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DUI Blood Testing in CA: How it works

BAC Graf

As part of the CA Implied Consent Law a person arrested for DUI is given the option of taking a breath or a blood test. For reasons I discuss elsewhere, blood testing is now my recommended choice of an implied consent test.  Remember: unless you are under 21 or on DUI probation you are free to refuse the handheld Breathalyzer given at the scene of the DUI stop AND I advise that you so refuse.

When you opt to take a blood test the cop takes you to a hospital or in some cases they take you to a phlebotomist (vampire) that they have on staff at the police station. The blood drawer is supposed to draw the blood by using accepted medical practices.

The cop then takes the blood vial and puts it in an envelope.  He puts the envelope in the unrefrigerated evidence room.  At some point, the blood sample is moved to the crime lab.  The blood is tested at the lab.  It usually takes 3-4 weeks before the lab technician gets around to testing the blood.  This creates a problem in that the blood becomes contaminated and the sample is ruined by fermentation.  I discuss fermentation elsewhere.

When it is finally time for the actual blood testing, the lab technician takes the vial and uses a syringe to draw off two 1 millimeter samples.  The samples are then put in an oven and heated.  The gas above the blood in these samples is called the headspace.  Once heated, the lab tech takes a syringe and draws off a very small sample of the headspace gas.  This gas is then transferred to the gas chromatagraph.

To this gas a constant amount of Isopropanol is added.  The Isopropanol is used as a standard. A carrier gas, usually hydrogen, is used in the tube.

The science behind blood testing relies on headspace gas chromatography flame ionization detection.  Gas chromatography is a process by which the various volatile organic compounds present in the gas sample are separated and measured. The gas is injected into a tube.  The different volatile organic compounds take different amounts of time to go through the tube.  For instance, in the sample chromatogram linked to below, ethyl alcohol (ETOH) comes through at 0.85 minutes.


(Notice in the diagram linked to there are two peaks to the left of the alcohol peak. These are other volatile organic compounds in the sample that inidicate that (1) this sample is contaminated and (2) the blood has fermented after it was drawn. The peak farther to the left, at about 0.64, is the injection peak which shows when the sample was injected into the tube.)

There are two samples and in the better method of blood testing two different tubes are used.  So that it takes each of the volatile organic compounds a different amount of time to go through each tube (the amount of time it takes to go through the tube is called elution).  When only one tube is used, you can’t tell whether the different volatile organic compounds are interfering with the measurement of alcohol no matter how many times you run the sample through (when two different substances come through the tube in the same time, this is called co-elution).

At the end of the tube, there is a flame.  When the alcohol (ETOH) comes through the tube it is burned off and the increased ionization is measured.  The hydrogen carrier gas has a base ionization rate.  When the various organic compounds come through, the increased ionization is measured.  The measurement creates a spike.  As I discuss elsewhere a good spike is symmetrical. A bad spike is not symmetrical or has a long foot at one side. The one linked to above has an asymmetrical foot on the right side. The spike doesn’t resolve to the base line until almost 1.0 minute.

The area under the spike is measured using calculus.  The numerical value is the amount of alcohol in the blood which is then translated into grams/210 milliliters. A more sciency explanation can be found here.

This is how blood is tested.  There are problems with blood testing which I discuss in my article Fermentation: The Big Problem With Blood Tests.

Blood Testing is now the implied consent test that I recommend which I discuss in my article Blood or Breath: Which Test Should I Take.

If you have questions about blood tests, it’s problems, which test to take or DUI in general call me at (213) 479-5322.  If you have an opinion, please leave a comment.

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