Accuracy of Ignition Interlock Devices (IIDs): The Truth

Ignition interlock devices (IIDs) are alcohol breath testers. They prevent a drivers from starting a vehicle if alcohol is detected in their breath. But what is the accuracy of ignition interlock devices?

As such, they have the same problems  plaguing other breath testers. Makers gloss over the many faults of the machines they sell. They’re frequently guilty of making claims not found by researchers. So what is the accuracy of ignition interlock devices? It varies widely.

Blood alcohol concentration (BAC) can only be measured by analyzing a sample of blood. Breath alcohol testers try to measure alcohol in the breath. The devices then use that to estimate BAC. That’s why not all states permit their use. Simply put, alcohol breath testers can convict completely innocent people.

It follows that the poor accuracy of ignition interlock devices can unfairly prevent people from driving their vehicles


I.  Some Possible Sources of Error

II. Summary & Conclusion

I. Some Possible Sources of Error

Incorrect Ratios

Breath testers (really, breath estimators) assume that that certain ratios are constant in the human body. But these ratios vary widely from person to person. And even within the same person during the day.

For example, many breath estimators assume a 2,100-to-1 ratio. That is, in converting alcohol in the breath to estimates of alcohol in the blood. But this ratio varies from 1,900 to 2,400 among people. And also the time of day. These variations will lead to false BAC readings.

Similarly, some breath testers assume a hematocrit (cell volume in blood) of 47%. Yet hematocrit values range from 42% to 54% in men. And they range from 38% to 46% in women. As a result, people with a lower hematocrit will have a falsely high BAC reading.

Many Compounds Detected as Alcohol

Many alcohol breath testers detect as alcohol any chemical compounds that contain the methyl group in its structure. Yet there are thousands of these compounds. Some occur naturally in the body and breath. Others are picked up. This can result from breathing fumes from alcohol, glue paint, paint remover, cleaning fluids, etc. Even from “new car smell.”

accuracy of ignition interlock devices
Ignition Interlock Device (IID)

Another source of false readings is smoking. Long-term heavy smokers are more likely to have falsely high BAC readings than non-smokers. That’s because smoking greatly increases the production of acetaldehyde. This substance can be detected and measured as alcohol.

Acetone on the breath is another cause of false BAC estimates. Research shows that acetone can be high enough in normal people to cause false high BAC readings.

Also, either fasting or dieting can cause much higher acetone levels. They can be high enough to cause a false BAC reading of .06.

Or a low-carbohydrate diet can increase acetone levels. The body makes more acetone as it tries to make up for the reduced glucose in low-carbohydrate diets. The Atkins and similar diets are designed to achieve a condition known as “low carb ketosis.”  This is to promote the burning of fat.

Ketones create isopropyl alcohol, which is then converted into acetone. Thus, a side effect of ketosis is that the high levels of acetone produced causes it to be exhaled. There it can be falsely identified as alcohol. This occurs among teetotalers.

These false readings are in addition to the presence of any other compounds in the methyl group. And there are over 100 of which can occur in the breath.

Endogenous Ethanol (Alcohol) Production

Alcohol exists in the breath of all living humans. It’s from from endogenous (that is, internally produced) ethanol. Natural bacteria, primarily in the intestines, produces alcohol 24/7. Carbohydrate-rich diets are linked with higher levels of such alcohol production.

The term “auto-brewery syndrome” describes people who show signs of intoxication because of endogenous ethanol production. This usually occurs after eating meals rich in carbohydrates. Some alcohol abstainers produce enough alcohol within their bodies to become legally intoxicated and arrested while driving.


Elevated body temperature is another source of error. Each one degree Centigrade above the standard body temperature can falsely raise the estimate by seven percent. The body temperature of a normal person will vary as much as one degree from the standard temperature. It can also vary by that much during a normal day.accuracy of ignition interlock devices

Also air temperature can cause false BAC readings.

Radio Frequency Interference (RFI)

RFI is another source of possible error. But the RFI detectors that available for use are based on technology that’s over half a century old. They aren’t reliable, and don’t solve the problem.

Lack of Precision

Even under ideal conditions, BAC testers lack precision. Law profesor Lawrence Taylor  explains that scientists universally recognize an inherent error in breath analysis. It’s generally plus or minus about .01% BAC. Courts across the U.S. accept this as fact. Hawaii courts recognize a larer inherent error of .015 BAC.

This means that under ideal conditions, a BAC reading of .08 actually reflects an actual BAC of anywhere from .07 to .09. And Hawaii recognizes that it could be from .065 to .095. That’s a margin of error of 20 to 30 percent in the accuracy of ignition interlock devices.

II. Summary & Conclusion: Accuracy of Ignition Interlock Devices

Ads for alcohol breath testing devices tout “unparalled accuracy” and contain other puffery. But they give no guarantee of accuracy in measuring BAC. Much more often, they disclaim it.

Sellers don’t guarantee that their devices accuracy measure BAC. That’s because the devices only estimate BAC. Faulty breath testers can put innocent people behind bars. Poor accuracy of IIDs probably cause few problems that serious. But problems can be serious. For example, a sober driver who can’t start her car to rush her baby to the emergency room. Or a sober man who can’t start his car may lose his job. The list of serious problems from IIDs is a long one.

This doesn’t mean IIDs shouldn’t be used. To the contrary, they prevent needless traffic deaths. But the alcohol detection needs to be high enough to avoid problems caused by faulty BAC estimates. The accuracy of ignition interlock devices is very important.