When police suspect impaired driving, they rely on scientific instruments and trained evaluators to measure intoxication. But how accurate are these methods? Understanding the science—and limitations—behind DUI testing reveals why these cases aren’t as straightforward as they appear.
Understanding Blood Alcohol Concentration
Blood Alcohol Concentration (BAC) measures the amount of alcohol in your bloodstream, expressed as a percentage. In Canada, the legal limit is 80 milligrams of alcohol per 100 milliliters of blood (0.08% BAC).
However, BAC is a proxy measurement. What really matters is brain alcohol concentration—the amount of alcohol affecting your cognitive and motor functions. The assumption that blood alcohol directly correlates to impairment has limitations, which creates opportunities for legal challenges.
Absorption and Elimination Rates
Alcohol doesn’t instantly appear in your blood. After consumption, it takes 30 to 90 minutes to reach peak absorption. This timing creates the “rising blood alcohol defence”—you might have been under 0.08 while driving but over the limit when tested later.
Your body eliminates alcohol at approximately 10 to 20 milligrams per 100 milliliters per hour, though this varies based on metabolism, body composition, food consumption, and other factors. These variables matter tremendously in court.
Roadside Screening Devices vs. Approved Instruments
Not all breath testing devices are created equal.
Approved Screening Devices (ASD)
These portable units are used roadside. In Canada, common models include the Dräger Alcotest 6810 and the Alco-Sensor IV. They provide a simple pass/fail reading or a general range rather than a precise BAC measurement.
ASDs are less accurate than station instruments because:
- They’re more susceptible to environmental factors (temperature, humidity)
- Mouth alcohol can create falsely high readings
- Calibration is less stringent
- They use different detection technology (fuel cell vs. infrared)
Critically, ASD results are generally not admissible as evidence of your actual BAC in court—they’re used to establish grounds for further testing.
Approved Instruments (Breathalyzer)
At the police station, you’ll blow into an approved instrument like the Intoxilyzer 8000C or the Datamaster C. These sophisticated devices use infrared spectroscopy to measure alcohol molecules in your deep lung air.
The testing protocol requires:
- Two samples taken 15 minutes apart
- Results within 20 milligrams of each other
- A 15-minute observation period beforehand (to ensure no mouth alcohol)
- Regular calibration and maintenance
- Qualified technician operation
Despite these safeguards, approved instruments have vulnerabilities that the best dui lawyers exploit regularly: software glitches, calibration errors, improper administration, and physiological factors that skew results.
Sources of Error in Breath Testing
Mouth Alcohol Contamination
Alcohol in your mouth—from recent drinking, belching, vomiting, or gastroesophageal reflux disease (GERD)—can create readings significantly higher than your actual BAC. This is why the 15-minute observation period is mandatory, though officers don’t always follow it properly.
Radio Frequency Interference (RFI)
Police radios, cell phones, and other electronic devices can interfere with breath instruments, causing false high readings. Proper testing protocols require eliminating RFI sources, but this doesn’t always happen.
Partition Ratio Assumptions
Breathalyzers assume a 2100:1 ratio between breath alcohol and blood alcohol. This ratio varies significantly between individuals (range: 1300:1 to 3100:1). Someone with a lower partition ratio could blow over 0.08 while their actual blood alcohol is under the legal limit.
Medical Conditions
Diabetes, hypoglycemia, and ketogenic diets can produce acetone, which some instruments misread as alcohol. Additionally, lung conditions affecting breath temperature or volume can skew results.
Temperature and Hematocrit
Breath temperature significantly affects readings—each degree Celsius above 34°C increases the reading by approximately 6.5%. Fever or simply being in a hot vehicle can artificially elevate results.
Drug Recognition Expert Evaluation
When police suspect drug impairment but not alcohol, they call a Drug Recognition Expert (DRE)—an officer with specialized training in detecting drug impairment.
The 12-Step DRE Evaluation
- Breath alcohol test (to rule out or confirm alcohol)
- Interview with arresting officer
- Preliminary examination (pupil reaction, eye tracking)
- Eye examinations (horizontal gaze nystagmus, vertical gaze nystagmus, lack of convergence)
- Divided attention tests (one-leg stand, walk and turn, Romberg balance, finger to nose)
- Vital signs (blood pressure, temperature, pulse)
- Dark room examinations (pupil size in different lighting)
- Muscle tone examination
- Injection site examination
- Subject’s statements and interview
- DRE’s opinion and conclusions
- Toxicological examination (blood or urine sample)
DRE Limitations
While DREs undergo 72 hours of training, the evaluation remains subjective. Many factors can mimic drug impairment:
- Fatigue and sleep deprivation
- Medical conditions (neurological disorders, inner ear problems)
- Anxiety from the police interaction itself
- Prescription medications
- Natural variations in pupil size and eye tracking
Research shows DRE accuracy rates vary significantly, with false positive rates as high as 10-15% in some studies. The subjective nature of these evaluations creates substantial room for legal challenge.
The Margin of Error
No scientific instrument is perfectly accurate. Approved instruments in Canada typically have a margin of error around ±10-20 milligrams. This means a reading of 90 could actually represent a true BAC between 70 and 110.
This margin of error is crucial for cases where readings are close to the legal limit. A reading of 85, for instance, could actually be below 80 when accounting for instrument uncertainty.
Why the Science Matters for Your Defence
Understanding breath testing science is essential because it reveals potential weaknesses in the Crown’s case. Technical errors, procedural violations, and physiological factors can all undermine the reliability of test results.
If you’re facing DUI charges, the test results showing you were over the limit may not tell the complete story. A toronto dui lawyer with expertise in the scientific aspects of breath testing can identify technical defences that might not be obvious to someone unfamiliar with the complexities of these instruments.
The Bottom Line
Breath testing and drug recognition evaluations are scientific processes, but they’re far from infallible. Equipment malfunctions, human error, physiological variations, and environmental factors all introduce uncertainty into results that might seem definitive.
The prosecution will present breathalyzer results as unassailable scientific fact. But the science is actually quite nuanced, and many readings can be successfully challenged by legal counsel who understands both the technology and its limitations.
If you’re facing impaired driving charges, don’t assume the test results are conclusive. The science behind DUI testing is complex, and skilled legal representation can identify weaknesses that make the difference between conviction and acquittal.
