Breathalyzer False Positive

Article Index

Criminal Law Resources

Find a DUI Lawyer

DRUG-CRIMES home

Breathalyzer Errors & False Positives - Defending a DUI arrest

Breath analyzers, often referred to as a Breathalyzer (formally a brand name), only estimate blood alcohol concentration (BAC).  They do not provide actual blood alcohol concentration results.  Analysis of a blood sample is required to provide an accurate result. Estimated blood alcohol readings obtained from a breath analyzer can be:

  • Accurate.
  • Inaccurate.
  • Provide a false positive due to alcohol detection with accurate or inaccurate readings.
  • Provide a false positive due to external, non-alcohol influences with accurate or inaccurate readings.

Breathalyzers are manufactured under various brand names including:

  • Intoxilyzer
  • Alcosensor
  • Alcoscan
  • BAC Datamaster

Different types of breath analyzers use different methods to estimate alcohol concentration which can produce different results depending upon various factors of the person being tested.  Larger breath analyzers are more accurate than smaller handheld models.  Some states do not permit breath analyzer readings obtained from handheld breath analyzers to be admitted as evidence in court.

Some breath alcohol testers, particularly infrared breath testers, detect any compound containing the methyl group structure as well as ethyl alcohol (or ethanol), the alcohol found in alcoholic beverages.  70-80 compounds with the methyl group structure can be found in the human breath at any one time and will be incorrectly detected as ethyl alcohol. 

Compounds in the methyl group include:

  • Acetic Acid
  • Butane
  • Butadiene
  • Dimethylamine
  • Dimethylether
  • Dimethylhydrazine
  • Ethane
  • Ethyl chloride
  • Isopropyl Alcohol
  • Methane
  • Propane
  • Propylene

Breath testers which detect a greater number of types of ethyl group substances will produce a higher false blood alcohol concentration reading than analyzers which are designed to test a specifically smaller group of substances, because of the cumulative effect when the breath alcohol tester adds all of the compounds together.

According to the National Highway Traffic Safety Administration (NHTSA), dieters and diabetics can have acetone levels hundreds and even thousand of times higher than that in other people.  Acetone is one of many substances that can be falsely identified as ethyl alcohol by some breathalyzers, thereby showing a false positive reading.

Some breath testers assume a hematocrit level in the blood of 47% without taking into account that hematocrit values range from 42 to 52% in men and from 37 to 47% in women.  A person with a lower hematocrit count will have a falsely high estimated blood alcohol reading.

In the other direction, you may be able to falsely lower your estimated BAC reading with strenuous exercise for several minutes.  Running up a couple of flights of stairs or doing jumping jacks for 15 minutes might do the trick.

Substances in the environment can also lead to false BAC readings. For example, someone working with products such as contact cement, oil-based paints, varnish, lacquers, paint removers, celluloid, gasoline, cleaning fluids and other similar materials can show to be legally intoxicated when tested with a breath analyzer.

Certain food, alcohol and other items in a person's mouth can also cause false BAC levels.  For instance, a sip of alcohol just prior to being tested or possibly even eating a chocolate or cake filled with liqueur.  One person who reported his blood alcohol test results at a party using a AlcoHAWK stated "Subjected to first test of the night was Meaghan. She had downed two "chocolate cakes", which are baked from vodka and Frangelico (hazelnut liqueur). Her breath revealed a .05 BAC."

Other items in a person's mouth which can cause false positives are tobacco smoke, blood, vomit, dirt, and moisture.

External influences which can produce false BAC positive ratings include electrical interference from a cell phone or police radio.  If standing directly under high tension wires, it may be possible for the EMF (electromagnetic field) to cause a false positive reading.  An EMF tester can later be taken to the exact location of the test to determine the strength of the electromagnetic radiation.

Breath analyzers are sensitive to temperature and may give false readings if not adjusted or recalibrated to account for ambient or surrounding air temperatures.

Were you sick at the time of your DUI arrest?  The temperature of the subject is also important. For each one degree of body temperature above normal, a breath alcohol tester will cause an increase of approximately 8% in the estimated BAC reading.

Many breath testing machines assume a 2,100-to-1 ratio when converting alcohol in the breath to estimates of alcohol in the blood.  The actual ratio among different people will vary from 1,900 to 2,400 and will also change over a period of time after drinking, leading to another cause of false blood alcohol readings.

Routine maintenance and recalibration, by the police officer or other police official, of a breath alcohol analyzer is vital.  Failure to maintain, recalibrate and test the breath analyzer will result in additional errors.

It has been reported that results of estimated blood alcohol content from breath test readings vary as much as 15% from the true blood alcohol concentration and that approximately 23% of people tested with a breath analyzer will show estimated blood alcohol content to be higher than the true blood alcohol content.

A blood test showing actual blood alcohol content can avoid false positives obtained from using a breathalyzer.

How much blood alcohol content equals one drink?  Etoh (abbreviation for ethanol) of 10g is approximately 1 standard alcoholic drink.

For information on alcohol research, see the Alcohol and Alcohol Problems Science Database