How Reliable is a Breathalyzer Test?

When a police officer asks you to blow into the machine and take a breathalyzer test, should you do it or should you refuse? Is the breathalyzer test reliable and can you trust it?

If you do take the breathalyzer test, will it “exonerate” you or will it become evidence that they will use against you in court? Will police let you walk away if you take the test and “pass?”

Below, I’ll answer some of these questions and discuss how the breathalyzer works, including how the machine is calibrated and how the machine generates a result that claims to be “blood alcohol content” (BAC) when it is testing your breath

Is the Breathalyzer Test Reliable?

Police and prosecutors want people to believe that the breathalyzer test – the “Datamaster” machine, in SC – is a magical “black box” that provides a reliable, accurate measure of a person’s blood alcohol content after a person blows into the machine.


If jurors knew the truth about breathalyzer machines, they would not convict nearly as many DUI defendants…

Although the test can provide an indication that a person has been drinking, and it might give an accurate result in any given case, it can also provide wildly inaccurate results for many different reasons. They don’t keep using these tests because they are infallible, they keep using them because it is difficult to convert breath samples to blood alcohol results and it’s the best they’ve got.

What Happens if You “Pass” the Breathalyzer Test in SC?

If you “pass” a breathalyzer test in SC, will police then release you and let you go home with an apology? Some officer somewhere might, but I’ve never seen it happen…

Police in SC do not use roadside PBTs (portable breath tests, which are even more unreliable than the Datamaster machine).

If an officer thinks that you have been drinking, they are going to arrest you before you take a breathalyzer test, handcuff you, transport you to the police station or the jail, and then take you to the “Datamaster room” where they will offer the breathalyzer test to you after a 20-minute observation period.

If you know you haven’t been drinking, you take the breathalyzer test, and you blow a 0.00, will the officer release you?

They should, but

If the officer releases you at that point with a kind apology, you might just sue him or her for wrongful arrest and assault and battery. 1) No cop wants to get sued, and 2) most cops don’t want to admit they were wrong, especially when their mistake was as serious as handcuffing and kidnapping an innocent person. So, what does the officer do?

Put you in a jail cell and charge you with DUI for driving while drugged… At this point, you probably aren’t going to agree to take yet another test (a blood draw at a hospital, for example), and, if you do, the officer won’t have the results until long after you’ve been booked and released from jail.

So, should you take the breathalyzer, knowing that your best case scenario is that you are going to be arrested anyway and your worst case scenario is that you will get an inaccurate result that says you were drunk?

What Can Go Wrong with a Breathalyzer Test?

What could affect the result of the breathalyzer test other than alcohol in your system?

  • Was the machine properly calibrated before the test was administered? If not, the results can be skewed.
  • What did you eat or drink before taking the test? Some foods (breads, for example) can increase the amount of alcohol detected on your breath, and food or non-alcoholic drinks can affect the rate at which your system processes alcohol.
  • Radio interference in the Datamaster room can impact or invalidate the results.
  • What was the expiration date on the simulator solution that was used for the pre-test? Was it expired?
  • What was the temperature of the pre-test simulator solution?
  • Were there any foreign objects in your mouth? Chewing gum, dentures, tongue piercing, bridges?
  • Did you burp at any point before blowing into the machine?

How can these things affect the result? Let’s look at how the machine works…

How Does the Breathalyzer Test Work?

You blow into a mouthpiece attached to the machine. The officer will tell you to blow, blow, blow, keep blowing, so that you will end up blowing out the air from the deepest parts of your lungs – the alveolar sacs where the lungs exchange oxygen with the capillaries.

The machine then tests your breath to calculate its alcohol content and prints a result for the officer. How do we know the machine is giving an accurate number? And how the heck do they get a blood alcohol result from a breath sample?

Calibration of the Breathalyzer Test

Just before the machine tests your breath, it runs a practice test with a “simulator solution.” The simulator solution should contain .08% alcohol within a small margin of error, it must be within a certain temperature range, and it has an expiration date.

If the machine gets .08% for the simulator solution, then it is assumed that it is calibrated and it calculates the alcohol content of your breath.

But how does the machine get a blood alcohol result from a breath sample?

Conversion of Breath Alcohol to Blood Alcohol Content (BAC)

Breathalyzer tests don’t test your blood alcohol. They test your breath alcohol, and then the machine must convert that number to arrive at a “blood alcohol content.”

How do they do that?

They multiply the breath alcohol content by 2100, and “Voila!” There is your blood alcohol content. Except, is it really?

Where did they get the 1/2100 ratio?

They settled on 1/2100 as a matter of convenience – the actual ratio for converting breath to blood alcohol, assuming there is an accurate breath alcohol result, is different for different people. It can vary from 1/1800 to 1/2500 depending on the person’s gender, size, metabolism, and even what they’ve had to eat or drink on that particular day…

So, the machine is using an average multiplier – not one that is specific to your body and your metabolism – which means that, unless a 1/2100 ratio just happens to be correct for your body, it is not an accurate result.

Does it make a difference? Of course, it does – 2100 is a huge multiplier, and a difference of a few hundred in the calculation could easily put you over the “limit.” How else can it make a difference?

Considering the size of the multiplier (2100), even very small inaccuracies in the breath alcohol number can make the difference between DUI or not DUI – this is why it is critical that there is no alcohol in the mouth when you take the breathalyzer test. Chewing gum, dentures, piercings, or any foreign material that could hold even the smallest amount of alcohol can skew the test results.

So, should you agree to take a breathalyzer test, knowing that 1) you will most likely still be arrested if you “pass” and 2) if the results are off, you are giving the prosecutor evidence they can use to convict you?

SC DUI Defense Lawyer in Conway, SC

If you have been charged with DUI or DUAC in Myrtle Beach, Conway, or anywhere in the Horry County area, you may have valid defenses to the charge or there may be grounds to get your case dismissed. You may also be able to challenge the results of the breathalyzer test, and we will review SLED’s records to find any error messages or problems with the machine that was used for your test.

Contact DUI Defense Lawyer Johnny Gardner now at (843) 248-7135 for a free consultation to find out how we can help.