A breathalyzer can read higher than a blood test, but it can also read lower. The direction of the difference depends primarily on timing: whether your body is still absorbing alcohol or already eliminating it. During the absorption phase (roughly the first 90 minutes after drinking), breath alcohol readings tend to match or exceed blood alcohol levels. Once your body shifts into elimination mode, the reverse is more common.
Why the Two Tests Give Different Numbers
Breathalyzers don’t measure alcohol in your blood directly. They measure alcohol vapor in air from deep in your lungs, then convert that number into an estimated blood alcohol concentration using a fixed mathematical ratio. That ratio, set at 2,100 to 1, assumes that 2,100 milliliters of exhaled air contains the same amount of alcohol as 1 milliliter of blood. If your personal ratio happens to be exactly 2,100:1, the breathalyzer estimate and a blood draw will match perfectly.
The problem is that this ratio varies from person to person and even within the same person over time. Studies have documented individual ratios ranging from roughly 1,700:1 to over 2,400:1. If your actual ratio is lower than 2,100:1, the breathalyzer will overestimate your blood alcohol. If it’s higher, the breathalyzer will underestimate it. This built-in variability is the single biggest reason the two tests can disagree.
Absorption vs. Elimination: Timing Matters Most
When you’re still absorbing alcohol, typically within the first 60 to 90 minutes after your last drink, alcohol concentration in arterial blood (which feeds the lungs) runs ahead of concentration in venous blood (which is what a blood draw collects). Since a breathalyzer measures gas exchange from arterial blood in the lungs, it can read noticeably higher than a venous blood sample taken at the same moment. Research on individuals with gastroesophageal reflux confirmed this pattern: during the absorption window, breath alcohol in milligrams per 210 liters tended to equal or exceed venous blood alcohol in milligrams per deciliter.
Once absorption is complete and your liver is steadily breaking down alcohol, the relationship often flips. Venous blood catches up to and can exceed the breath estimate. So the same person tested at different points in the evening could see the breathalyzer read higher early on and lower a couple of hours later.
Body Temperature and the 8.6% Effect
Your core body temperature directly changes how much alcohol evaporates from your blood into your lung air. For every 1°C (about 1.8°F) your body temperature rises above normal, breath alcohol concentration increases by roughly 8.6% relative to actual blood alcohol. A fever, vigorous exercise, sitting in a hot car, or even just being in a warm environment can push your reading higher than a blood test would show. This isn’t a flaw in the machine. It’s physics: warmer liquid releases more vapor.
Breathing Patterns Can Shift Readings
How you breathe in the moments before and during a test also influences the result. In a controlled study, hyperventilation immediately before blowing dropped the average breath reading from 0.104 to 0.086, a decrease of nearly 0.02. That’s because rapid breathing floods the lungs with fresh air from passages that don’t participate in gas exchange, diluting the alcohol vapor. Five and ten minutes after hyperventilating, readings partially recovered but remained slightly lower than baseline.
Holding your breath does the opposite. Keeping air in contact with lung tissue longer allows more alcohol to diffuse into it, which can push the reading up. Modern evidential instruments try to account for this by requiring a steady, sustained blow and monitoring the alcohol concentration curve in real time, only capturing the reading once the level stabilizes.
How Modern Breathalyzers Try to Stay Accurate
Evidential breath testing devices, the ones used at police stations rather than roadside screening, rely on either infrared absorption or fuel cell technology. Infrared instruments pass infrared light through your breath sample. Alcohol absorbs specific wavelengths of that light, and the amount absorbed tells the machine how much alcohol is present. Fuel cell devices oxidize the alcohol chemically and measure the electrical voltage produced.
Both types are designed to analyze only deep lung air, not air from your mouth or throat. As you blow into an infrared device, it continuously tracks the ethanol concentration. The reading climbs as the breath sample gets deeper, and the machine waits for the level to plateau before recording a result. If it detects a sudden spike early in the blow, which signals mouth alcohol rather than lung air, the test is automatically aborted. A blood sample is then required instead. The entire process takes up to 5 minutes on infrared instruments and 10 to 15 minutes on fuel cell models.
Mouth Alcohol and Medical Conditions
One persistent concern is that acid reflux (GERD) could push stomach alcohol vapors into the mouth and inflate a breath reading. The mechanism sounds plausible: a reflux episode brings stomach contents upward, potentially adding alcohol vapor to what the machine reads. However, a study specifically testing this scenario in people with diagnosed GERD concluded that the risk of gastric reflux falsely increasing an evidential breath test result was “highly improbable.” The 15-to-20 minute observation period that officers are required to conduct before an evidential test exists partly to let any residual mouth alcohol dissipate.
That said, recent use of mouthwash, breath spray, or certain medications containing alcohol can temporarily create mouth alcohol that skews a reading upward. This is why the observation period matters, and why a suspiciously high roadside screening result is typically followed by a more controlled evidential test or a blood draw.
Which Test Is More Reliable?
Blood testing by gas chromatography remains the gold standard. It measures the actual concentration of alcohol in a blood sample rather than estimating it from a proxy measurement. Courts and the National Highway Traffic Safety Administration treat blood alcohol evidence as critical in impaired-driving cases. Following the U.S. Supreme Court’s decision in Birchfield v. North Dakota, police generally need a warrant to compel a blood draw unless circumstances make waiting impractical, such as when a suspect is unconscious (as clarified in Mitchell v. Wisconsin, 2019).
Breath testing is faster, less invasive, and produces immediate results, which makes it practical for roadside enforcement. But its reliance on the assumed 2,100:1 ratio, combined with sensitivity to body temperature, breathing patterns, and absorption timing, means it will sometimes read higher and sometimes lower than a true blood measurement. The direction and size of the discrepancy depend on the specific circumstances at the moment of the test. For any individual reading, differences of 10 to 15% in either direction are well within the range that normal biological variation can produce.