TAC Monitor Case Study #3: ​

Identifying a Problem with Device Operation

From - Counterpoint Volume 8; Issue 3 - Article 6 (December 2024)

An article for participants in the myCAMprogram

Jan Semenoff, BA, EMA
​Forensic Criminalist

Article information:

Article - 1100 words (approximately 6-7 minutes)

Introduction

This case study comes from a probation revocation hearing I attended. The person involved had been on a transdermal alcohol monitor for six years without a single positive alcohol reading. Then, one day, the device reported a single Transdermal Alcohol Concentration (TAC) of 0.031 g/dL. Every reading before and after was zero.
​Even so, the prosecution asked for at least 10 more years of monitoring — and argued they could legally request up to 99 more years. The person had already been returned to jail for five months by the time of the hearing, waiting for this one reading to be resolved.
This case is important because:
  1. It shows how a TAC device can give false-positive readings when it is not working properly.
  2. It explains what records should be requested if you or your lawyer ever need to investigate a TAC reading.
​In similar matters for the purposes of these events, I typically assume the TAC device was in proper operational condition, calibrated correctly, and functioning properly. Unfortunately, in this matter, I was not able to make that assumption. A single positive TAC reading and the unexplained “tamper events” with concurrent fluctuating IR voltages do not allow for a blanket presumption of reliability. The only indication of operational reliability of the device was that “CAM Bracelet XXXXXX was last serviced and calibrated at [manufacturer] on XX/XX/2024”, about one month prior to the event in question.

Understanding TAC Technology

TAC devices are types of alcohol testing devices worn on the ankle.
  • How it works: The TAC device takes readings every 30 minutes, measuring alcohol that comes out through the skin (transdermal alcohol).
  • Fuel cell sensor: Inside the device is a fuel cell sensor, similar to those in handheld breath testers. The reading is multiplied by 100 to estimate an equivalent Blood Alcohol Concentration (BAC).
  • Limitations: It assumes that about 1% of the alcohol in the blood comes out through the skin. But this is just an approximation — not a direct measurement.
  • Data collected: TAC devices also measures skin temperature and infrared (IR) light readings. The IR readings are used to detect possible “tampering,” such as blocking the sensor.
A strange TAC history
​I need to point out that this case reported a single TAC reading of alcohol followed by what is interpreted and reported as a non-compliance tamper event without establishing details as to how the tamper may have occurred. The report identified a single episode where the TAC device had detected ethanol, or a substance chemically indistinguishable by the device as ethanol and reported a single TAC reading (0.031g/dL). Thirty minutes later, and with all subsequent TAC readings, the TAC value was reported at zero. It is the interpretation of the single TAC reading that is critical. Typically, when analyzing SCRAM cases, I am confronted by a number of readings, both TAC, temperature, and IR voltage. 

The Problem in This Case

The official report showed:
  • One single TAC reading of 0.031 g/dL at 9:48 PM.
  • All other readings before and after were 0.000 g/dL.
​Right after this reading, the report also flagged a “tamper event” based on changes in IR voltage. But there was no detail explaining what the tamper was supposed to be.
If this TAC reading were correct, the person would have had to fully absorb alcohol from at least one drink within 30 minutes — and then have all of it disappear from the body in the next 30 minutes. That’s not how alcohol works in the human body.

How Alcohol Moves Through the Body (ADME)
Alcohol follows a process called Absorption, Distribution, Metabolism, and Elimination (ADME):
  1. Absorption: Alcohol enters the stomach and small intestine, then moves into the bloodstream.
  2. Distribution: Alcohol spreads evenly through the body’s water content.
  3. Metabolism: The liver starts breaking down alcohol almost immediately after drinking starts.
  4. Elimination: Alcohol leaves the body at a steady rate, mainly through the liver, with small amounts lost through breath, urine, and skin.
The speed of absorption depends on:
  • How much and what type of alcohol you drink.
  • Whether you’ve eaten food.
  • How fast you drink.
  • Your body’s metabolism.
Studies show that for most people, alcohol takes between 15 minutes and 3 hours to be fully absorbed. Once absorption is complete, alcohol is eliminated at a much slower rate — usually between 0.010 and 0.025 g/dL per hour, with a typical average of 0.017 g/dL per hour.

