Ethanol (Alcohol)

The ethanol/alcohol test is used for both medical and legal purposes. Samples and results for each use are usually collected and tested separately.

Medical: The doctor looks for alcohol to help treat the patient's symptoms. For medical purposes, blood, and sometimes urine, alcohol tests are used to detect the presence of ethanol and to measure its concentration. One or more of these tests may be requested when a patient is seen in the Accident and Emergency department with symptoms suggesting alcohol poisoning. The symptoms may include confusion, staggering, vomiting, and unconsciousness. Other tests, such as a Full Blood Count (FBC), glucose and electrolytes are often ordered at the same time as there are a variety of other conditions that can cause similar symptoms. Additional drug testing and testing for the presence of other, more toxic, alcohols (such as methanol and isopropyl alcohol) may also be performed if the use of other substances is suspected.

Legal: Legal testing must be done by specially trained analysts and must have a strict chain-of-custody (a paper trail that records sample movement and handling). Testing may be requested to determine whether an erratic driver has a blood alcohol concentration that is over a legal limit, to determine whether an under-age child has been drinking, or to find out whether drinking alcohol has contributed to an accident. Post-mortem ethanol testing may be done to discover whether alcohol contributed to a person’s death. Legal ethanol testing may also be done randomly or as part of an employer's drug testing programme to see whether an employee has alcohol in their system.

Samples tested for legal purposes may include blood, breath, urine, and/or saliva. Breath testing is the most common alcohol test performed on drivers. It uses the tail end sample of breath from deep in the lungs and employs a conversion factor to estimate the amount of alcohol in the blood. Blood alcohol testing may be requested to confirm or refute other evidence, and/or be used as an alternative to breath testing. Urine testing on a random urine sample may also be performed as an alternative, especially in employment settings. Testing saliva for the presence of alcohol is not as widely used but may be used as an alternative screening test.

Medical ethanol testing is used when a patient has symptoms that suggest intoxication and/or ethanol toxicity, such as flushed face, glassy or watery eyes, slurred speech or drowsiness. Medical testing may also be done on a regular basis to ensure that a person being treated for alcoholism is not continuing to abuse alcohol.

Legal ethanol testing may be requested whenever there is reason to suspect that a person has not complied with a drinking-related law, and whenever there has been an accident and/or unexpected death – to determine whether alcohol played a role. Employment alcohol testing may be performed randomly or when the employer suspects that an employee has alcohol in their system while at work.

For medical testing, the detection of ethanol in a sample indicates that a person is likely to have been drinking, and the concentration present can give an indication of the severity of alcohol intoxication. However, symptoms and complications at a given ethanol concentration may vary significantly from person to person, depending on the individual’s general health, age, tolerance to alcohol and other medications or drugs that he or she is taking. The ability to remove alcohol from the body also depends on a healthy liver and the availability of the necessary liver enzymes.

For legal testing, results obtained are compared to legal allowable limits. In England, Wales and Northern Ireland (November 2021), the legal limits for driving are 80 milligrams/100 mL of alcohol in blood, or 107 milligrams/100 mL of urine. The legal limit in breath samples is 35 micrograms of alcohol in 100 mL of breath. The alcohol limits for drivers in Scotland have been stricter than in the rest of the UK since 2014, and are 50 milligrams/100 mL of blood, 67 milligrams/100 mL of urine and 22 microgrammes/100 mL of breath. Blood alcohol concentrations in the range 80 to 400 mg/100 mL cause increasing impairment and depression of the central nervous system.

Concentrations above 400 mg/100mL blood are likely to reduce in loss of consciousness and may prove fatal.

Results from different types of ethanol sample are not interchangeable. Breath and urine samples are good estimates of blood alcohol concentrations in most people but can be affected by a variety of factors. Breath sample concentrations can be affected by alcohol consumption immediately before the test, ketones (released into the breath by some diabetics and dieters), and by other substances that contain alcohol such as mouthwash and cough syrup. Urine concentrations lag behind blood concentrations due to the delay in excreting alcohol. Peak concentrations in urine are reached approximately 45-60 minutes after alcohol ingestion. Urine samples that contain both glucose (such as may be seen in diabetics) and bacteria or yeast, should not be left at room temperature for long periods of time because the glucose can ferment and produce ethanol. Fermentation can also be seen in post-mortem samples. Occasionally two other tests are used which measure serotonin metabolites, 5-HIAA (5-hydroxyindoleacetic acid) and 5-HTOL (5-hydroxytryptophol), which can help confirm whether alcohol was drunk. An increased ratio of 5-HTOL/5-HIAA can indicate alcohol consumption.

Children may develop low blood sugar (hypoglycaemia) in association with alcohol poisoning, so health practitioners may request blood glucose tests along with ethanol concentrations if they suspect ethanol toxicity in children.

No. Alcohol must be metabolized and eliminated by the liver, and the rate at which this happens is fixed in an individual. Food will slow the absorption of alcohol, and coffee may make someone who is intoxicated feel more alert, but neither will accelerate the elimination rate.