many labs because they are widely accepted and set the bar for purity.

How it knows how much of a substance is in the sample

Measuring the amount of substance is different than identifying what it is, but it isn’t magic, either. At the tail end of the GC machine’s tube is a flame ionization detector (FID). I know, I know, that sounds like science. Relax. It’s just a flame burning at the end of the tube, and it burns whatever comes out. The FID measures the ions electronically. The measurement corresponds to a quantity of the substance. (This Youtube video helps visualize the FID process starting at the 3:20 mark.)

Imagine tossing a half-inch square piece of paper into a fire. It will vanish quickly. If you throw a one-inch square piece of paper into a fire, it will flash and then burn longer. Similarly, the FID electronically measures the size of the flash and burn and assigns a value.

If you’ve gotten this far, you know more about blood testing than most. If you can understand this, so can a jury.

In part two, I’ll share some common challenges I’ve used to contest a blood sample result.

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