Why the Reading Was Impossible

If the single TAC reading was correct, then the subject had to have completely absorbed a single drink within 30 minutes, or very shortly thereafter. Not out of the question, but we should expect to see some sort of TAC reading during subsequent sampling. None was observed.
We have a single reading of 0.031 g/dL at 9:48 PM, following by a TAC reading of 0.000 g/dL at 10:19 PM.

Clearance Rates – Metabolic Elimination Rates in the Human Body

In this case, the TAC went from 0.031 g/dL to 0.000 g/dL in only 30 minutes.
To put that in perspective:
  • Normal elimination: 0.010–0.025 g/dL per hour.
  • This device’s reading: 0.062–0.078 g/dL per hour — three to six times faster than humanly possible.

A Forensically Impossible Reported Elimination Rate Value

Extrapolating from this data, the TAC device has indicated an impossible elimination rate (0.062 - 0.078 mg/dL per hour). The subject has gone from 0.031 g/dL to zero in only 30 minutes.
I was quite concerned about the reported elimination from 9:48 PM (identified in Table 1 arbitrarily as Test 2) to the next reading of 0.000 g/dL at 10:19 PM (Test 3). This period of elimination does not follow the medically or forensically acceptable elimination rate range addressed earlier.
Specifically, we have an elimination of alcohol in 30 minutes of between 0.031 – 0.039 g/dL. This is equivalent to an hourly elimination rate of 0.062 – 0.078 g/dL per hour, which is essentially a medically impossible elimination rate for ethanol in the human body.
​No medical or forensic study has ever shown a person eliminating alcohol that fast. This points to a measurement error, not actual drinking.
Metabolic elimination rates or clearance rates of ethanol are dependent upon the type and amount of ADH enzyme (Alcohol Dehydrogenase Enzyme) found in the specific person. In a healthy person, the rate of clearance of alcohol from the blood by liver is roughly 0.010 – 0.025 grams/dL per hour, with a median rate of about 0.017 – 0.018 grams/dL per hour.
People who are on low-protein diets, or who are malnourished, are reported to have lower rates of elimination. Others, especially those chronically exposed to alcohol, have higher clearance rates. High-level, long-term alcoholics may have a clearance rate as high as 0.036 – 0.040 grams/dL per hour (Jones, 1996), but this is considered an upper-end extreme in a very small (less than 2.9%) of the population (Neuteboom & Jones, 1990).
Dr. A.W. Jones reported a range of 0.009 – 0.029 grams/dL per hour (for 95% of the sample of 1200 subjects, with a median and mean of 0.019 grams/dL per hour [Jones 2010]). Less than 1.6% of all individuals have clearance rate higher than 0.040 grams/dL/hour. Less than 0.4% of all individuals have clearance rate higher than 0.048 grams/dL/hour (Neuteboom & Jones, 1990).
To my knowledge, the medical and forensic literature has never reported an individual with an elimination rate of 0.078 g/dL per hour.
The reported TAC data and its corresponding reported elimination is clearly NOT a metabolic elimination rate measurement. However, it DOES correspond generally with the evaporation of alcohol from atmospheric contamination.
I was cross examined about the tamper masking the subsequent TAC reading. In effect, with no evidence to support the device was tampered with (just the +12% IR voltage readings), they were putting forth the notion that the client had successfully masked their consumption over the subsequent 8 hours.

Discovery Issues

I have noticed a recent trend whereby TAC reports contain abbreviated data, insufficient to perform an appropriate analysis for reliability. Specifically, the TAC reports have redacted the previously available data set showing TAC readings, IR voltages, temperature and reading times. This data is imperative to identify aberrant readings, reported elimination rates, and the IR voltage which is supposed to be indicative of a tamper event.
Picture
Figure 1 - The reported TAC data only supplies this graph with no underlying data. It is insufficient to properly analyze the event.
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Figure 2 - In order to investigate the occurrence, we need access to the raw data produced by the TAC device. You may need to subpoena these records in order to receive this data.

