Partition Ratio in DUI Breath Testing

Today, we’re talking about breath testing in DUI cases and specifically a very important concept known as a partition ratio.

This is the ratio between the amount of alcohol in a person’s breath and the actual amount of alcohol in a person’s blood.

In Arizona and in most states, it’s not illegal to drive with a breath alcohol content. When you get stopped for a DUI, they’re testing your blood alcohol content.

They want to know how much alcohol is in your blood because that’s what actually impairs you. But, in many cases, they don’t actually test your blood.

They just take a breath sample and what they do is they take you in, then they take a sample from you through an intoxilyzer device and that machine is taking the amount of alcohol in your mouth or in your breath and then it’s calculating what it thinks is actually in your blood.

That ratio between what’s in your breath and in your blood is called the partition ratio.

Here’s how it works. In the United States, the breath testing machines, they’re saying that if they find one part of alcohol in your breath that they can presume and work backwards that there are 2100 parts in your blood. It’s a 2100 to one ratio from blood to breath. That’s what the machine says. It’s saying that in your breath if they are calculating that there’s one little molecule of alcohol or one part of alcohol in there, they can presume that there are 2100 parts in your blood. They’re saying that there’s a connection between these two things. In reality, that number, that ratio, 2100 to one, is not the same for everybody. It’s not the same at all for anybody. It can range very, very drastically. It can be down to 800 to one or it can be much higher, like 3500 to one.

The numbers can change dramatically and the way that works is practically, if you just do the math on it and you say you want to start at a BAC of, I did the math, BAC of a 0.1 BAC, which is over the legal limit, and you want to divide that by 2100, so that they’re saying this is your blood alcohol content.

You divide that by the 2100 because that’s the partition ratio, then you multiply that number. Since we just divided it, we’re going to multiply it by a new partition ratio.

Let’s say that the actual ratio between breath to blood is 1500. You take that number, you divide it by 2100, you multiply it by the new partition ratio of 1500. You can do the math at home if you want, but the BAC will now drop to a 0.071 which is under the legal limit.

This is why the partition ratio is so important. The machine doesn’t change. The machine doesn’t care whether you’re my size, somebody else’s size. Whether you’re female, male. Whether you’re of a certain racial ethnicity.

It doesn’t really matter. It doesn’t change for any of those specific factors and all of these different things may impact what your actual ratio is.

If you were charged with DUI and they an intoxilyzer device and they made you blow into it and they got a sample, this is one of many, many defenses that we can use to show that that number was actually not your real number. That your ratio is not 2100 to one.

It’s actually much lower than that. If the machine is saying your BAC was very high, because your ratio is much lower for a whole multitude of reasons that we’re not going to get into in this video, but that number, your actual number, the actual amount of alcohol that’s in your blood is significantly lower than what that machine says.

It’s very important to understand that this is something that we can use in our defenses or our negotiations with the prosecutor’s office.

If it’s something you want to discuss further, you’ve been charged with DUI, and they gave you a breath test, and you want to challenge that number, or you think it sounds high, or you just want to sit down with us and discuss the best way through your case, give us a call.

We offer free case evaluations. We’re happy to meet with you.

Thanks for watching.

A very common test that is used in DUI cases is the breath test. Essentially, police officers will have you blow into a machine that will analyze how much alcohol is in your system from that breath sample. However, the key metric used in DUI breath testing is unreliable. It is unreliable because it does not factor in the partition ratio. The partition ratio is a metric that can be so detrimental to the state’s cases against DUI’s that some judges try to prevent lawyers from even saying the words “partition ratio” in court.

The partition ratio is the ratio between the amount of alcohol in a person’s breath and the actual amount of alcohol in a person’s blood.

In Arizona and most other states, it is not illegal to drive with any specified breath alcohol content (BRAC). It is the blood alcohol content (BAC) that matters when it comes to the law because that is what impairs your ability to operate a vehicle. Consequently, in DUI cases, the state will attempt to build a case based on your BAC.

However, in many cases, they don’t test your blood. Instead, they will take a breath sample. This is done with a machine that detects the amount of alcohol that is in your breath, and then calculates what it thinks your BAC is based on that breath sample. The ratio between what is in your breath and in your blood is the partition ratio.

In the United States, the breath testing machines are calibrated to presume that if there is 1 particle of alcohol in your breath, there are 2100 particles of alcohol in your blood. This would be a 2100:1 blood-to-breath ratio. However, this ratio of 2100:1 is not the same for everyone. The partition ratio can range drastically from person to person. For example, a person whose partition ratio is 1500:1 would test at 0.1% BAC (over the legal limit) while their actual BAC is 0.071% (under the legal limit). The machine doesn’t consider the variance in the partition ratio from person to person. There is a multitude of different factors that affect the partition ratio of different individuals. Some of these factors would include size, body type, sex, ethnicity, and many other genetic factors. Consequently, the partition ratio is a very important factor in a DUI case.

One of the main defenses in a DUI case is this disparity between what the alleged partition ratio is versus the actual partition ratio. The reason this defense is so potent is that it challenges the legitimacy of the alleged BAC, which is what the state’s case hinges on.

If you have been charged with a DUI, contact our office immediately. We offer free case evaluations and will put together a plant that will work for you.