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BAC Tests: Least to Most Accurate

June 11, 2019

So it's probably a question you've asked before, what's the best way of measuring how much alcohol is in your system? We've made a list of all the common methods people use to stab at their BAC, in order of least to most accurate.

Method 1: The Field Sobriety Test

This one estimates intoxication based on performance on a series of coordination tasks, e.g. walking in a straight line or standing on one leg.

Thoughts: It kind of works. Alcohol impairs your motor skills and coordination, so it makes sense that if you're intoxicated past certain level, you'll fail on these tasks. There are some massive issues though. If you're at the stage of failing these tasks, you probably already know you're really drunk. But even if you pass the test fair and square, it means surprisingly little; in Australia the legal driving limit in every state is set lower than the threshold at which you'd normally start failing these tests.

Verdict: Useless

Method 2: Standard Drink Counting

Works by calculating your BAC based on the amount of alcohol you consume over a time period. There's a bunch of different methods for this, some better than others.

Thoughts: It's not as easy as it sounds. To do this effectively you have to carefully track everything that you drink. That means noting the volume and alcohol content of everything you put in your mouth. Good luck if someone's topping up your drink. Another problem, how well does your body tolerate alcohol? Have you been drinking on an empty stomach? How about medication you're on, or your liver function? It's impossible to account for all the variables that affect how you metabolise alcohol.

Verdict: Works as a rough guide for someone on a standard licence, but not good enough if you're on a 0 BAC requirement.

Method 3: The Personal Breath Tester

This is a handheld device which measures the alcohol you exhale. They can work in different ways depending on the type you're using.

Thoughts: Like most technology, personal breath testers aren't perfect. In fact, some are really bad. When buying one, choose something that uses Fuel Cell Technology. This is the gold standard for breath alcohol measurement. Not only is it technology in your alcohol interlock, but it's also what the police use in roadside testing. Fuel cell technology isn't enough by itself though, your device needs to be regularly calibrated, every six months to maintain accuracy.

Verdict: Only as good as your device. A recently calibrated fuel cell device gives a pretty good indication. Our recommended Personal Breathalyser here

Learn More about Personal Breathalysers:

Method 4: Alcohol Interlock Device

This one is just for argument's sake, because there's no way you'd want to test your BAC on these devices. If you blow a positive reading, it's recorded. And can cause your program to be extended. Your alcohol interlock device is however, a pretty sensitive, accurate piece of fuel cell technology.

Thoughts: Alcohol interlock devices all have to pass strict government standards. They use the same fuel cell technology as police breathalysers, and are calibrated every time you go in for service. They have a small margin of error though. That's why the limit for alcohol interlock devices is set at 0.02% even though interlock drivers must have a true BAC of 0. It's also why readings on your alcohol interlock are not used as evidence of criminal offence. But it's still good enough that when you trigger the threshold, your interlock program can be extended.

Verdict: Accurate with a small margin of error (don't actually use it to casually test your BAC though).

Method 5: Blood Test

Requires drawing a sufficient amount of blood intravenously, storing it properly, and sending it to a government approved lab for processing.

Thoughts: If you want to know the blood alcohol content of a person, measuring the blood directly makes sense. And just as you'd expect, this is by far the most accurate method of measuring BAC. Mostly this happens after you've given a police breathalyser reading above what your licence allows. You'll be taken to a police tester van, station or hospital for a blood test to confirm your BAC. Although it's the most accurate method, the higher ups seemed to think it would be a bit draconian to use blood testing in alcohol interlocks (not to mention unhygienic and inconvenient).

Verdict: Perfect accuracy, but a pain (literally).


The only certain way to measure alcohol content in blood directly is with a blood test. But as we mentioned, that's a lengthy process which can take weeks to process and more sterilised needles than you probably have. The next most accurate method is the use of accredited Fuel cell breathalysers which are regularly calibrated (e.g. police breathalysers, alcohol interlock devices, and certain personal breath testers). So maybe next time you're standing on one leg trying to touch your nose, ask yourself how useful that really is. Also, you look ridiculous!