False-Positive Alcohol Readings on Ankle Monitors: Two Real Cases
From - Counterpoint Volume 2: Issue 4 - Article 6 (May 2018)
An article for participants in the myCAMprogram
Jan Semenoff, BA, EMA
Forensic Criminalist
Article information:
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1000 words (approximately 5-6 minutes)
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Introduction
Ankle monitors check for alcohol through your skin 24 hours a day. The readings are sent to the monitoring agency and may be used in court to decide if you followed your program rules.
Sometimes, these devices show a “positive” result for alcohol even when the person has not been drinking. These are called false positives. This article explains two real cases where that happened, and why.
How Transdermal Alcohol Devices Work
Figure 1 - A representative TAC device
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Transdermal Alcohol Monitors read the amount of alcohol in or around your body. They are worn on your ankle and uses a fuel cell sensor to detect alcohol vapor coming from your skin.
Instead of giving a blood alcohol concentration (BAC) like a breath test, it reports a transdermal alcohol concentration (TAC). TAC readings are then used to estimate what your BAC might have been.
Most of the time, alcohol in TAC readings comes from alcohol that was drunk and passed into your sweat. But sometimes, alcohol from outside sources — like sprays or vapors — can be detected instead.
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Case Study 1 – Static Guard™ Spray
In this case, the participant wore a brand-new blanket that had just been sprayed with Static Guard™, a product used to reduce static electricity. That night, the TAC device reported a positive alcohol reading.
An Important Note:
I need to point out that this article does NOT identify the failure of the TAC devices overall, nor does it indicate faulty or malfunctioning devices. It DOES identify potentially false-positive readings that are more a failure of the interpretation of the data presented, and probably an inappropriate applicationof the readings obtained.
I’ve written before about sub-standard acts, practices or conditions leading to substandard results. This is one of those cases, where the substandard condition (the application of Static Guard™) may have created a substandard (and therefore unreliable) reading. As for Case Study #2, the cause is unknown.
I need to point out that this article does NOT identify the failure of the TAC devices overall, nor does it indicate faulty or malfunctioning devices. It DOES identify potentially false-positive readings that are more a failure of the interpretation of the data presented, and probably an inappropriate applicationof the readings obtained.
I’ve written before about sub-standard acts, practices or conditions leading to substandard results. This is one of those cases, where the substandard condition (the application of Static Guard™) may have created a substandard (and therefore unreliable) reading. As for Case Study #2, the cause is unknown.
Why this was suspicious
Why this was suspicious
- The first positive reading was very high — equal to a BAC of 0.260 g/dL — with no gradual rise before it.
- To reach that BAC in 30 minutes, the person would have had to drink the equivalent of 13 standard drinks (about 20 ounces of hard liquor) and absorb it completely. Science shows this is extremely unlikely.
- The “elimination rate” (how fast the reading dropped) was much faster at the start, then slowed over time. This does not match how the human body metabolizes alcohol. Humans metabolize alcohol at a steady rate - called a Linear Elimination by toxicologists.
Let’s start by looking at the raw data. In this case, the non-compliance report provided the following Transdermal Alcohol Concentration (TAC) data:
Table 1 - Data from the SCRAM report. I've indicated 1/2 hour increments in red and blue, and indicated the elimination rates, broken down by the hour. Notice the changed in the reported elimination rate over time.
It is perhaps easier to visualize the reported elimination graphically:
Chart 1 - The reported Transdermal Alcohol Concentration at various times during the night in question. Notice the elimination does NOT follow the anticipated linear rate, as shown in the blue dashed line. Is this indicating the metabolism of alcohol in the human body?
Understanding Elimination Rates
The body removes alcohol at a steady rate — usually somewhere between 0.010–0.025 g/dL per hour for most people, and rarely up to 0.040 g/dL per hour for long-term heavy drinkers. This is called Zero Order Kinetics — the rate stays the same until BAC gets very low.
In this case:
- The first two hours showed elimination rates of 0.081 and 0.075 — far higher than the human body can achieve.
- Later hours showed much slower elimination.
- The pattern matched evaporation, not metabolism.
What This Means for You
Figure 2 - This is what would have had to be CONSUMED AND FULLY ABSORBED between 12:35 AM and 1:05 AM in order for the first SCRAM reading to be reliable. Possible? Sure... Likely?
