In a Montana DUI case, the number on your paperwork is an alcohol concentration: grams of alcohol per 100 milliliters of blood, or grams of alcohol per 210 liters of breath (MCA § 61-8-1001(3)). Which definition applies depends on whether you gave a blood sample or a breath sample. This page explains where each number comes from. It is part of our Montana DUI Guide.
What the Statute Means by Alcohol Concentration
Most people say BAC. The Montana Code does not. It defines one term, alcohol concentration, with two units of measurement, and then uses that term in every limit: 0.08 for most drivers, 0.04 in a commercial vehicle, 0.02 under age 21, and 0.16 for aggravated DUI (MCA §§ 61-8-1001(1), 61-8-1002(1)). Our page on Montana's legal limits sets those out.
The statute says the concentration is "shown by analysis of the person's blood, breath, oral fluid, or other bodily substance" (MCA § 61-8-1002(1)(b)). So the legal question is always what a particular sample showed when it was analyzed.
Four Kinds of Test, Four Different Roles
| Test | What it measures | How it is used | Source |
|---|---|---|---|
| Breath test at the station | Grams of alcohol per 210 liters of deep lung breath | Reported as an alcohol concentration and used as evidence | MCA § 61-8-1001(3); ARM 23.4.212(8) |
| Blood test | Grams of alcohol per 100 milliliters of blood | Analyzed by a laboratory and used as evidence | MCA §§ 61-8-1001(3), 61-8-1018(1)(b)(ii) |
| Roadside breath test (PAST) | An estimate of alcohol concentration from a handheld device | Recorded only as positive or negative | MCA § 61-8-1016(1)(b); ARM 23.4.225 |
| Oral fluid | Named in the statute as a sample that can show alcohol concentration | Added for alcohol testing in 2025 | MCA §§ 61-8-1002(1)(b), 61-8-1016(1)(a) |
The officer, not the driver, chooses which test or tests are given (MCA § 61-8-1016(2)(b)).
Breath: Grams per 210 Liters of Deep Lung Air
The Department of Justice's Forensic Science Division writes the rules for breath testing. Those rules require every breath result to be reported "as grams of alcohol by weight per 210 liters of deep lung breath" (ARM 23.4.212(8)). They define deep lung air as air from the deeper section of the lung that contains a portion of alveolar air, and they describe alveolar air as the breath on which "the 2100:1 breath blood ratio is established" (ARM 23.4.201(3), (14)).
Two practical points follow. First, the sample that counts is deep lung air. NHTSA's officer training manual says breath testing should be done on alveolar breath, obtained by having the person blow until the air in the lungs is expelled. Second, because the statute defines a breath-based concentration in its own units, a breath result does not have to be translated into a blood number before it counts.
Only instrument models approved by the division may be used (ARM 23.4.209), and the test must be given by a person the division has certified (MCA § 61-8-1018(1)(b)(i)). The rules also say all breath analyses must follow the procedures the division sets (ARM 23.4.201(25)). For the device and the steps, see the breath test machine and breath test procedure.
Blood: Grams per 100 Milliliters
A blood result is the more direct measurement. For the State to use it, the sample must be drawn by a physician, registered nurse, or other qualified person acting under their supervision, and analyzed in a laboratory operated or certified by the Department of Justice or one exempt from certification (MCA §§ 61-8-1018(1)(b)(ii), 61-8-1019(1)).
One detail matters more than people expect. The rules define "blood" to include whole blood, serum, or plasma (ARM 23.4.201(6)). In State v. Hala, 2015 MT 300, a hospital had tested the driver's plasma for medical purposes, and a later sample sent to the State Crime Lab was tested as whole blood. The Montana Supreme Court noted testimony that a plasma test typically reads about 10 to 15 percent higher than a whole blood test drawn at the same time. So the first question about any blood number is what kind of sample it was. See blood test procedure and hospital blood tests after a crash.
The Roadside Test Is an Estimate
The handheld device used on the roadside is a preliminary alcohol screening test, or PAST. The statute describes its purpose as "estimating the person's alcohol concentration," and both the officer and the device must be certified (MCA § 61-8-1016(1)(b), (c)). The rules say its results are reported "only as positive or negative" (ARM 23.4.225(5)). It helps an officer decide whether to arrest. It is not the number the 0.08 charge rests on. Our guide to the roadside breath test covers it.
How the Instruments Are Checked
- Field certification. Breath instruments must be field certified for accuracy at least once every 31 days, and a certification is valid when results on a known alcohol standard fall within plus or minus 10 percent of the target (ARM 23.4.213(1)).
- Laboratory certification. Instruments also go back to the division for a laboratory certification that is valid for 365 days. It requires accuracy within 5 percent or .005, whichever is greater, and includes a review of the instrument's sensitivity for detecting interfering substances (ARM 23.4.214(1)).
Those tolerances are part of why a result close to a legal line deserves a careful look.
The Number Is Taken After the Driving
Every test happens some time after the stop. For an impairment charge, the statute attaches its inferences to a sample "taken within a reasonable time after the alleged act" (MCA § 61-8-1002(2)). What that gap means for the result is its own subject, covered in our page on rising alcohol levels.
This page explains Montana law in general as of the 2025 Montana Code Annotated and the Administrative Rules of Montana. It is not legal advice about your specific case.