Are breath test results always accurate?


DWI Lawyer and former Assistant District Attorney Brad Smith of Arnold & Smith, PLLC answers the question: Are breath test results always accurate?

Question: "Are breath test results always accurate?"

Brad Smith:

The manufacturers of the various breath test machines will tell you that they absolutely are because they have an interest in sort of convincing the public that they are accurate because they make lots of money selling their various pieces of technology to law enforcement agencies all across this state.

The simple reality though is that a lot of people don’t recognize that breath testing isn’t measuring or sort of transferring your breath alcohol concentration into your blood alcohol concentration, it’s measuring your breath alcohol concentration. And the way that’s done is when you blow into the instrument you are blowing ethanol molecules, if you’ve been drinking, into the machine. And just as you suck in oxygen and the harder you blow out the more carbon dioxide molecules come out of your lungs, the harder that you blow into this instrument, if you have been drinking anything at all, the more ethanol molecules go into the instrument and then the higher the reading would be.

The breath test machine uses certain technology in order to basically measure these ethanol molecules, and there are all sorts of different things that throws off this piece of technology. There is something that affects it called RFI, or Radio Frequency Interference, so if the police officer is wearing a walky talky, or if he has a cell phone in his pocket, those are all different sort of instruments or pieces of technology that the police officer is wearing that have been proven to affect the actual reading on the test. The other thing that affects breath testing that somebody is presumably charged with an impaired driving offense that doesn’t know when they go in to do a breath test is the calibration of the instrument, how old is the instrument? How many times has it been used? How many people have put their mouth around that mouth piece? Or blown into the tube that goes into the machine before this actual breath test.

So, When you are being asked to blow into a machine that you don’t know anything about, you don’t know how old the machine is, you don’t know whether or not its been calibrated, or what the police officer has on his utility belt, whether his microphone is on, or anything of that nature, is it really something smart, something you should blow into without asking these sorts of questions and without understanding the breath test, and the simple answer to that question is no.