Serum ethanol levels vary from 1.09 - 1.18 times greater than
whole blood alcohol levels (9-18% or an average of 1/7th greater).
This is due to the removal of the red and white blood cells and & leaving
of the watery serum which has that increased of a proportion of
We do ethanol-specific serum ethanol levels, and these are the
results reported as "blood" alcohol levels. This method
is not known to have false positives due to some other cross-reacting
We’ve asked the hospital attorney for an opinion (in 1990s) as to whether
we should specify the nature of the result reported relative to
legal intoxication levels. The opinion suggested that we continue
to do as we have been doing, leaving it up to the legal system
to emphasize differences if these are relevant.
I’m of the impression that the state law specifies "blood
alcohol" levels as a lay person, non-scientific term for what
we actually measure. I think that virtually everyone measures serum
alcohol levels and that that was probably what the law intended
to specify, using a less precise, non-scientific term.
The practical difference in court would be approximately 0.01%
which may keep someone with a "blood" (serum) alcohol
level of .10 - .11% dodge from conviction as this would precisely reflect
a whole blood alcohol of approximately 0.09%, a result less than
the legal limit. Persons tested by our (serum) alcohol to be 0.12%
or greater would be above the legal limit whether or not the term
serum or blood was applied. I’m not sure in reality how many
convictions are obtained at the break point of .10 - .11%.
[The S. C. legal limit as of 2006 is now 0.08%.]