In analyzing blood test results in a DUI case, it is critical to first know what type of testing was performed upon the blood sample.  Measurement of the alcohol concentration in a sample of blood is usually performed through an enzymatic assay test or a static headspace gas chromatography with flame ionization detectors.

            Understanding the Basics of Enzymatic Assay Testing for Alcohol

            In most hospitals, blood tests for alcohol concentrations are conducted by what is called enzymatic testing. Enzymatic assay testing can be very confusing, but once you understand the basic principles and the terminology involved, you will have a better idea how these test results are obtained and can be attacked. 

            In an enzymatic assay test, the analysis for alcohol concentration is performed on serum or plasma samples, rarely on whole blood. It is important to be mindful of the context within which hospital blood tests are performed. The hospital serum or plasma ethanol test is used as a clinical screening test to determine if a patient has altered mental status due to ethanol, some other  condition (such as closed had trauma), or a combination or ethanol and some other medical condition.  Quite simply, in the context of hospital blood tests, medical personnel are trying to confirm whether they administer certain treatments which may not safely be performed when alcohol is present in a patient. 

            To measure the alcohol concentration through an enzymatic assay test, the serum, plasma, or whole blood sample is mixed with the enzyme alcohol dehydrogenase to form acetaldehyde. During this process, nicotinamide adenine dinucleotide (NAD) is converted to NADH. The concentration of NADH is measured to determine the alcohol concentrtion. It is critical to note the indirect nature of this test. The hospital blood test does not simply measure the concentration of alcohol in the blood. The amount of NADH, which is an enzyme, is measured spectroscopically at the 340 nanometer wavelength. The amount of NADH present is proportionate to the amount of alcohol converted by ADH to acetaldehyde.

Some Problems With Enzatic Assay Testing: Indirectness and Lack of Specificity for Alcohol 

            Enzymatic testing is troubling on numerous levels, but attacks upon enzymatic testing are generally based upon it as an indirect measure of alcohol and its lack of specificity for alcohol. 

            Serum and plasma contain more water per unit volume (milliliter) than whole blood. Ethanol dissolves in water. Since serum and plasma contain more water than a whole blood sample, more ethanol will be present in a milliliter of serum or plasma as compared to a milliliter of whole blood. The amount of ethanol in a serum or plasma sample may be 20 percent or more compared to whole blood. The greater amount of ethanol contained in serum and plasma is variable and unpredictable. Due to that variability, a whole blood sample is more reliable and is the preferred sample for testing for ethanol in the forensic setting.    

In Potter v. State, 301 Ga. App. 411, 687 S.E. 2d 653 (2009), the Georgia Court of Appeals held that the evidence presented at trial supported the finding that the method of converting serum alcohol concentration into blood alcohol concentration had reached a scientific stage of verifiable certainty, and thus testimony from the toxicologist as to her opinion of Potter's blood alcohol concentration, based on a blood serum test, was admissible. In this case, the state's expert witnesses testified that it was generally accepted in the scientific community that ethyl alcohol concentrations measured in a serum sample are between 14 and 20 percent higher than those in a whole blood sample.