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If your monitor ever shows a sudden, high alcohol level without any gradual climb before it, that’s a red flag. In one real example, the reading jumped from 0.000 to 0.260 in just 30 minutes.
For that to be true, the person would have had to drink the same as 13 standard drinks — about 20 ounces of hard liquor — all at once, and have it completely absorbed in half an hour. That’s not how the body normally works. Alcohol usually takes anywhere from a few minutes to over 3 hours to be fully absorbed. And the more you drink at once, the slower your body tends to absorb it.
But, we are assuming the poison of choice was hard liquor. Can you imagine drinking the equivalent to 13 glasses of wine in 30 minutes? That is about 2 1/2 bottles. Perhaps a dozen or more beer in 30 minutes... You decide...
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A Standard Drink is typically considered to be 12 ounces of beer at 5% Alcohol by Volume (ABV), 5 ounces of wine at 12% ABV, and 1.5 ounces of spirit liquor at 40% ABV.
Why this matters:
- A sudden jump like this is often a sign of an error, not actual drinking.
- You should document these kinds of readings right away and let your lawyer or supervision officer know.
- Keep track of what you were doing before the reading — your food, activity, and any products or chemicals you were around — so you can help explain unusual results.
Evaporation vs. Metabolism
Alcohol from products like Static Guard™ evaporates into the air and can be absorbed by the SCRAM sensor. Evaporation happens quickly at first, then slows down — a logarithmic pattern — which is different from the constant rate of alcohol metabolism in the body.
Static Guard™ contains 60–100% ethanol, plus propane, isobutane, soap, and fragrance. The ethanol in it is “denatured,” meaning a bittering agent is added so it can’t be drunk, but it still reacts like alcohol in a sensor.
Based on the chemical content and the elimination pattern, the reading was much more likely from contamination than from drinking.
Chart 1 - Again...
The evaporation rate of ethanol
The obvious question, therefore, is, “What is being indicated by the data obtained?
Ordinary evaporation of volatile [4] substances follow a complex logarithmic mathematical model, referred to in both thermal physics and chemistry as a Boltzmann Constant. When demonstrated graphically, it shows the following pattern:
Chart 4 - The evaporation rate of ethanol based on its concentration
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Chart 5 - The evaporation rate of 98% Ethanol.
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Note that in Chart 4, the higher concentrations of ethanol have a steeper arc [100%, black at bottom] than the 10% ethanol solution [Gray dashed, second from top], (Hasegawa, 2016).
Chart 5 shows the logarithmic elimination pattern of 98% pure ethanol in red (Tyukodi, 2012).
Both support that, graphically, ethanol elimination is NOT linear. Also, the time of the elimination can be measured in seconds, minutes, hours or in days. The logarithmic pattern is the same regardless of the duration of the study.
Chart 5 shows the logarithmic elimination pattern of 98% pure ethanol in red (Tyukodi, 2012).
Both support that, graphically, ethanol elimination is NOT linear. Also, the time of the elimination can be measured in seconds, minutes, hours or in days. The logarithmic pattern is the same regardless of the duration of the study.
[4]. Volatile in the chemical sense refers to any substance that evaporates at room temperature. Ethanol is considered a volatile chemical.
The chemical components of Static Guard ™
The Material Safety Data Sheet for Static Guard™ indicates the following chemical composition:
Table 2 - The chemicals found in Static Guard, taken from the manufacturer's Material Safety Data Sheet (MSDS). Note that the actual percentages of the formula are a trade secret. Federal regulations allow this in published MSDS. (Poison Control Physicians can access more detailed information under emergency conditions by accessing a secure repository of published chemical information data).
Denatured alcohol is simply ethanol that has a bittering compound added to it (typically Bitrex™ - a completely safe compound that is the Guinness Book of Records holder as the bitterest known compound in the world) that makes it taste extremely bitter and unpalatable to consume orally. The chemical nature of the ethanol itself is not otherwise altered. The propane and isobutane are used as a propellant in the compressed gas canister and are also volatile. Therefore, the formula is primarily ethanol, with a little surfactant soap (the quaternary compounds) and volatile fragrance.
Based on the chemical composition published by the manufacturer, we can conclude that the chemical composition is between 60-100% ethanol. The concentration of the ethanol itself is not listed. Most denatured ethanol used for industrial purposes is between 70 -100% Alcohol by Volume Concentration. We would therefore expect the evaporation rate to follow somewhere in the 70-100% range indicated in Figure 3.
