On Going Metric
by Myrton F. Beeler,
The Commission on Continuing Education of the
American Society of Clinical Pathologists voted
unanimously in Chicago in July, 1973, gradually to
adopt the International System of Units (SI), with the
proviso that units currently in use, thought to be
useful clinically but not adequately provided for in
the new system, be retained indefinitely; that controversial proposals for new units, such as that for the
new enzyme unit, not be adopted until the situation
has been further clarified internationally.
In this article, we will explore the reasons for this
action, the history of the metric system, the nature
of the present proposals, some of the problems
involved in their adoption, and how we hope to
accomplish the change with minimal disruption.
Material objects and phenomena have physical
properties which can be measured. A system of
measures can be regarded as a catalogue of the units
used to make these measurements. The material
properties themselves and the units need names and
symbols for clarity and efficiency of communication.
The units must rest on standards which can be
determined accurately and precisely and w h i c h , to a
Myrton F. Beeler, M.D., is Professor, Department of
Pathology, Louisiana State University, New Orleans.
This article was published
originally
in ASCP's
Technical Improvement Service, 1974.
6
LABORATORY MEDICINE • VOL. 7, NO. 10, OCTOBER 1976
M.D.
greater or lesser extent, permit the units to have
the same values in different places at different times.
There are several attributes of a system of
measures which we regard as being highly desirable
— t h e units should be useful, convenient, and precisely definable. " C o n v e n i e n t " implies that the size
of the units should lie w i t h i n or close to the range of
sizes of the properties we intend to measure.
It also w o u l d be a major advantage if the system is
" c o h e r e n t , " which means that conversion factors
among units w o u l d always unify. Coherence also
implies that, after the list of fundamental measurable
properties has been chosen and a formula for each
derived property has been arrived at (in terms of
fundamental units) and the choice of fundamental
units has been made, all other units will be fixed.
For the most part, unlike the case in many other
spheres of activity in which the British units are still in
wide use, laboratory measurements largely are
reported already in metric units.
It was originally intended that metric units be
based on a natural scale and that they be used w i t h a
decimal division system. The first international
standard for a unit was that for length. In 1735, the
Academy of Sciences of France sponsored an expedition to South America which was to determine the
length of a pendulum which w o u l d have a half-period
of one second at the equator. This standard is kept
—
^
—
—
Table 1
Constituent
Normal range
(old units)
(Swiss)
Normal range
(new units)
(Swiss)
Albumin
Bilirubin
Calcium
Cholesterol
Creatinine
Glucose
Phosphate
Protein
Urea nitrogen
Urea
Uric acid
3.1-4.1 g/100 ml
.20-1.17 mg/100 ml
9.0-11.0 mg/100 ml
130-330 mg/100 ml
0.51-1.13 mg/100 ml
77-106 mg/100 ml
2.3-4.8 mg/100 ml
6.5-7.6 g/100 ml
10-28 mg/100 ml
21-60 mg/100 ml
3.0-7.6 mg/100 ml
450-600 umol/l
3.4-20.0 umol/l
2.25-2 74 mmol/l
3.36-8.53 mmol/l
44-100 mmol/l
4.3-5.9 mmol/l
0.80-155 mmol/l
65-75 g/l
Not to be used
3.5-10.0 mmol/l
180-450 umol/l
today in Paris and its length has recently been determined to be slightly over 1.949 metres. (Metre means
a measure.) In 1795, the metre was legally defined as
the fraction (1/10 -7 ) of the distance f r o m the North
Pole to the equator.
In 1799, Talleyrand assembled an international
group to develop a new system of weights and
measures. The unit of length was to be based on
measurement of the meridian arc f r o m Dunkirk to
Barcelona, and the measurement of weight was to be
based on the specific gravity of water. A metre and
kilogram of platinum were taken to be the definitive
standards, thus departing f r o m " n a t u r a l " standards.
Although it is now known to be shorter than was
intended, it is still accepted t h r o u g h o u t the w o r l d
and rests in the Archives Nationales in Paris.
Subsequently, in 1889, a new " r u l e " was made, as
closely like the old as possible, of p l a t i n u m - i n d i u m ,
which allows comparisons to be made reproducible
to better than 0.2 microns. It remained official
until 1960.
The metric system, in its early f o r m , gained gradual
acceptance on the continent during the 19th
Century, but was not accepted in Great Britain or
the United States. O n the other hand, Great Britain
authorized use of metric units in 1864, and in 1866
their use was made legal (but optional) in the United
States. The intention that the units be based on
decimal subdivisions and multiples was unable to be
achieved totally because the division of the terrestrial day into hours, minutes, and seconds was
already universally used.
