Blood samples obtained in drunk driving cases are generally — but not always — analyzed as whole blood (sometimes called "legal blood"). If the sample is withdrawn for medical purposes, however, the test will probably be done with serum (often referred to as "medical blood"). Serum is the clear yellowish fluid obtained from separating whole blood into its solid and liquid components (usually by centrifuging the sample); the liquid portion of the blood is called plasma, which is similar to serum. A third method involves precipitating proteins from the blood sample and centrifuging it; the result is a clear liquid called "supernatant," which is then analyzed.
Will analysis of serum/plasma or supernatant result in the same blood-alcohol readings as analysis of the whole blood? In a study entitled Distribution of Ethanol: Plasma to Whole Blood Ratios, Hodgson and Shajani, 18 Forensic Science journal 73 (1985), scientists attempted to determine the answer to this very question. The conclusion: Blood-alcohol concentrations in plasma were approximately 11 percent higher than that of whole blood, and those in supernatant samples were about 5 percent higher.
Thus, for example, evidence of a subject's blood-alcohol analysis indicating a .10 percent BAC may in fact reflect a true .09 percent if the plasma separation method of analysis was used. This has been confirmed in another study in which researchers concluded that a "person with an ethanol concentration of [.09 percent] in whole blood could have a reported concentration above [.10 percent] if either serum or plasma is analyzed." Winek & Carfagna, 11 Journal of Analytical Toxicology 267 (1987). Since many states permit the three types of "blood samples" to be used interchangeably in blood-alcohol analysis, counsel should certainly determine which type was actually used.
A simple technique for visually demonstrating the concept of testing blood that has aged and been subject to possible fermentation is to bring in a fresh vial of blood and compare it to the evidentiary sample withdrawn months earlier from the defendant. The fresh blood will be bright red, while the test sample will be nearly black.
For a study that found that serum-alcohol concentration can be up to 20 percent higher than blood-alcohol concentration, see Frajola, Blood Alcohol Testing in the Clinical Laboratories: Problems and Suggested Remedies, 39(3) Clinical Chemistry 377 (1993).
For legal case decisions, see Commonwealth v. Wanner, 605 A.2d 805 (Pa. Super. 1992), where the defendant appealed his DUI conviction on the grounds that the evidence of his blood-alcohol concentration was based on tests conducted on blood plasma rather than on whole blood, as required by statute. The appellate court agreed, citing the Bartolacci opinion; although that case addressed the use of blood serum rather than plasma, both involved tests on only portions of the blood. The court further found that tests on plasma resulted in BACs 15 to 20 percent higher than tests on whole blood.
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