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How the “Blood-Breath Partition Ratio” Affects Your DUI Breath Test Results

DUI Breath Tests & The “Blood-Breath Partition Ratio”

A DUI breath test machine is used to calculate your blood alcohol concentration or BAC, but it is only able to detect the amount of alcohol vapor in your breath because it does not actually test your blood. Because DUI breath test machines have to calculate your BAC-based entirely on breath results from alcohol vapor, the results can sometimes be completely wrong.

DUI breath test machines use a ratio called the “blood-breath partition ratio” to use your breath results to calculate your actual blood alcohol concentration. The machines use the same blood-breath partition ratio for everyone, despite the fact that the ratio is not always accurate for every person.

What is the Blood-Breath Partition Ratio?

The blood-breath partition ratio, or BBP ratio, is a number used to reflect how much alcohol your lungs absorb from the blood. In Ohio, the BBP ratio is set at 2100:1, or assuming that 2100 milliliters of alveolar air (deep lung air) will have the same quantity of alcohol as 1 milliliter of pulmonary arterial blood when the person exhales air at the average temperature of 34 degrees Celsius. Ohio continues to use this ratio despite ongoing scientific debate regarding the validity of this ratio and suggesting that the actual ratio for some people can vary from person to person as much as 1117:1 to 7289:1. The proper BBP ratio can also depend on specific bodily factors. For instance, one study suggested that the mean BBP ratio for healthy males with “normal” body temperatures was 2280:1, while the ratio could potentially be different for women, unhealthy persons, or any person with a variance in body temperature.

For instance, according to one study, a person with a partition ratio of 1555:1 would cause the Intoxilyzer 500 breath test machine to overestimate by 26% the blood alcohol concentration compared to that of a simultaneous blood sample. Because the breath test machines do not measure a person’s blood breath partition ratio but instead are simply calibrated with the assumed value of 2100:1, we cannot know what the person’s actual partition ratio was at the time of the breath test. As a result, these breath test machines can often be inaccurate indicators of a person’s actual blood alcohol concentration any time the partitioning of alcohol from their blood deviates from the assumed 2100:1 ratio.

What are Potential Problems with the Blood-Breath Partition Ratio?

Ultimately, one problem with the blood-breath partition ratio that can be used by your attorney to defend against your DUI breath test results is that the ratio may not be accurate for everybody all the time. Because different people’s lungs absorb alcohol at very different rates, the BBP ratio can be significantly wrong in any given case. If it was wrong in your case, you could have had a high breath test result despite not being that intoxicated or truly over the legal limit.

For every person, the blood-breath partition ratio indicating the rate at which their lungs absorb alcohol can change under different circumstances. Some factors that may cause the BBP ratio to be “off” for you include if your breath temperature was higher than 34 degrees Celsius for any reason, which would result in a higher BAC reading form your breath test nearly 23 percent higher for every 1 degree Celsius increase in body temperature. If you had a fever at the time of your breath test, you could have had a false high BAC reading, or if you were dehydrated for any reason the reading could be artificially high. In addition, every person’s individual metabolism is different, which can cause significant variation from one individual’s BBP ratio to the next. Finally, hematocrit levels, or the ratio of the solid component of blood to the liquid, have also been reported by various studies to affect the validity of the blood-breath partition ratio.

DUI Defense & The Blood-Breath Partition Ratio

Without being given both a blood test and a breath test at the same time, it is impossible to know for sure whether the BBP ratio was accurate for you when you were given the DUI breath test. However, your DUI attorney can try to introduce evidence that suggests your BBP ratio could have been different at the time of your breath test, such as that you were sick or running a fever; you were dehydrated; you were engaged in an activity that may have raised your body temperature and breath temperature; or your car, the weather, or the precinct where you were tested was particularly warm at the time you were given the breath test. While questioning the BBP ratio’s accuracy for you at the time of your breath test is not guaranteed to have your breath test results suppressed, it could make an important difference in weakening the prosecution’s case against you, especially if your BAC was close to .08%.

Ultimately, a qualified DUI defense attorney can at least look to use any available evidence to suggest that your BBP ratio deviated from the 2100:1 average at the time of your breath test, causing the breath test to report an inaccurately high reading.