The choice of natural units also was f o u n d to be
impractical because of limitations of the measuring
methods available, but they were close to being
natural units in that a quarter of the meridian is
approximately equal to 10,000 kilometers, and the
density of water is close to 1 gram per milliliter.
M.W.
Factor for
converting
new units to
old units
Factor for
converting
new units to
old units
60,000
585
40
387
113
180
31
.0059
.0587
4.00
38.8
.0113
180
3.01
167
17,1
.250
.0258
88.4
.0556
.23
—
—
—
—
—
—
60
158
6.00
.016
.167
63
W i t h the development of electrical and mechanical
physics and engineering, additional units were
needed and added, so that by the end of the first
decade of this century, the " C G S " (centimeter, gram,
second) metric system had been rather fully
elaborated.
Other versions of the metric system have been
proposed for one purpose or another since that time
on grounds of greater convenience, including the
meter, kilogram, second (MKS) and the meter, t o n ,
second (MTS) system. During the same period of
t i m e , additional units from the fields of photometry
and ionizing radiation have been added.
In 1960, the General Conference of Weights and
Measures, now a 41-national organization, gave
official approval to the "Système International D
Units (SI). It differs f r o m previous versions of the
metric system largely in that it is " c o h e r e n t . "
In 1964, Dybkaer and Jorgensen published a
monograph entitled Quantities and Units in Clinical
Chemistry. A Proposal made on behalf of the Danish
Society for Clinical Chemistry and Clinical Physiology.
It represented a comprehensive attempt to bring the
measuring system used in the clinical chemistry
laboratory into line w i t h the SI. The proposals have
subsequently been modified and published as tentative recommendations by the Commission on
Quantities and Units of the International Union of
Pure and Applied Chemistry's Section on Clinical
Chemistry.
Meantime, the National Committee for Clinical
Laboratory Standards of the United States has, on the
recommendation of the Subcommittee on Quantities
and Units of the Area Committee on Clinical
Chemistry, accepted these tentative recommendations as new proposed standards.
Early in 1973, fourteen different metric conversion
LABORATORY MEDICINE • VOL. 7, NO. 10, OCTOBER 1976
7
bills were introduced into the 93rd Congress. A
Metric Association has been established and incorporated by enthusiastic supporters. Its address
is "Sugarloaf Star Route, Boulder, Colorado 80302."
Annual dues for individual members are $2, for
corporate members $25, and for sustaining members
$100. Metric Association regions also have been
set up.
The stated mission of the Metric Association is to
" p r o m o t e increased usage of the modernized metric
system (SI) in the United States, with the ultimate
objection of complete conversion to i t . " For this
purpose, they are attempting to determine effective
methods for metric conversion, to publish information outlining advantages of early metric adoption, to
broaden and sustain metric interest through meetings and publications, and to encourage offering of
awards and citations for outstanding contributions
and p r o m o t i n g metrication. They publish the Metric
Association
Newsletter.
the mole. The proposed name of the unit for catalytic
activity is the katal.
The symbols for these units are, respectively, m,
kg, s, A, K, c d , m o l , and kat.
Factors are used which create multiples or submultiples of units, to make their use convenient over
a w i d e range of magnitude of measurable properties.
They are as follows:
Decimal Factors
Number
Name
Symbol
1012
10s
tera
g'ga
mega
kilo
T
G
M
k
hecto
deca
deci
centi
h
(la
d
c
mil li
micro
nano
pico
femto
atto
M
n
P
Í
10 H
10 : )
10 2
10'
10-'
10' 2
io-3
10G
IO' 9
IO' 1 2
10-15
IO' 18
1 pouw£*
kilo
T pound
I received a letter f r o m the President, Louie S.
Sokol, in July, 1973, on a sheet of metric paper
measuring 210 x 297 m m , representing an area of
1/16th of a square meter. In case you cannot
intuitively grasp the significance of this immediately,
it is slightly longer and slightly narrower than what
we have regarded as standard typewriter-size paper
in the United States.
Returning to the SI, six fundamental measurable
properties of matter chosen for the SI were length,
mass, t i m e , electric current, thermodynamic temperature and luminous intensity. To this has been
added, fairly recently, amount of substance; Dybkaer
originally started to add catalytic amount. Very
recently this suggestion has been withdrawn in favor
of the concept of catalytic activity.