The Tamper Event Data Problem

In addition to the impossible elimination rate (0.062 - 0.078 mg/dL per hour) the unit showed a series of infrared voltage spikes and drops. A TAC device is considered “tampered” when its IR Voltage surpasses 12% of its baseline value.
However:
  • The IR voltage also had large drops and spikes both before and after the alleged tamper.
  • There were almost 15 times more voltage drops than spikes during this time.
  • After the “tamper,” the voltage readings bounced back and forth for hours.
​This is not normal and suggests the device was not working under standard conditions. It could be a sensor fault or another technical problem — but the report didn’t explain it.
As an aside, I'm still waiting to hear a meaningful explanation as to how a "tamper" is initiated by a test subject that creates an IR voltage spike. I've yet to hear testimony or an explanation from any  TAC device manufacturer regarding the correlation of IR voltage to tampering with the TAC device.
​We were able to question the legitimacy of the tamper event by the IR voltage data. It was clearly elevated beyond the + 12% from baseline that they uses as an indicator of tempering. However, in this case, after the tamper event, the IR voltage dropped well below the threshold value, and within 8 hours after that, started bouncing back and forth, low and high. Something was clearly not operating under standard conditions.

Why You Need the Full Data

​A big problem in recent years is that TAC reports often leave out key details. They may only give you a summary graph instead of the full spreadsheet of readings.
If you don’t have the complete data, you can’t:
  • Check the TAC readings at every 30-minute interval.
  • Compare temperature and IR readings alongside TAC.
  • Spot patterns that suggest environmental contamination.
​Practice Tip: If you or your lawyer are ever defending against a TAC reading, insist on the raw data — not just the summary report. You may need to subpoena this information.

Other Causes of False Positives

Transdermal alcohol monitors can detect alcohol from sources other than drinking. These include:
  • Workplace chemical exposure.
  • Cleaning products, disinfectants, or hand sanitizer.
  • Personal care items like perfumes or colognes.
  • Automotive products, paints, and solvents.
The fuel cell in the device can react to other alcohols — such as isopropanol or methanol — not just ethanol from beverages. False positive readings are known to occur, whether from environmental factors, such as occupational exposure to workplace chemicals to incidental exposure from fumes from cleaning agents or disinfectants. Abstaining from the use of these sorts of contaminates is therefore a condition of TAC monitoring.
In 2007, the U.S. National Highway Traffic Safety Administration (NHTSA) found that external sources of alcohol can and do cause positive readings in these devices. In one controlled study, 43% of the positive readings were not from drinking. They concluded, “In general, the sensitivity and accuracy of these devices was poorer than we expected”.
[1] Marques, P. R. and McKnight, A. S., “NHTSA - Evaluating Transdermal Alcohol Devices, Final Report”, National Highway Traffic Safety Administration, November 2007, Page 20.

Outcome of the Case

Based on the impossible elimination rate, unexplained IR voltage changes, and lack of supporting evidence, it was clear this was not proof of drinking.
The court eventually agreed and released the person from custody. They had already spent five months in jail for an event they did not commit.

Key Takeaways for Participants

  • One reading alone does not prove drinking — especially if the elimination rate is impossible.
  • Always request the full device data if you need to challenge a reading.
  • TAC devices can pick up alcohol from environmental sources.
  • Device malfunctions can and do happen.
  • Documentation and expert review can make the difference in court.
  • KEEP A LOGBOOK: We still do not know what caused the false reading. A logbook may have cleared that up a lot sooner and easier.

