An alcohol test is any method used to detect ethanol or its byproducts in your body, whether through breath, blood, urine, hair, or saliva. These tests serve different purposes: a roadside breathalyzer gives a snapshot of current impairment, while a hair test can reveal drinking patterns over the past three months. The type of test used depends on why it’s being done and how far back it needs to look.
Breath Tests
Breathalyzers are the most familiar alcohol test. They measure how much ethanol is in the air you exhale and then convert that number to an estimated blood alcohol concentration (BAC). The conversion relies on a standard ratio: on average, 1 milliliter of blood contains 2,100 times more ethanol than 1 milliliter of air from your lungs. This 2100:1 ratio is baked into every breathalyzer’s calculation, though the actual ratio in any given person can range from 1500:1 to 3000:1, which is one reason breath tests aren’t perfectly precise.
There are two main sensor types. Fuel cell breathalyzers use a platinum electrode that generates an electrical current when ethanol hits it. The stronger the current, the higher the alcohol concentration. These are common in portable devices used by law enforcement. Infrared spectroscopy breathalyzers, like the Intoxilyzer 8000, shine infrared light through your breath sample at specific wavelengths. Ethanol molecules absorb that light in a predictable pattern, and the device reads the absorption to calculate your BAC. Infrared models are typically found at police stations rather than on the roadside because the equipment is larger and more sensitive.
Both types can detect BAC levels from .000 up to .500 g/dL, with a margin of error around ± .01 at the .100 level. A breathalyzer only tells you what’s happening right now. Alcohol clears the breath quickly, so these tests are useful for determining current impairment but can’t tell anyone about last week’s drinking.
Blood Tests
A blood draw is the most accurate way to measure how much alcohol is currently in your system. It directly measures ethanol concentration rather than estimating it from breath. Blood tests are standard in hospitals, legal proceedings where precision matters, and situations where a breath test isn’t possible.
Ethanol itself disappears from blood relatively fast. Most people eliminate alcohol at a rate that varies by a factor of three to four between individuals, depending on body composition, sex, genetics, and liver health. Women tend to reach higher peak BAC levels than men given the same dose per kilogram of body weight, largely because women carry a higher percentage of body fat and a smaller volume of water in which alcohol can dilute. Whether you’ve eaten recently matters too: the liver contains more of the primary alcohol-processing enzyme in a fed state, which speeds up clearance.
Heavy, chronic drinking actually increases the metabolic rate for alcohol in the short term, but advanced liver disease from prolonged use eventually slows it down. These biological differences mean two people drinking the same amount can have meaningfully different BAC readings on the same blood test taken at the same time.
Urine Tests
Urine testing doesn’t typically look for alcohol itself. Instead, it detects ethyl glucuronide (EtG), a metabolite your body produces when it breaks down ethanol. EtG is far more persistent than alcohol: it can show up in urine for 24 hours or more after just one or two drinks, and for two to four days after heavier consumption. This makes urine testing a tool for verifying short-term abstinence rather than measuring current impairment.
EtG urine tests are widely used in probation, custody cases, workplace monitoring, and addiction treatment programs. They are extremely sensitive, which is both their strength and a significant limitation. Ethanol appears in over 400 common household products, including hand sanitizers, cleaning sprays, mouthwash, and certain medications. Research has shown that simply inhaling the vapors from antibacterial hand sanitizer can elevate EtG levels enough to produce a positive result, even without any alcohol consumption. This vulnerability to false positives has drawn criticism, particularly when the tests carry legal consequences.
Blood Biomarker Tests
For a longer view of someone’s drinking habits, clinicians use phosphatidylethanol (PEth) testing. PEth is a substance that forms in red blood cell membranes whenever ethanol is present in the bloodstream. It accumulates with repeated drinking and clears slowly, with a half-life of about six days. A single blood draw can reflect approximately two weeks of drinking behavior, making PEth the most sensitive biomarker for detecting sustained alcohol use.
PEth and EtG complement each other in clinical settings. PEth reflects long-term patterns while EtG captures the last few days. If both are negative, a clinician can confidently conclude sobriety. If PEth is low but EtG is positive, the picture points to occasional, sporadic drinking. If PEth levels indicate heavy use but EtG is negative, the person has likely stopped drinking very recently after a period of heavy consumption. This layered approach gives treatment providers a much more detailed and reliable picture than self-reporting alone.
Hair Tests
Hair testing offers the longest detection window of any alcohol test, covering up to 12 weeks of use. As your body metabolizes alcohol, two types of markers get deposited into growing hair: EtG and fatty acid ethyl esters (FAEEs). EtG reaches hair through the bloodstream, while FAEEs are lipophilic (fat-soluble) compounds that get incorporated into hair through the oils secreted by your scalp.
A standard hair test uses the first 3 centimeters of hair closest to the scalp, representing roughly three months of growth. Labs look at concentrations of four specific FAEEs. When the combined level exceeds 0.5 nanograms per milligram of hair in that 3-centimeter segment, it strongly suggests chronic excessive drinking.
Hair tests have their own accuracy issues. Regular use of alcohol-containing hair products, like certain lotions or styling sprays, can push FAEE levels into the range associated with heavy drinkers, even in someone who doesn’t drink at all. For this reason, experts recommend interpreting EtG and FAEE results together. A positive FAEE result combined with a negative EtG result may indicate external contamination rather than actual consumption.
How BAC Is Estimated From Drinking
When a direct test isn’t available, BAC can be estimated using a formula originally developed by the Swedish scientist Erik Widmark. The calculation accounts for how much you drank, the strength of the drink, your body weight, a distribution factor (called the Widmark factor), and how long ago you started drinking. It then subtracts the amount your body has already eliminated based on an hourly clearance rate.
The Widmark factor differs between men and women and varies with body mass index. For men, it decreases as BMI rises, and the same is true for women, though women start with a lower baseline value. This is why online “BAC calculators” ask for your sex and weight. These estimates are rough guides at best. The three-to-fourfold variability in alcohol elimination rates between individuals means any formula can be significantly off for a given person.
Which Test Is Used When
- Roadside or workplace screening: Breath test, for immediate results on current impairment.
- Emergency rooms and legal evidence: Blood draw, for the most accurate current BAC reading.
- Probation or treatment monitoring: EtG urine test, to verify abstinence over the past one to four days.
- Clinical assessment of drinking patterns: PEth blood test, reflecting roughly two weeks of behavior.
- Custody cases or licensing reviews: Hair test, covering up to three months of alcohol use history.
The choice always comes down to the question being asked. A breath test answers “are you impaired right now?” A hair test answers “have you been drinking regularly over the past several months?” No single test does both, which is why multiple types exist and are sometimes used in combination.