The names of the units for these measurable
properties, respectively, are the meter, the kilogram,
the second, the ampere, the kelvin, the candella, and
8
LABORATORY MEDICINE • VOL. 7, NO. 10, OCTOBER 1976
ITI
There are, in addition, a large variety of derived
measurable properties which can be expressed in
terms of these fundamental units. They include mass
concentration (kilogram per liter), mass fraction
(kilogram per kilogram), volume fraction (liter per
liter), substance concentration (mole per liter),
molality (mol per kilogram), mole fraction (mole per
mole), number concentration (reciprocal liter),
pressure (pascal, for newton per square meter),
density (kilogram per liter), and so o n .
You will notice that these derived units depart f r o m
the coherent system because the liter is used as the
unit for v o l u m e , rather than the cubic meter. According to Dybkaer, the " l i t e r " was retained because of
chemical practice and is now classified by the International Committee of Weights and Measures as " a
unit in use with S I . " Because 1 mg/l equals 1 g/m 3 , a
subsequent change to the coherent system could be
made w i t h o u t changing the numerical values; since
we wish to get on with the conversion, we appear for
the moment to be stuck w i t h this approach.
The 11th General Conference devised a new,
fourth definition for the meter, with an uncertainty
of only 0.01 m i c r o n . The meter is n o w the length
equal to 1,650,763.73 wavelengths, in vacuum, of the
radiation corresponding to the transition between
the levels 2p 10 and 5d 5 of the krypton—86 atom. Thus,
the standard now has a natural d e f i n i t i o n , but the
meter, being an uneven multiple of it, is the largest of
the prefixes, but is too small for some astronomical
measurement. It is not adequate for minute particles
either, as the smallest division, the attometre, is
replaced in 1799 by a platinum standard. Again it
replaced in 1889 by another made of platinumi r i d i u m , with a volume of 46.40052 cm 3 . The unit,
named gram, (from a Greek w o r d meaning a small
weight via a Latin w o r d referring to 1/24 of an ounce)
is the unit for the weight of 1 cm 3 of water.
In the SI, as already described, the fundamental
unit is the kilogram. The standard now agreed upon
by chemists and atomic physics is 1/12th of the atom
of carbon 1 2 . Derived properties include density
(kg/1) and relative density (the density of the system
divided by the density of a reference system under
specified conditions). Since the reference substance
is almost always water, one can calculate it approximately by dividing the mass of the substance by its
volume.
Whenever the numerical value of a property of
matter is divided by another numerical value of the
same property of matter, the units cancel out and the
result is regarded as a pure number, having no definable dimension. We believe that this confuses its
derivation, and that the original units should be
maintained.
equal to 10~9 meter, whereas the approximate
diameter of a photon is 10~15 meter. Derived units
include area (m2) and volume (m 3 ).
The noncoherent unit of volume, the liter, was
defined in 1795 as a unit of the magnitude of the
content of a cube each side of which measures a 10th
of a meter. In 1901, at the Third General Conference,
it was redefined as the volume occupied by one
kilogram of water at its maximum density under a
pressure of one atmosphere. Unfortunately, this
caused the volume of the liter to be equal to 1.00028
dm 3 . The original definition was restored by the
Twelfth General Conference in 1964.
The w o r d mass comes from the Greek w o r d for
bread, via the Latin w o r d meaning "that which
adheres together (like d o u g h ) . " Mass is actually a
quantity of matter or objects (nucléons), although it
is frequently confused in the minds of lay people
with weight. Weight, of course, is proportional to
mass, but the astronauts certainly do not maintain
their earth weight w h e n walking on the m o o n .
The gram was defined in 1795 as the " a b s o l u t e "
weight of a vol ume of water equal to a cube with sides
measuring one centimeter at a temperature equal to
that of melting ice. Because a standard one hundred
times greater was more practical, the kilogram was
chosen for the standard; because the initial
experimenters were unable to obtain 0 °C, this
definition was altered to refer to water at its maxim u m density. Already noted, this standard was
Danlous-Dumesnils distinguishes between time
and duration, but acknowledges that the w o r d
" t i m e " is usually used when " d u r a t i o n " is meant.
The true solar day is that interval of time (duration)
separating two consecutive passages of the sun
across a designated meridian, and it varies f r o m time
to time during the year, so that a more useful expression has been devised, the mean solar day, equal to
1.002737909 sidereal day (which is determined by
observing the stars). The Greenwich meridian was
chosen as the point of reference during the second
decade of this century by most countries.
The Julian calendar designated years, months,
and days; each day is subdivided into hours,
minutes, and seconds. Until 1945, the General Conference of Weights and Measures defined the
second as the duration of 9,192,631,770 periods of the
radiation corresponding to the transition between
two hyperfine levels of the ground state of the
caesium—133 atom. As has already been m e n t i o n e d ,
the second is taken to be the unit for time in the
SI, and the fact that it is not decimally related to the
hour and the day is, in part, compensated for by using
the prefixes already discussed. Frequency is a
derived property of time applying to periodic phenomena, such as the human pulse rate. The unit of
frequency is the hertz. It can be standardized very
precisely by reference to atomic oscillators.
