Breathalyzer slope detection is a software feature inside evidential breath testing machines that watches the shape of your breath-alcohol curve during a single exhale, looking for the spike-and-drop pattern that signals alcohol lingering in your mouth or throat rather than coming from deep in your lungs. When the curve looks wrong, the device refuses to record a number and shows an error instead. The feature exists to protect against falsely high readings, but peer-reviewed research has found it catches contamination only about half the time, and manufacturers keep the exact logic hidden as a trade secret.
What the Software Is Actually Watching
Alcohol concentration in a breath sample doesn’t stay flat. It rises steeply at first as air from the upper airways passes the sensor, then climbs more gradually as deeper lung air, called alveolar air, reaches it. Alveolar air gives the most accurate reading because it has been in close contact with blood-rich capillaries. A valid sample curve rises and levels off into a plateau.1Texas District and County Attorneys Association. Breath Testing for Prosecutors
Slope detection tracks the rate of change on that curve in real time. A positive slope means the concentration is still climbing or holding steady. A negative slope means it has started to fall. On the DataMaster DMT, for example, a positive slope is any comparison of consecutive data-point averages where the trend is not declining, and the device produces an “Invalid Sample” message when it detects three consecutive negative comparisons after first recording at least six positive ones.2Vermont Department of Health. VT DMT Explanation of Status Codes and Their Limits A rise followed by a drop is the classic signature of mouth alcohol.
How the Device Reads the Breath
Evidential breath testers use one of two sensor technologies, sometimes both. Infrared devices shine light through the sample and measure how much gets absorbed at wavelengths where ethanol absorbs strongly, typically 3.4 microns and sometimes 9.5 microns. The sensor takes multiple readings per second to build a continuous concentration curve.
Fuel cell devices use an electrochemical reaction instead: alcohol oxidizes on a platinum electrode and generates a small current proportional to concentration. For mouth alcohol detection, fuel cell instruments often use duplicate sensors positioned to measure the breath at slightly different moments during the exhale, and flag the sample if the two readings diverge beyond a set threshold.3PubMed Central. The Limitations of Mouth Alcohol Detection Systems in Breath Alcohol Testing – Case Reports
Before slope analysis even begins, the device enforces minimum sample requirements. The Draeger Alcotest 9510 requires a starting airflow of at least 8.0 liters per minute, a sustained minimum flow of 4.0 liters per minute, at least 5 seconds of blowing, and a minimum breath volume of 1.5 liters. Plateau detection then kicks in, with a plateau defined as an alcohol concentration increase of 4% or less within one second.4Washington State Patrol. Draeger Alcotest 9510 for Legal Professionals Only after the breath clears all those thresholds does the instrument record a result.
Why Mouth Alcohol Is the Main Target
Residual alcohol sitting in the mouth or throat is what slope detection is built to catch. After a recent drink, a belch, or vomiting, concentrated alcohol droplets can linger on the tongue, cheeks, or in the throat. Those droplets hit the sensor immediately and produce a sharp spike that dwarfs what deep lung air would show. The spike then falls off rapidly as the mouth alcohol gets blown past the sensor and lung air arrives.
Deep lung air produces a smoother curve: a steady climb into a plateau, no dramatic drop. The algorithm is looking for that difference. The stakes are real. Mouth alcohol can push a breath test well above someone’s actual blood alcohol level, and someone genuinely below the legal limit can register above it if contamination from a recent sip, a burp, or alcohol-containing mouthwash reaches the sensor.
How Reliable Slope Detection Actually Is
This is where the comfortable version of the story breaks down. One peer-reviewed investigation found that a slope detector’s ability to identify mouth alcohol contamination was approximately 52%, essentially a coin flip.3PubMed Central. The Limitations of Mouth Alcohol Detection Systems in Breath Alcohol Testing – Case Reports Roughly half the time mouth alcohol was present, the device recorded a result anyway.
The reliability problem is compounded by secrecy. Manufacturers treat their slope detection algorithms as proprietary trade secrets, which keeps independent scientists from scrutinizing exactly how the software decides whether a sample is valid.3PubMed Central. The Limitations of Mouth Alcohol Detection Systems in Breath Alcohol Testing – Case Reports There is no industry-wide consensus on the parameters that should trigger a mouth alcohol flag. Different devices from different manufacturers use different thresholds and different detection methods, yet courts treat the results as scientifically reliable.
Defense attorneys have pushed back on the secrecy. In State v. Chun (2008), defense experts in New Jersey obtained and reviewed the source code of the Alcotest 7110 and identified numerous software errors. Massachusetts followed with Commonwealth v. Camblin (2015), where the state’s highest court ordered evidentiary hearings on whether source code errors made the same device unreliable, whether it was sufficiently specific to alcohol versus other compounds, and whether calibration procedures were adequate. Those decisions opened a path for defendants elsewhere to demand similar scrutiny.
What Else Can Trigger a Slope Error (Or Slip Past It)
A slope flag doesn’t mean the subject just took a drink. Several medical conditions and substances produce the same spike-and-drop pattern, and the same conditions can also produce false readings that the algorithm fails to catch.
