A breath test number can feel like the end of the conversation in a Maryland DUI case, but it isn’t. In short: Maryland breath testing is governed by strict regulatory procedures — a 20-minute observation period, two analyzed samples, annual instrument certification, and specific timing rules — and a breakdown at any step can affect whether the result holds up. The roadside handheld device police use during a stop is legally different from the evidentiary machine used at the station, medical and environmental conditions can distort a reading without any alcohol being involved, and refusing the test triggers separate, automatic license penalties under Maryland’s implied consent law. This article walks through how Maryland breath testing actually works, where it can go wrong, and how that evidence gets challenged.
How Does Maryland Breath Testing Work?
Maryland’s evidentiary breath test — the one used as courtroom evidence of blood alcohol concentration (BAC) — is governed by Maryland Transportation Code § 16-205.1 (the implied consent statute) and by detailed testing regulations under COMAR 10.35.02. It’s administered at the police station or a testing facility, not on the roadside, using an approved evidentiary breath testing instrument (an Intoximeter- or Intoxilyzer-type device, depending on the jurisdiction).
The process isn’t a single breath into a machine. It’s a regulated procedure: a mandatory observation period before testing, at least two separate breath samples analyzed by the instrument, and a paper trail documenting that the machine itself was working correctly on the day of the test. Maryland’s DUI laws set the legal thresholds the test result is measured against — 0.08 or above supports a DUI charge, and 0.07 to under 0.08 supports the lesser charge of driving while impaired. But the number only matters if the process that produced it was followed correctly.
Breath Test vs. Preliminary Breath Test
One of the most misunderstood points in a Maryland DUI stop is the difference between the small handheld device an officer may use roadside and the machine used later at the station. They are legally and functionally different tools.
| Preliminary Breath Test (PBT) | Evidentiary Breath Test | |
|---|---|---|
| Where it happens | Roadside, during the initial stop | Police station or testing facility |
| Purpose | Helps establish probable cause to arrest | Produces the BAC evidence used at trial |
| Admissible in court to prove BAC? | No | Yes |
| Regulation and calibration | Less frequently calibrated; more susceptible to field conditions | Subject to COMAR-mandated annual certification and strict maintenance standards |
| Observation period required first? | Not to the same standard | Yes — a documented 20-minute observation period is required |
The practical effect of this distinction matters more than most drivers realize. A roadside PBT result is not what gets used to prove intoxication in court — it’s investigative, used to help the officer decide whether there’s probable cause to arrest. The evidentiary test at the station, administered under the stricter COMAR procedures described below, is what actually produces the number a prosecutor relies on. Refusing a roadside PBT and refusing the station’s evidentiary test can also carry different consequences, which is a distinction worth understanding before deciding how to respond at either stage.
Observation and Testing Procedures
Maryland regulation COMAR 10.35.02.08 requires a specific, documented procedure before any evidentiary breath sample is collected:
- A 20-minute observation period. For at least 20 minutes before the breath sample is taken, the person being tested may not eat, drink, smoke, or have any foreign substance in their mouth or respiratory tract. An officer is required to observe the person and check their mouth during this window.
- Two breath samples, minimum. The regulation requires two separate breath samples to be collected and analyzed.
- A third sample if the first two don’t agree closely enough. If the difference between the first and second sample results exceeds 0.020 g/210L, a third sample must be collected.
This procedure exists precisely because breath testing is sensitive to contamination. If an officer doesn’t actually observe the full 20 minutes — gets distracted, steps away, or starts the clock late — or if the person burps, regurgitates, or has something in their mouth during the observation window, the reading can be artificially inflated by mouth alcohol that has nothing to do with actual blood alcohol concentration. Whether that 20-minute period was properly documented and observed is one of the first things worth examining in any breath test case.
Equipment Calibration and Maintenance
The instrument itself has to meet ongoing regulatory standards, not just work correctly the first time it’s installed. Under COMAR 10.35.02.08, a state toxicologist must certify approval of each evidentiary breath testing instrument before it’s used, and that approval requires the instrument to meet defined precision and accuracy standards — a combined average standard deviation no greater than 0.0042 g/210L, and accuracy within 5 percent or 0.005 g/210L, whichever is greater. Instruments must then be recertified by the toxicologist at least annually.
For a DUI defense, this creates a concrete paper trail worth requesting: calibration logs, maintenance records, and recertification dates for the specific instrument used in a given case. A machine that was out of its certification window, overdue for maintenance, or previously flagged for inconsistent results is a legitimate basis to question the reliability of whatever number it produced.
Timing Issues
Blood alcohol concentration isn’t static — it rises as alcohol is absorbed into the bloodstream and falls as the body metabolizes it. That creates a real timing problem for breath testing: the reading reflects BAC at the moment of the test, which can be well after the moment of actual driving.
If a driver’s BAC was still rising when they were stopped — because they had a drink shortly before getting behind the wheel — it’s possible for the test result taken 30, 45, or 60 minutes later (after the stop, field sobriety testing, transport, and the mandatory 20-minute observation period) to be higher than it actually was at the time of driving. This is sometimes called a “rising BAC” issue, and it’s a legitimate area of scientific dispute in cases where the timeline between the traffic stop and the actual test is long enough for absorption to meaningfully change the number.
