In most hospitals, blood samples are for alcohol concentration are conducted by what is called enzymatic testing.

In an enzymatic test, the test is usually performed on serum or plasma samples, which is only a portion of the whole blood.  In order to establish a violation of Georgia's DUI per se law, the state must establish that the whole blood had a concentration of alcohol at or above the legal limit. In an enzymatic test, the serum or plasma 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 concentration.

The Georgia Court of Appeals recently ruled upon the admissibility of a serum conversion to whole blood 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. Therefore, 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.

So, yes, the state may attempt to test only a portion of a person's blood in its attempt to prove a DUI per se violation. However, there are numerous problems with this type of testing.  Contact DUI lawyer Ben Sessions to discuss the defenses to this type of testing.