Dear Richard,
Although David has essentially answered your pertinent comments I would like to take the opportunity of these early, peaceful morning hours to put a bit more flesh on the
bones.
1. The choice of upper and lower case characters for official abbreviations of SI units is much more serious than you would think. Rather than handing it over to NLMC it
should really go to IUNPU or even CIPM and BIPM and of course ISO, IUPAC, and possibly consulting with IFCC if of LM interest and importance.
The rules for the abbreviation of SI units state that capitals can only be used for units which have been named after a person, e.g. Celcius, sorry Centigrades, Bequerel and
Siemens. In the attached article you will find the rational for capital L for Litre which also explains the controversy litre vs liter. Litre is not a ase unit but a derived unit and originally defied as the volume of pure water of plus four degrees C with
a mass of one kg. This differed only a little from one thousandth of a cubic meter, i.e one cubic decimetre. and the definition has been updated accordingly. Consequently one ccm equals one mL. As Prof Ian Mills has described the meter and thus the litre
is now defined to the 13th decimal which may be enough to fit our purposes.
2. The IU or International Unit. This is an arbitrary unit although endorsed by WHO. It is based on the biological potency of a preparation – often of a hormone, e.g. Vitamin
D – in a vial stored by WHO. When the stock of vials is exhausted a new set is prepared and compared with previous, the potency assigned and the preparation given an IS number. “IS” stands for International Standard. In many cases the IU has been abandoned
when the measurement procedure can be traceable to a defined calibrator and expressed in SI units i.e. 40 IU Vitamin D is equal to 1 µg Vitamin D.
Now, there are different degrees of “arbitrary” and any laboratory can define its own arbitrary unit which is often the case when a new quantity is launched as useful in the
clinic. In history this has created major problems (remember Hybritec’s PSA units?) if the home-brewed measurement procedure of the quantity is widely accepted and laboratories begin compare results with a view to harmonize results or standardize measurement
procedures. This problem was thoroughly addressed by the 1st IFCC-Bergmeyer conference, published as a Supplement to the Scandinavian Journal of Clinical and Laboratory Investigation ( 1.
Bergmeyer Conference : Improvement of Comparability and Compatibility of Laboratory Assay Results in Life Sciences: Principles of Assays in Medical Sciences : 1988, This has not been digitized by the publisher of SJCLI but is available at Google Books- Possibly
the IFCC office in Milan still has a number of copies). The issue of arbitrary units is also addressed in the ISO 17511 and its updated version and the CLSI EP 32, (previous C 59) both of which
are on their way. Unfortunately, due to the cost of these documents and the financial restrictions they may not be as widely circulated and read as they and the profession deserve.
If the unit is completely arbitrary or only meant to be locally used e.g. limited to use in the UK, any abbreviations would be just fine. Considering the increasing globalization
use of arbitrary units may not be so fine any longer, after all.
3. The “mcg” is simply ridiculous. Does it stand for millicentigram? That may be convenient and understandable for somebody trained to convert inch to feet, yards or miles
but hopefully it will disappear with that generation.
4. Let us try the rules we have just been alerted to, to give a sustainable answer to the comment no 4. One more rule, or fact, needs to be exposed to the reader; units can
never be in plural. Thus, a dollar is a dollar, the monetary unit, whereas “dollars” would refer to the coin or bill. The rational for this is that a result is expressed as a
number times a unit, i.e. 10 m is 10 times longer than the unit “meter”.
The abbreviation of hour is “h” and thus “hr” is denounced as well as “hrs”. The “24 h” would be deprecated because there should be no prefix in the denominator. Eh?? Well creatinine clearance is expressed as mL/min or rather mL/s although measured during
a urine collection period of 24 h. Although perhaps convenient this should be made clear in the SOP and handled by the IUNPU.” /d” is not a very good suggestion due to the arbitrary definition of day. How about the night and would it change with the latitude?
Best regards
Anders
Anders Kallner MD, PhD
Associate Professor
Dept Clin Chem Bldg L205
Karolinska University Hospital
SE 171 76 Stockholm, Sweden