How Does Blood Serum Compare To BAC?

In Connecticut, police officers can pick a test of your blood, breath or urine if they suspect you were driving while intoxicated. Most of my clients underwent breath tests. Blood tests occur most often when my client was taken to the hospital due to injuries in an accident. Non-accident blood tests are rare, in my experience.

Breath tests measure alcohol by volume (sort of, since it approximates).

Blood tests measure the weight of the alcohol. It generates a plasma or serum level, which needs to be converted to blood alcohol content ("BAC"). In Connecticut, like most states, a BAC over 0.08 is illegal.

Statutes like Connecticut's are based on "whole blood" levels, which coincide with breath tests, the most common used. A blood sample contains red blood cells, most of which are "spun out" of the sample, leaving a clear fluid on top. This is known as "serum." This happens when the blood sample sits without any kind of anti-coagulant.

If an anti-coagulant is added, due to the weight of the liquids, "plasma" is left on the top of the sample. The red blood cells are also "spun out" of the sample.

Since there is considerably less volume in both of these "spun out" samples, they do not equate to breath tests. And, since the red blood cells are not present, there is a much higher concentration of alcohol present in the serum or plasma than in the entire blood sample.

So, what we do is reduce the blood alcohol content (it usually is labeled "ETOH" in hospital records) by 17% to get the approximate result that would have occurred if a breath test were administered.

Is it exactly accurate?

No. But neither are breath test readings, since they use a complex algorithm to equate those results to what is actually in your blood at the time of operation.

Think of it like this: your body is metabolizing alcohol while you drink it and then as time passes. Netting all these times out, trying to equal the breath versus blood results and getting the times right is a best guess that the law, at least in Connecticut, appears to allow.