Whole Blood Bac Conversion

Psychiatry to go

It is always the greatest joy in court when a breath alcohol content or blood alcohol content is stated in mg / dl or in g / L in the file and one of the parties involved equates this with the "alcohol level". Unfortunately, that is not the case, and the convincing conversion is then up to the expert. I tried to collect all possible pitfalls:

1. "Conversion" breath alcohol concentration into blood alcohol concentration

It makes a huge difference if I have the alcohol content in the breathair or the alcohol content in the blood. You can't actually convert the alcohol content in the breath to the alcohol content in the blood using a formula. After all, units are not simply converted here, but rather two biologically related, but not the same, biological states. You have to imagine it this way: alcohol is dissolved in the subject's body, mainly in the blood. A small part of this alcohol evaporates within the lungs into the air space of the lungs. Part of it is exhaled. The alcohol concentration in this exhaled air naturally gives an indication of how much alcohol is dissolved in the blood of the test person. The two values ​​correlate quite well under standardized conditions even over certain distances. But they are neither linearly related nor are they independent of certain factors such as lung diseases, duration and speed of drinking and others. The blood alcohol passes into the air shortly after drinking at a different rate than half an hour after the last sip. So you cannot convert the values, you can only derive an estimate of the other value from one value. In its great foresight, however, the legislature has put a simplification into the world that comes very close to reality:

A breath alcohol concentration of 0.25 mg / L is legally assessed as a blood alcohol concentration of 0.5 ‰ (g / kg).

If you want to read the biological details, you can do so here, for example.

It increases the confusion that the normal breath alcohol test devices, which the policeman hands through the window on the side of the road, measure the breath alcohol concentration. However, they show depending on the setting on the device either the actual breath alcohol concentration (AAK) in mg / Lor the extrapolated blood alcohol level in per mille using this formula. The value in mg / L can usually be found in the court record.

2. Conversion of the blood alcohol concentration from g / L to per mille

Not all blood is created equal. You can measure the alcohol content in whole blood, serum or plasma.

And there are different units in which you can specify the alcohol concentration. Traditional and well-known are the per mille, which refers to the weight percentage of alcohol in grams of alcohol per kg of blood. Then there are the SI units, namely g / l. Here the alcohol percentage is given in grams of alcohol per liter of blood. Now a liter of blood does not weigh exactly 1 kg; then everything would be a little easier. But blood is a little heavier than water. Blood has a specific gravity of 1,057 g / cm³. Therefore, one liter of whole blood weighs about 1057 grams = 1.057 kg.

According to Wikipedia, the following rules of thumb apply to the conversion:

  • BAK (‰) = ethanol in whole blood (g / L) / 1.057
  • BAK (‰) = ethanol in serum or plasma (g / L) / 1.2312

You can of course rearrange these formulas so that the SI units are at the front:

  • Ethanol in whole blood (g / L) = BAK (‰) * 1.057
  • Ethanol in serum or plasma (g / L) = BAK (‰) * 1.2312

Nice. Let's try a table. Mean:

  • Breath alcohol concentration: Breath alcohol concentration in the breath in mg alcohol per liter air (mg / L).
  • Blood alcohol concentration: Blood alcohol concentration of alcohol in whole blood in grams of alcohol per liter of whole blood (g / L).
  • Blood alcohol concentration in ‰: Concentration of alcohol in whole blood as grams of alcohol per kilogram of blood.
  • In each case, the value that is assumed is formatted in bold. If it is a matter of converting breath alcohol concentration into blood alcohol concentration, this is an estimate. The resulting values ​​are therefore marked with "approximately". If it is a question of converting the blood alcohol concentration from one unit to the other, this is a conversion, so it does not result in an "approximately".
Breath alcohol conc. in mg / LBlood alcohol conc. in g / LBlood alcohol conc. in ‰ (g / kg)
1mg / Labout 2.144 g / Labout 2 ‰
about 0.473 mg / L1 g / L0,946 ‰
about 0.5 mg / L1.057 g / L1 ‰


  • There is a major pitfall: when in the court record one breathalcohol level is 1 mg / L, but then the patient is 2 ‰ bloodalcohol content twice as drunk as you initially think ... So x mg / L breath alcohol concentration = 2 * x blood alcohol concentration per thousand. Or x mg / L AAK = 2 * x BAK in ‰
  • The difference between the two measures for the alcohol content in the blood, namely g / kg (per mille) or g / L, with its conversion factor of 1.06, is more of an academic nature. You mainly have to understand it in order not to use the conversion factor of2 to be confused when converting breath alcohol concentration to blood alcohol concentration (AAK-> BAK).

I hope I've cleared up any ambiguity and made no mistake. If I did happen to do one thing: I'll take no responsibility for its consequences and ask for correction!

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Forensics, psychiatry, psychopharmacology AAC, alcohol, breath, breath alcohol, breath alcohol concentration, blood, blood alcohol, blood alcohol concentration, per mille, serum concentration, conversion