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Breath Tests

The Validity Of Breath Testing

The validity rests upon a vast array of assumptions that can be attacked by an experienced DUI attorney in a wide variety of cases:


  • The machine is calibrated and in proper working order.
    Even if everything is in proper working condition, there can be an acceptable variation of +/- .02. If your result is a .10% that’s a 20% error factor. How would you feel if the altimeter of an airplane was 20% off?
  • The sample was properly attained.
    The law requires that you be under continuous observation for 15 minutes prior to your breath test to prevent “MOUTH ALCOHOL” If the operator of the breath machine did not have you under continuous observation for 15 minutes prior to testing, your test may be invalid. Even if you were observed, you may have medical conditions, which make you an improper candidate for breath testing. These conditions include common ailments such as heartburn and gum disease.
  • The sample was obtained after you absorbed all the alcohol.
    Breath testing assumes that the subject is post absorptive. This is important because the machine is calibrated to multiply the breath result by 2100 to calculate your blood alcohol level. If you were still absorbing alcohol, this would tend to significantly over report your true blood levels.
  • The temperature of your breath was 34 degrees centigrade.

This is an assumption that is made in every breath test in Orange County. State of the art breath testing equipment actually measures the temperature of your breath and will subtract any false high result due to elevated breath temperature. Studies done with this equipment establishes that the real average breath temperature for arrestees in DUI cases is closer to 35.5 with readings actually over 37. This alone would cause a false high of between 10-20%

10 Common Challenges To Breath Tests

  1. The machine was not properly operated
  2. The machine was not properly maintained
  3. The machine was not properly calibrated
  4. The machine was affected by radio frequency tnterference (Police Radio etc.)
  5. The machine has an “acceptable” range of +/- .02%
  6. Your body temperature was greater than 98.6F
  7. You were tested within 2 hours from your last drink
  8. You have medical or dental problems that make you an improper candidate for breath testing
  9. The officer did not have you under continuous observation for 15 minutes prior to giving you the breath test
  10. Your symptoms and drinking pattern are inconsistent with the test result

For example, the officer could have simply held the machine with his hand incorrectly if it was an Alcotest 7410 Plus. It has an exit port (a hole for your air to escape after it passes through the machine).  If the officer’s hand covered the hole even a little, it may have increased the reading.  See this memo from the Attorney General’s office: 7410 exit port flaw AG memo

The breath test instrument should have a “slope detector” to help ensure that the breath sample measured was not effected by “mouth alcohol”. See Slope Detectors The handheld devices used in Orange County are not equipped with slope detectors.

Low carbohydrate diets can cause false-high readings, adding an expected .02% to your true BAC. See Adkins – False Positive Breath

Other articles related to breath testing:

Physiological Aspects of Breath Alcohol Measurement

Partition Ratio – Blood sample compared to Breath

mouth alcohol

GERD by Jones AW

Breath tests and GERD

Breath Temperature assumptions and truth

Breath test during absorption