The term blood alcohol is a noun created in the twentieth century and consists of the word alcohol to which has been added the suffix -emia, from the Greek haima, which means blood. The alcohol level therefore reveals the presence of ethyl alcohol in the blood. Alcohol level, or the level of alcohol in the blood, is measured in grams per liter of blood. A decree dated August 29, 1995 lowered the maximum blood alcohol level authorized for driving to 0,5 g / L.

When someone needs to be on the road, it is essential that they check their alcohol level before getting behind the wheel. Driving under the influence of alcohol is the second leading cause of death on the roads. In the 35-44 age group, in particular, alcohol and narcotics factors are over-represented: alcohol is in this segment of the population at the origin of 25% of fatal accidents. In the event of a blood alcohol violation, but also when the driver wants to retake a new license after a cancellation, or recover it following a suspension, it often happens that a blood test is taken. 

It is mandatory to have a breathalyzer in your vehicle. It is therefore the preferred method in order to know if you are fit to drive after consuming alcohol. There is, however, if you do not have this test, a mathematical formula to calculate it, depending on weight and gender: 

• For a man: (V * T * 0,8) / (0,7 * M)
• For a woman: (V * T * 0,8) / (0,6 * M)

In these two formulas:

• V corresponds to the volume drunk, in mL;
• T is the degree of alcohol in percentage (for a beer at 5 °, it will be 0,05%);
• 0,8 corresponds to the density of alcohol;
• 0,7 or 0,6 is the diffusion coefficient depending on whether one is a man or a woman;
• M represents the weight in kg.

To calculate easily, you can optionally save an Excel sheet on your smartphone. Mobile applications, allowing to calculate the alcohol level in the blood, also exist. The breathalyzer, on the other hand, measures the alcohol level in the exhaled air.

While the breathalyzer is only used to detect a blood alcohol level without measuring the precise alcohol level, the verification of the precise alcohol level in the blood can be performed by blood sampling and medical examinations. This blood test can be done for medical or forensic purposes. 

This medical biology test is used to determine if a person has consumed alcohol, and to measure the amount that is present in the body. 

• It is asked when a patient is suspected of ethyl intoxication, in particular if he presents the symptoms;
• It is also required when a person is suspected of violating the law on alcohol consumption, or in association with the search for illicit drugs;

• The maximum rate authorized by law when driving is 0,5 grams per liter of blood. In addition, under several conditions, such as a probationary period or accompanied driving, this maximum authorized alcohol level is set at 0,2 g / L of blood. 

Ultimately, there are therefore two main situations for which a dosage of alcohol in the blood is necessary:

• Has medical purposes : the presence or absence of alcohol can help explain certain symptoms present in patients, and allow a good adaptation of a treatment, for example. Medical BAC may be requested if a patient presents with symptoms suggestive of acute or chronic alcohol poisoning;
• For purposes forensic : forensic blood alcohol level is requested when a subject is involved in a traffic or work accident, or in the context of an act of violence against others, in the event of death suspect. Police and gendarmerie, in particular, frequently carry out blood alcohol tests on the roads. 

Concretely, the blood test makes it possible to precisely measure the level of ethanol, but can also help to evaluate the consumption of alcohol thanks to several biological markers.

Ethanol, or ethyl alcohol, can be measured in a blood sample and in exhaled air. The latter must be analyzed immediately.

Exhaled air

The exhaled air sample is obtained by blowing. The breathalyzer remains the only legal instrument to measure the precise level in exhaled air for legal action. It is used by the police and gendarmes and must be reliable, precise, and follow the AFNOR NF X 20 701 standard. For the test to be valid, this breathalyzer must be checked once a year. It is a measuring device which establishes the concentration of ethanol in the exhaled air based on the comparison of two luminous fluxes. 

In the event that there is a physical inability to breathe or during a bodily injury, a blood test is always taken, giving rise to two samples, in order to be able to request a cross-check.

Blood sample

The blood sample is taken by means of a venipuncture, with a needle placed in a vein at the crease of the elbow. The blood test for drinking and driving is done in the laboratory. The test carried out is an assay of the ethanol from this blood sample. When the analysis is intended for legal proceedings, it should be performed in an accredited laboratory using a recommended technique. 

These tests are able to determine the level of alcohol in the blood on an empty stomach. In addition, a blood test will allow the analysis of the liver function, the organ where almost 95% of alcohol is eliminated, by means of the measures:

• of the rate of GT range (Gamma Glutamyl Transferase) ;
• du VGM (Mean corpuscular volume) ;
• the rate of CDT (Carbohydrate Deficient Transferrin) ;
• of the transaminases (ASAT and ALAT) and triglycerides.

The blood alcohol results have consequences for a driver, depending on the level that has been measured in the blood is:

• Less than 0,5 g / L of blood (ie 0,25 mg per liter of exhaled air): the control is negative, the driver is not penalized;
• Between 0,5 and 0,8 g / L of blood : the driver is sanctioned. Failure to comply with the authorized blood alcohol limit results in a fourth class fine with a withdrawal of six points from the license;
• Greater than 0,8 g / L of blood (or 0,4 mg per liter of exhaled air): according to article L243-1 of the Highway Code, “even in the absence of any obvious sign of drunkenness, driving a vehicle under the worsening of an alcoholic state characterized by an alcohol concentration in the blood equal to or greater than 0,8 grams per liter or by an alcohol concentration in the exhaled air equal to or greater than 0,4 mg / L is punished by two years’ imprisonment and a fine of 4500 euros ”. The additional penalties incurred are, in particular, the suspension, for a period of three years at most, of the driving license.

Moreover, for a doctor, if the presence of ethanol in a sample clearly proves that the patient has consumed alcohol, the measured concentration also makes it possible to give an indication of the extent of the impregnation. In addition, the ability to eliminate alcohol from the body depends on the efficiency of liver enzymes, but also on the integrity of the liver.

And thus, the results of the biological markers are interpreted as follows:

• GT range : the gamma GT blood test to screen for chronic alcoholism only seems to be 75% reliable. If the blood test comes back positive, it is supplemented by a CDT assay;
• CDT : Carboxy-deficient transferrin (CDT) is very specific to alcohol consumption, and in particular excessive consumption. An excessively high level of CDT is measured by electrophoresis when it is greater than or equal to 1,6% (with a doubtful “gray” area of ​​between 1,3 and 1,6%). Its dosage returns to the norm after two to four weeks after stopping alcohol consumption.
• VGM : after two months of excessive alcohol consumption, the volume of red blood cells increases. However, other factors can lead to an increase in the size of the blood cell, such as tobacco or anti-vitamin K;
• Enzymes ASAT, ALAT and triglycerides : these biological markers are also measured but they turn out to be less specific to alcohol consumption.

Finally, you should know that as a general rule, an alcoholic drink usually takes an hour to eliminate, but there are significant variations between individuals according to ethnicity, sex and weight. On average, the blood alcohol level decreases by 0,15 g of alcohol per liter of blood per hour.