The unit for electric current is the ampere, defined
as the constant current w h i c h , if maintained in two
straight parallel conductors of infinite length, of
negligible circular section, placed one meter apart in
(continued on page 12)
LABORATORY MEDICINE • VOL. 7, NO. 10, OCTOBER 1976
(continued
from page 9)
Table II
Constituent
Normal range
(old units)
Normal range
(new units)
Chloride
Potassium
Sodium
CO,(Carbonate)
100-108meq/l
4.0-5.0 meq/l
135-150 meq/l
23-30 meq/l
100-108 mmol/l
4.0-5.0 mmol/l
135-150 mmol/l
23-30 mmol/l
a vacuum, w o u l d produce between them a force
equal to two times 10~7 newton per meter of length.
(The newton ¡s a unit of force, defined as mass times
acceleration.)
Electricity being a bit peripheral to o u r present
concern, we will just mention that the unit for the
property-potential is volt; for resistance, ohm; for
charge, coulomb;
for capacitance, farad; and for
inductance, henry.
The w o r d " t e m p e r a t u r e " comes from a Latin w o r d
meaning to mix or soften by mixing. In 1694,
Renaldeni proposed a scale fixed at two p o i n t s — t h e
boiling point of water and the melting point of ice.
Fahrenheit in 1721 was the first to devise an instrument filled with mercury for measuring temperature.
The Fahrenheit scale is used even now in the United
States. In 1741, Celsius divided the interval between
the melting point of ice and the boiling point of
water into 100 degrees, but his thermometers read
100 ° in melting ice and 0 ° in boiling water. In 1750,
Stromer altered the scale to read 0 ° in melting ice.
Similar thermometers had been in use in France as
early as 1743. This is the centigrade scale, renamed
the Celsius scale in 1961.
The Kelvin scale, based on the science of thermodynamics, has a unit equal to 1/273.16 of the thermodynamic temperature of the triple point of water. O n
this scale that point is 273.16 °K. (The triple point of
water is, approximately, the temperature of melting
ice at a pressure of one atmosphere. Under similar
conditions the boiling point of water is 373.15 K.)
A degree on the Celsius scale is equal to a degree on
the Kelvin scale minus 273.15.
The candella (the unit of luminous intensity) has
been defined since 1967 as the luminous intensity, in
the perpendicular direction, of a surface, measuring
1/600,000 square meter, of a black body at the temperature at which platinum freezes under a pressure
of 101,325 newtons per square meter. In other w o r d s ,
it is the energy emitted f r o m a point source of light
in a perpendicular direction for unit duration which
the human eye records as a luminous sensation. This
makes it dependent on the human eye—a consider-
12
LABORATORY MEDICINE • VOL. 7, NO. 10, OCTOBER 1976
Factors
(for converting
new units to
old units)
Factors
(for converting
old units
new units)
able weakness, since the measurable property is
therefore clearly physiological.
Finally, w e come to the property amount of substance, and its unit, the mole. Avogadro's number is
the number of atoms in twelve g of 12 C. Avogadro's
constant (N) is defined as the number of atoms or
molecules per mole, and the mole is the amount of
substance of a system which contains as many
elementary units as there are carbon atoms in 0.012
kg of the pure nuclide 12 C.
Parenthetically, there is also a recent decision by
the IUB/IUPACS Commission o n Biochemical
Nomenclature to define "enzyme activity" as a
derived kind of quantity with the unit " m o l s / s " or
" k a t a l . " Thus, the previous proposal for a new
property named "catalytic a m o u n t " is to be w i t h drawn, happily.
Having now reviewed the history and content of
the metric system in general and the SI in particular,
let us now look more closely at implications of its
adoption for the clinical chemistry laboratory in
particular, but also for other divisions of the clinical
pathology laboratory.
IUPAC Information
Bulletin—Number
21, prepared by the Commission on Quantities and Units of
lUPAC's Section on Clinical Chemistry and by the
Expert Panel on Quantities and Units in Clinical
Chemistry, Committee on Standards of International
Federation of Clinical Chemistry, contains extensive
recommendations regarding the names to be used
for chemical constituents of biologic material. They
are to contain three segments of i n f o r m a t i o n : the
kind of system, the c o m p o n e n t , and kind of quantity.
