There is another advantage to using the deciliter as the denominator in
assays. A deciliter is of course 100 ml and, using the quite good
approximation that 1 mL of water or serum weighs 1 g, a deciliter may
also be taken as 100 g. This makes a nice base for using percentages.
Thus, for example, if a blood assay finds 0.08 g of alcohol in a
deciliter or 0.08 g/(100 g), it might be reported as 0.08 %.
Jim
On 2013-05-20 12:05, mechtly, eugene a wrote:
Martin,
Historically, units were often *chosen* so that numerical values would lie
conveniently between 1 and 99, without decimal markers and without superfluous
zeros.
In SI, the desire for convenience is met by the system of prefixes, preferably
in the numerators of compound units, not in the denominators.
The "deciliter" in the denominators of many medical laboratory measurements
perpetuates the ancient practice of picking the range of acceptable numbers first, and
choosing a combination of units second, that will enable the expected range of numerical
values.
I am happy that medical laboratory measurements now almost universally use SI
units or units accepted for use with SI, even though they sometimes do not
apply the principal of *no prefixes in the denominators.*
At least the units used in medicine and pharmacy are no longer grains,
scruples, ounces, etc. which are completely outside SI.
Eugene Mechtly
________________________________________
From: [email protected] [[email protected]] on behalf of Martin
Vlietstra [[email protected]]
Sent: Monday, May 20, 2013 2:10 AM
To: U.S. Metric Association
Subject: [USMA:52795] Re: B12 values
In the UK blood sugar is measures on mmol/L and in Germany it is measured in
mg/dL, so the difference in the way things are measured is not restricted to
just the US.
Somewhere I read that the reason for using mg/dL was to ensure that there
were no decimal points in the result and also that the resultant number did
not have a superfluous number of zeros.
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf
Of Paul Trusten
Sent: 19 May 2013 22:54
To: U.S. Metric Association
Cc: U.S. Metric Association
Subject: [USMA:52786] Re: B12 values
Jim, it is weird. Call it an American affectation. From what I can gather
from my brief study of medical laboratory science, there is a US use of
laboratory units that differs from those in the rest of the world. I think
the difference lies in the perception in the US that if SI *BASE* units are
used in expressing the concentration of the substance being measured, the
LABORATORY units are considered to be SI, while the units used in the US are
"not" SI. I think that if they knew what they were talking about, they
would say "non-US," not "non-SI." Actually, I think (I could be wrong) that
the US units are called either "US" or "standard."
For example, the unit used to measure blood glucose in American labs is
mg/dL, while outside the US, it is mmol/L. Blood glucose meters usually have
a switch on them to allow the patient to toggle betwee mg/dL and mmol/L,
presumably depending upon the country of usage.
So, it is SI to them only if they use an SI base unit (mol) in the numerator
of the concentration. Nyuk, nyuk, nyuk.
It just seems to me the keepers of the medical laboratory units may need to
brush up on how SI is applied.
Paul Trusten, Reg. Pharmacist
Vice President
U.S. Metric Association, Inc.
Midland, Texas USA
www.metric.org
+1(432)528-7724
[email protected]
On May 19, 2013, at 15:55, James Frysinger <[email protected]> wrote:
I was doing some research for my sister's use of vitamin B12 supplements
and came to this page:
http://www.webmd.com/diet/vitamin-b12-15239?page=2
It contains a small chart:
Vitamin B12Normal:
More than 200-835 picograms per milliliter (pg/mL)
148-616 picomoles per liter (pmol/L) (SI units)
Interestingly, WebMD apparently considers picomoles per liter to be in SI
units but picograms per milliliter not to be in SI units.
Of course, the non-SI unit is the liter (or milliliter) but that's
acceptable for use with the SI. So, in my mind neither value statement is
"more SI" than the other.
How do you view this, Paul Trusten?
Jim
