What are the differences among testing of “whole blood,” plasma, and serum? How does that affect the blood alcohol concentration (BAC) reading? The BAC results will differ depending on what is tested.
“Whole blood” is exactly as it sounds — whole blood. It contains red and white blood cells. It has not been clotted and has not had its cells separated. Blood that has been clotted results in serum, and blood that has its cells separated results in plasma. Blood tests are to be done with “whole blood” samples, and not the resulting liquids of serum or plasma. Testing of serum or plasma of that blood sample will result in a higher BAC reading.
In a given blood sample, the “whole blood” version of that sample will contain less water than the serum or plasma versions of that sample. Because of the difference in water content, and the solubility of alcohol in water, the tests of the more water filled serum or plasma will yield a higher BAC reading than the test of the less water filled (and red and white cell filled), “whole blood.”
If serum or plasma have been tested, then the results must be converted to reflect the lower BAC of “whole blood.” Though there is a range, the average ratio of serum or plasma to “whole blood” is 1.16 : 1.
Labs generally use gas chromatography to test “whole blood” in order to determine the blood’s alcohol content. Serum and plasma immunoassay testing might take place in hospitals or clinical labs, but they might not relay that information in their reports.
If it is not known, learned, or discovered what has been tested, then a higher reading of serum or plasma may be unfairly imputed to a driver who may not had violated any laws.
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