Forensic toxicologists sometimes use “retrograde extrapolation” — working backward from a known test result to estimate what BAC likely was at an earlier point in time, such as the moment of driving. That method depends on assumptions about a person’s absorption and elimination rates, which vary from person to person and even from day to day for the same person depending on food intake, body composition, and how much time has passed since the last drink. When the State relies on this kind of backward calculation, or when a timeline shows meaningful time between the stop and the test, that gap is worth scrutinizing rather than assuming the test result and the driving-time BAC are the same number.
Medical or Environmental Factors Affecting Results
Breath testing instruments are designed to measure alcohol from deep lung air, but several medical and environmental conditions can interfere with an accurate reading:
| Factor | How It Can Affect Results |
|---|---|
| GERD or acid reflux | Can bring stomach contents containing alcohol up into the mouth, inflating the reading with “mouth alcohol” rather than true blood alcohol |
| Diabetes or ketogenic diets | Can produce acetone and other compounds on the breath that some instruments may misread as ethanol |
| Dentures or dental work | Can trap residual alcohol or food particles, interfering with an accurate sample |
| Mouthwash, breath spray, or cough medicine | Many contain alcohol that can cause a falsely elevated reading if used shortly before testing |
| Occupational or environmental exposure | Exposure to certain solvents, paints, or fumes shortly before testing has been raised as a potential interference in some cases |
| Body temperature (fever) | An elevated body temperature can affect the partition ratio the instrument relies on to estimate BAC from breath |
None of these factors automatically invalidates a test result. But when one or more genuinely applies to a specific driver, it becomes a legitimate, fact-specific basis to question whether the number on the printout accurately reflects blood alcohol concentration — separate from any procedural issue with how the test was administered.
What Happens if You Refuse?
Refusing the evidentiary breath test doesn’t avoid legal consequences — it triggers a separate, automatic set of penalties under Maryland’s implied consent law that apply regardless of what happens in the criminal case. Our detailed breakdown of Maryland DUI refusal laws covers this in depth, including the DR-15 Advice of Rights form officers are required to read before testing and the escalating driver’s license suspension periods for a first versus a subsequent refusal. Those administrative penalties run through the Motor Vehicle Administration and are separate from — and can apply even faster than — anything that happens in front of a judge. Our companion article on Maryland’s implied consent law explains the underlying legal doctrine that makes a refusal carry consequences in the first place.
Can Breath Test Evidence Be Challenged?
Yes, and the challenges generally track the procedural and scientific issues covered above rather than simply disputing the number in the abstract. Because Maryland’s breath testing regulations create a documented paper trail — observation logs, calibration certificates, maintenance and repair records, and operator certification records — a defense review typically starts by requesting that paperwork through discovery rather than arguing about the number itself. Gaps, missing entries, or inconsistencies in that paper trail can carry as much weight as a scientific argument about the reading. A defense attorney reviewing a breath test case typically examines:
- Whether the required 20-minute observation period was actually completed and properly documented
- Whether the instrument’s calibration and recertification records were current at the time of the test
- Whether the officer administering the test was properly trained and certified to do so
- Whether any medical or environmental factor could plausibly explain an elevated reading
- Whether the timing between the stop and the test raises a legitimate rising-BAC issue
- Whether the underlying traffic stop and arrest were themselves lawful — since a breath test obtained after an unlawful stop can be challenged on that basis as well, separate from the test’s own accuracy
Breath test evidence often works alongside field sobriety testing as part of the State’s overall case, and the same kind of procedural scrutiny applies to both. Our related article on Maryland field sobriety test errors covers the parallel administration and scoring issues that can affect that evidence. For a first DUI or DWI charge specifically, understanding how a challenged breath test interacts with sentencing and program eligibility is also worth reviewing in our overview of Maryland first-offense DUI laws.
None of this means a breath test result is automatically thrown out — most aren’t. But because the entire process is built around specific, documented regulatory steps, a genuine breakdown in any one of them is exactly the kind of detail a Maryland DUI defense attorney reviews closely before accepting a breath test number at face value.
Frequently Asked Questions
Is a roadside breath test the same as the one used as evidence in court? No. The handheld device officers sometimes use roadside is a preliminary breath test (PBT), used to help establish probable cause for an arrest. It is not admissible in court to prove blood alcohol concentration. The evidentiary breath test given later at the police station, under stricter regulatory procedures, is what produces the BAC evidence used at trial.
What is the 20-minute observation period, and why does it matter? Maryland regulation requires an officer to observe a person for at least 20 minutes before an evidentiary breath sample is taken, during which the person can’t eat, drink, smoke, or have anything in their mouth. This prevents mouth alcohol or contamination from artificially inflating the reading. If that period wasn’t properly completed or documented, it can become a basis to challenge the result.
Can a medical condition cause a falsely high breath test reading? Yes, potentially. Conditions like GERD or acid reflux, certain diabetic conditions, and even some dental work can interfere with an accurate breath reading. Whether a specific condition affected a specific test result is a fact-specific question that a defense attorney can evaluate.
How often does Maryland require breath testing machines to be recalibrated? Under COMAR 10.35.02.08, evidentiary breath testing instruments must be certified by a state toxicologist before use and recertified at least annually, with defined precision and accuracy standards. Maintenance and calibration records for the specific machine used in a case can be requested and reviewed.
Does refusing the breath test avoid a DUI charge? No. Refusal doesn’t prevent a DUI charge from being filed, and it triggers its own separate, automatic license suspension under Maryland’s implied consent law — generally more severe than the suspension for a standard failed test. Refusal can also be used against you in certain ways during the criminal case itself.