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For further study:

  1. American Academy of Forensic Sciences, Academy Standards Board, Best Practice Recommendation for Performing Alcohol Calculations in Forensic Toxicology, 2024.
  2. Best Practice Recommendations for Performing Alcohol Calculations in Forensic Toxicology – 2024 Update, 2024 IACT Workshop, April 2024.
  3. Center for Substance Abuse Treatment. The Role of Biomarkers in the Treatment of Alcohol Use Disorders. Substance Abuse Treatment Advisory. Volume 5, Issue 4, 2006.
  4. Dubowski, K., Absorption, Distribution and Elimination of Ethanol: Highway Safety Aspects, Journal of Studies on Ethanol, Supplement No. 10, July 1985.
  5. Dubowski, K.M. Absorption, Distribution and Elimination of Alcohol: Highway Safety Aspects, 10 J. Stud. Alcohol Suppl. (1985). 
  6. Friel, P.N., Bear, J.S. and Logan, B.K., Variability of Ethanol Absorption and Breath Concentrations During a Large-Scale Ethanol Administration Study, Ethanolism: Clinical and Experimental Research, Volume 19, Number 4, August 1995, Pages 1055-1060.
  7. Ganert, P.M. and Bowthorpe, W.D., Evaluation of Breath Ethanol Profiles Following a Period of Social Drinking, Can. Soc. Forens. Sci. J., Vol. 33, No. 3 (2000), pp. 137-143.
  8. Jones, A.W. and Andersson, L., Comparison of Ethanol Concentrations in Venous Blood and End-Expired Breath During a Controlled Drinking Study, Forensic Science International 132 (2003), pages 18-25.
  9. 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.
  10. Jones, A.W. and Jonsson, K.A., Food-Induced lowering of Blood-Ethanol Profiles and Increased Rate of Elimination Immediately After a Meal, Journal of Forensic Sciences, JFSCA, Vol. 39, No. 4, July 1994, pp. 1084-1093.
  11. Jones, A.W., Biochemical and Physiological Research on the Disposition and Fate of Ethanol in the Body, Garriott’s Medicolegal Aspects of Alcohol, 5th Edition, Chapter 3, pages 47-156.
  12. 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.
  13. 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.
  14. Jones, A.W., Jonsson, K.A. and Kechagias, S., Effect of High-Fat, High-Protein, and High-Carbohydrate Meals on the Pharmacokinetics of a Small Dose of Ethanol, Br. J. Clin. Pharmacol, 1997; 44: pp 521-526.
  15. Jones, A.W., Jonsson, K.A. and Neri, A., Peak Blood-Ethanol Concentration and the Time of Its Occurrence After Rapid Drinking on an Empty Stomach, Journal of Forensic Sciences, JFSCA, Vol. 36, No. 2, March 1991, pages 376-385.
  16. Jones, A.W., Variability of the Blood:Breath Ratio in vivo., Journal Studies of Ethanol 39, 1978, pages 1931-39.
  17. Marques, P. R. and McKnight, A. S., NHTSA - Evaluating Transdermal Alcohol Devices, Final Report, National Highway Traffic Safety Administration, November 2007.
  18. 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.
  19. Powers, D. N. and Glad, D., The SCRAM Tether as Seen Through the Eyes of Davis-Frye and Daubert, The Michigan Bar Journal, June 2006, 35-38.
  20. Salomone, A., et al, Occupational Exposure to Alcohol-Based Hand Sanitizers: The Diagnostic Role of Alcohol Biomarkers in Hair, Journal of Analytical Toxicology, Volume 42, Issue 3, April 2018, Pages 157-162.
  21. Semenoff, J., Alcohol Calculations – The New Paradigms; The New AAFS Guidelines, Counterpoint, Volume 8, Issue 3, Article 2, October 2024.
  22. Semenoff, J., An Introduction to Human Alcohol Physiology; How Alcohol Moves Through the Human Body, Counterpoint, Volume 8, Issue 1, Article 3, April 2024.
  23. Shen, D, Toxicokinetics, Chapter 7, Casarett & Doull’s Toxicology - The Basic Science of Poisons, Klassen, C., Editor, pp 305-326.