GERD and Reflux
Gastroesophageal reflux disease is probably the most commonly raised medical issue in breath test challenges. GERD lets stomach contents, including alcohol vapors, travel up the esophagus and into the mouth. Hiatal hernias and chronic acid reflux cause the same problem. Even a single silent reflux episode during a test, which the subject may not feel, can send concentrated alcohol vapor into the breath stream and produce an erratic curve that either triggers a flag or, given the 52% rate, passes through undetected. Courts generally expect medical records documenting the condition and often expert testimony from a forensic toxicologist tying the condition to the specific test at issue.
Dental Work
Bridges, dentures, deep fillings, and other dental appliances can trap small amounts of liquid alcohol in crevices. Those pockets release concentrated vapor during a test, creating the same contamination pattern. Whether the slope system catches it depends on how fast the trapped alcohol dissipates relative to the sampling window.
Asthma Inhalers
A study of 60 volunteers found that common asthma inhalers containing salbutamol, salmeterol, and fluticasone produced positive breath alcohol readings on the Alcotest 7110-E immediately after use, even though the inhalers contained no ethanol. Readings decreased linearly and returned to zero at about 10 minutes. Researchers identified the propellant gases, specifically chlorofluorocarbons, as the likely cause.5PubMed Central. Using Asthma Inhalers Can Give False Positive Results in Breath Tests Using a rescue inhaler shortly before a traffic stop can produce a reading that mimics mouth alcohol, or one the slope detector lets through.
Ketosis and Acetone
People on very low-carbohydrate diets, fasting, or managing uncontrolled diabetes produce elevated acetone through fat metabolism. Breath acetone in ketosis can range from 2 to 40 parts per million.6National Center for Biotechnology Information. The Ketogenic Diet – Breath Acetone Sensing Technology Modern infrared instruments have improved specificity, but the federal standard for approved evidential devices only requires that they distinguish alcohol from acetone at the 0.02 concentration level.7U.S. Department of Transportation. Approved Evidential Breath Measurement Devices Acetone interference at very low apparent BAC levels can still slip through.
What Happens on the Screen When It Flags
When slope detection catches an invalid pattern, the device stops the test and displays an error rather than a BAC number. The DataMaster DMT shows “Invalid Sample” for a declining concentration trend.2Vermont Department of Health. VT DMT Explanation of Status Codes and Their Limits Other models may show “Mouth Alcohol Detected” or “Check Ambient Air” depending on the manufacturer and the anomaly.
Standard protocol requires the officer to restart the pre-test observation period after an error. That period is typically 15 to 20 minutes, during which the officer is supposed to watch the subject continuously and make sure they do not eat, drink, smoke, belch, or vomit. If any of those happens, the timer resets. The point is to let any residual mouth alcohol dissipate before the next attempt.
Once the observation period finishes cleanly, the officer can run the test again. Most jurisdictions require two valid breath samples that agree within a set tolerance, often 0.02 g/dL, for the results to be admissible. If the samples don’t agree, more tests follow.
When Repeated Errors Look Like a Refusal
Multiple failed breath tests can start to look like a test refusal from the state’s perspective, even when the subject is genuinely trying to blow. Nearly every state has established separate penalties for refusing a BAC test, and the model DWI code recommends setting more severe penalties for refusal than for failing the test.8National Highway Traffic Safety Administration. BAC Test Refusal Penalties Typical refusal penalties include administrative license revocation or suspension, and at least 12 states treat test refusal as a separate criminal offense.
Some states explicitly define repeated deficient breath tests as a refusal by statute. A person with GERD, dental work, or breathing difficulties who triggers multiple slope errors through no fault of their own can face the same license suspension as someone who flatly refused. Telling the officer about any medical condition that could affect the test, and asking that the explanation be documented, creates a record that may help later at an administrative hearing or in court.
The officer may also offer or require a blood test as an alternative after repeated breath test failures. Following Birchfield v. North Dakota, a warrant is generally required for a blood draw unless exigent circumstances exist.8National Highway Traffic Safety Administration. BAC Test Refusal Penalties A blood test eliminates the mouth alcohol issue entirely, which can work in the subject’s favor if the breath instrument was reading artificially high.
What a Passing Slope Check Actually Tells You
Slope detection is often described in court as a fail-safe: if mouth alcohol had been present, the machine would have caught it. That argument carries less weight once you know these systems catch contamination roughly half the time, their internal logic sits behind trade secret claims, and multiple state courts have found enough concern about software reliability to order full evidentiary hearings.
A result that passed slope detection is not necessarily free of mouth alcohol. It means the contamination pattern, if present, didn’t cross that particular device’s algorithmic threshold on that particular blow. Given the variation in detection methods across manufacturers and the absence of standardized parameters, the fact that an instrument accepted a sample tells you less about sample quality than prosecutors tend to suggest.
Anyone challenging a breath test on these grounds should look for the device’s maintenance and calibration records, the officer’s observation period documentation, any medical conditions that could introduce contaminants, and whether the jurisdiction has allowed source code review of the specific instrument model. Slope detection is a useful engineering feature. Treating it as scientifically infallible overstates what the research supports.