The elimination rate reported in the first few hours is ridiculously high when compared to known human metabolism and tapers off over time in a non-linear fashion. This data clearly follows the pattern of alcohol evaporationrather than alcohol metabolismand is more likely than not a false-positive contamination rather than an indication of alcohol consumption.
I think it is always important that forensic investigators rely upon as complete a picture as possible in order to arrive at a logical conclusion. This case involves the data set from the device used, including the usage logs, and in comparison, to the known science of both evaporation and metabolism. I would respectfully suggest that the data supplied by the TAC device supports the normal evaporation of ethanol outside the body and does not support a reasonable assertion that the metabolism of ethanol internally was correctly identified and reported by the device.
Case Study 2 – Unknown Cause
In the second case, the transdermal device showed large changes in TAC during an overnight period. The readings went up and down in a way that suggested both drinking and eliminating alcohol at the same time — but that’s impossible to measure accurately.
Near the end:
- One hour showed elimination rates of 0.023 and 0.022 (possible for some people).
- The last hour jumped to 0.036 — not consistent with normal human metabolism.
Table 3 - The SCRAM report data from Case Study #2, showing TAC levels at the time of their measurements/
The Key Fact:
At the same time the transdermal device showed alcohol, the person blew 0.000 on a car ignition interlock device — and was able to start the vehicle. Both devices cannot be correct.
Why Context Matters
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Neither of these cases proves that the transdermal devices “don’t work.” Instead, they show why it’s important to:
Without knowing about the Static Guard™ spray or the zero reading on the car device, the transdermal results could easily be misunderstood as proof of drinking.
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What This Means for You
If you are in a CAM program:
- Know what can cause false positives — sprays, cleaners, industrial chemicals, and some personal care products.
- Keep good records — note what products you used, your activities, and any other alcohol testing you did (like a breath test).
- Report other test results — if another device shows zero alcohol at the same time as your CAM device shows positive, this matters.
Key Takeaways
- Ankle monitors measure alcohol through your skin, but can also detect alcohol from external sources.
- True alcohol metabolism has a steady elimination rate; contamination often shows a fast-then-slow drop.
- Environmental alcohol can come from sprays, cleaners, fuels, or other products.
- Always document anything that could affect your readings.
Practice Tip
If you get a positive result:
- Perform a retest right away and record any products, sprays, or chemicals you were near.
- Keep copies of other alcohol test results from the same time.
- Use your personal breath tester to test yourself. Video the test if possible.
Final Reminder
Your CAM device is only part of the story. Without context, results can be misunderstood. By keeping records and understanding how your device works, you can help protect yourself from false positives.
Send me your questions or comments:
Comments and questions will be posted here with their responses:
Comments and questions will be posted here with their responses:
For further reading:
- Hasegawa, K., Abe, Y. & Goda, A., Microlayered Flow Structure Around an Acoustically Levitated Droplet Under a Phase-Change Process.npj Microgravity. 2. 16004. 10.1038/npjmgrav.2016.4. (2016).
- Jones, A.W. and Andersson, L., Influence of Age, Gender and Blood-Alcohol Concentration on Disappearance Rate of Alcohol from Blood in Drinking Drivers, Journal of Forensic Science 1996; 41(6), pages 922-926.
- Jones, A.W., Evidence-Based Survey of the Elimination Rates of Ethanol from Blood with Applications in Forensic Casework, Forensic Science International 200 (2010) 1-20.
- Jones, A.W., Disappearance Rate of Ethanol from the Blood of Human Subjects: Implications in Forensic Toxicology, Journal of Forensic Sciences, JFSCA, Vol. 38, No. 1, January, 1993, pages 104-118.
- Jones, A.W., Biochemical and Physiological Research on the Disposition and Fate of Ethanol in the Body, Garriott’s Medicolegal Aspects of Alcohol, 5thEdition, Chapter 3, pages 47-156.
- Neuteboom, W. and Jones, A.W., Disappearance Rate of Alcohol from the Blood of Drunk Drivers Calculated from Two Consecutive Samples; What Do the Results Really Mean?,Forensic Science International, 45 (1990), 107-115.
- Tyukodi, B, et al, The Boltzmann Constant from a Snifter, European Journal of Physics, 33 (2012) 455-465.
