The test results that are least often challenged are the results of blood tests. The reason for this is because those tests are seen as a direct measurement of blood alcohol concentration. For breath and urine tests, it is necessary to measure the concentration of alcohol in the breath and urine and multiply that figure by a factor in order to determine the concentration of alcohol in the blood. But even the results of blood tests given by law enforcement are often not a direct measurement of blood alcohol concentration. The reason why is because law enforcement laboratories often use a method call headspace gas chromatography.
In headspace gas chromatography, the laboratory technician starts with the blood in a vial. The space between top of the blood sample and the top of the container is called the “headspace.” The technician draws a sample of the vapor from the headspace and injects the sample into an instrument called a gas chromatograph. The gas chromatograph tests the concentration of alcohol in the headspace. The prosecution’s expert witness assumes that there is a certain ratio between the concentration of alcohol in the blood and the concentration of alcohol in the headspace, multiplies the concentration of alcohol in the headspace according to the ratio, and tells the jury that this is the blood alcohol concentration of the defendant.
If the prosecution intends to rely on a sample of your blood as evidence of your blood alcohol concentration, then a good San Diego DUI lawyer will demand a sample of the blood the laboratory tested and have the blood retested. The attorney will have your blood retested by a laboratory that will use direct injection gas chromatography rather than headspace gas chromatography.
In direct injection – as opposed to headspace – gas chromatography, a sample of the blood, instead of the vapor from the headspace above the blood, is injected into the chromatograph. If a defendant has his blood retested, and the expert witness who retests the blood testifies at trial that the retest showed a lower BAC then the initial test, the prosecution will argue that the alcohol in the blood must have evaporated, and that a decrease in BAC is normal when the blood is stored for a period of time. A case like this will require the jury to make a decision between the credibility of the prosecution’s expert, and the credibility of your expert, the person who retested the blood.
An experienced San Diego Drunk Driving attorney should know that if the retest does not show a lower BAC, then other tactics have to be used to attack the prosecutions blood test result. One of the most important concepts to emphasize at a DUI trial in San Diego where a blood test must be contested is that the alcohol concentration in blood can go up over time because of fermentation. In the same way that the sugar in grapes ferments into wine, the sugar in potatoes ferment into vodka, and the sugar in grains ferments into beer, the sugar in blood – “blood sugar” – can ferment into alcohol.
Because a defense expert is so important when it comes to explaining the scientific aspects of blood and breath testing, prosecutors will often try to suggest to the jury that the defense expert is a “hired gun” who’s opinion has been bought. The prosecution has a team of experts on staff who testify for them at their trials. So defense experts rarely testify other than for defendants. The prosecution will often make an issue of the fact that a defense expert does not testify for the prosecution. There are a couple of ways to take the sting out of the prosecutor’s accusation. One is for the defense expert to tell the prosecution that he is available to testify for prosecution as a witness, and that the prosecution has never asked him to. Another way is to use an expert who has in fact testified for the government. One good expert who can be used to testify in DUI trials in San Diego is named Dewayne Beckner. He is a former California Highway Patrol officer, who testified countless times for the government before he began to work in the private sector as an expert witness. Another expert who can be used is John Woodward. Mr. Woodward worked for years as an alcohol testing expert in the military and, although he was never a prosecution expert in DUI trials, he has provided consultation to numerous government agencies in the design of their alcohol testing programs and standards, and has testified at government hearings in regard to the issues within his expertise. Visit our DUI attorney San Diego page for more information.