Included is an extensive list of names, recommended
conventions and abbreviations. Thus we may refer to
"((f Pt)B)S, sodium i o n , m o l , " meaning "serum from
blood of a fasting patient drawn in the m o r n i n g . "
While this uniformity seems to represent a desirable goal, an attempt to force all laboratory data into
that inflexible format at the same time that the SI is
being introduced it may be biting off more than w e
can, collectively, chew. For that reason, I am going to
take the liberty, in the remainer of this article, to
ignore these recommendations in order not t o confuse everybody on the more essential points.
journals should initiate the transition by beginning to
discuss, define, and refer to new units. Data should
be provided, side by side, in old and new units.
First, all of those substances now reported in the
units mg/100 ml or g/100 m l , are in the future t o be
reported in decimal multiples of the unit mol/1 —
mole per liter. What clearly will be needed, immediately, are factors by which new units are convertible t o o l d f o r each constituent. A brochure
listing some of this information was prepared by the
Société Suisse De Chimie Clinique in 1972, and a
nomogram has also been distributed by the Schweitzerische Gesellschaft Fur Klinishche Chemie.
Perhaps initially the new values and units should
follow the old and be enclosed in parentheses. After
a period of years, the order should be reversed, old
units f o l l o w i n g t h e new and enclosed in parentheses.
At that time (and not before then), I believe that
laboratories should begin to report data in new units.
They should, however, make conversion nomograms, charts, or graphs freely available to attending
physicians using the data before taking this step, until
there is no further demand for t h e m .
It is our understanding that Donald Young, M.D.,
of the Clinical Center of NIH is preparing a computerized list of such data which will be made available. Additionally, w e need to have the ranges
generally accepted as normal for each of these substances in the new units for side by side comparison
with similar ranges in the o l d units. Table I is a
table of such information for some of the substances
of major interest.
The conversion will not be inexpensive, rapid or
easy. Probably tragic mistakes will occur during the
transitional periods because of lack of familiarity with
the new units and lack of an intuitive understanding
of values in t h e new units. W h e n we can all contemplate in our minds, bust, waist, and hip measurements of Miss Metric America of 1990 of 91-61-91
x 10~2 and have a real intuitive feel for the significance of these values, the problems will lie largely
behind us.
Notice that in Table I, in the case of protein, an
expression for mass concentration is used because of
the impossibility of establishing a molecular weight
for a heterogenous group of substances. Notice,
also, that the values listed by the Swiss are not
exactly convertible by the theoretical factors. We do
not know the reasons. They may include rounding
off, for convenience.
Bibliography
1.
2.
3.
Electrolytes are also t o be expressed in new units,
abandoning milliequivalents per liter (Table II). This
will not change the numerical values f o r univalent
ions, so the problem here will not be great.
4.
5.
6.
For the present w e w i l l , by choice, avoid much
consideration of enzyme nomenclature and units. As
m e n t i o n e d , there is a proposal for a derived quantity
to be used as a unit of activity, the katal (mol/sec).
The conversion from the present international unit
w o u l d be as follows: 1 U = 16.67 n kat.
7.
8.
9.
10.
We believe that t h e colleges and universities,
authors of textbooks, and the scientific and technical
11.
Beeler MF: An editorial. The metric system and clinical chemistry. Am J
Clin Pathol 59:3, 1973
Danloux-Dumesnils M: The Metric System. A Critical Study of its
Principles and Practice. Univeristy of London, The Athlone Press, 1969
Dybkaer R, Jorgensen K: Quantities and Units in Clinical Chemistry. A
proposal made on behalf of the Danish Society for Clinical Chemistry and
Clinical Physiology. Copenhagen, 1964
Dybkaer R, Jorgensen K: A primer of quantities and units in clinical
chemistry. Copenhagen. 1966
Gresky AT: Letter to the editor. "To metric? Why not to natural?" Chem
Eng News, July 19:5-6, 1971
Information Bulletin. Appendices on Tentative Nomenclature, Symbols,
Units and Standards—Number 20. Quantities and units in clinical
chemistry. International Union of Pure and Applied Chemistry, 1972
Information Bulletin. Appendices on Tentative Nomenclature, Symbols,
Units and Standards—Number 21. Quantities and units in clinical
chemistry. International Union of Pure and Applied Chemistry, 1972
SI Units in Pathology. An editorial. J Clin Pathol 23:743, 1970
The use of SI in reporting results in pathology. Am J Clin Pathol
56:771-773, 1971
The use of SI in reporting results in pathology. J Clin Pathol 23:818819, 1970
Vanter SM, DeForest RE: The international metric system and medicine
LTD
JAMA 218:723-726, 1971
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LABORATORY M E D I C I N E • VOL. 7, NO. 10, OCTOBER 1976
13