Download Editorial T he M etric S ystemand C linical C hemistry slowly toward

Editorial
T h e
Metric
System
a n d Clinical
Chemistry
AD H O C COMMITTEE ON NOMENCLATURE, QUANTITIES, AND UNITS OF MEASURE
FOR THE AMERICAN JOURNAL OF CLINICAL PATHOLOGY °
Coincidental with the move toward
adoption of the SI, proposals have been
developed for reform of quantities and
units of measure in clinical chemistry.
Originally published by R. Dybkaer and
K. Jorgensen in 1964,1 they 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,4' 5 and in other publications. 7 ' s In
part, they represent an attempt to bring
the measuring systems used in clinical
chemistry (and other areas of laboratory
medicine) into line with the SI. In their
present form, they would not completely
accomplish this; and, furthermore, they contain additional proposals, such as those for
enzyme units, not presently part of the SI.
All discussions about adoption of a metric
system for clinical chemistry should clearly
distinguish, therefore, between the SI and
these proposals, insofar as they differ from
the SI.
T H E UNITED STATES appears to be moving
slowly toward adoption of a metric system
of measures. T h e movement started in
1866, when Congress passed a law allowing
use of the metric system in this country.
It now appears likely that, through further action by Congress, the Federal Government and its agencies will be mandated
to use a metric system by the 1980's. If this
happens, it seems likely that state and
local governments, educators, scientists,
and industry will follow. (In fact, the National Bureau of Standards has already
adopted a form of the metric system in all
of its publications except when its use
would impair communication.) T h e costs
of complete conversion are almost incalculable. T h e costs of not converting, stemming in large part from adverse effects
upon our commerce, might be greater.
There are several metric systems, including the well-known and already widelyused centimeter-gram-second (cgs) system,
the modified metric meter-kilogram-second
system (mks), and the Systeme International d'Unites (SI) which is also an mks
system. T h e SI had its origin in Paris in
1870. It was given official status by the
11th General Conference on Weights and
Measures, now a 41-nation organization, in
Paris in 1960. It is this latter system which
is gaining international acceptance.
* Members o£ this committee include: Myrton F.
Beeler, M.D., Bradley E. Copeland, M.D., S. Raymond Gambino, M.D., and Edward R. Powsner,
M.D.
Received November 20, 1972.
Requests for reprints should be sent to: Myrton
F. Becler, M.D., Department of Pathology, Louisiana State University Medical Center, 1542 Tulane
Avenue, New Orleans, Louisiana 70112.
Published opinions regarding the desirability of conversion to the SI and how it
should be accomplished, have varied from
those expressed by Dybkaer and Jorgensen
in 1966,2 stating that it would appear advantageous to make a clean break from the
present system rather than to make changes
stepwise and slowly; to those expressed in
an editorial in the British Journal of Clinical Pathology in 1970 a calling for gradual
introduction with as few changes in the numerical result as possible (and stating that
"the only costs involved with be in altering a number of laboratory report forms");
277
278
EDITORIAL
A.J.C.P.—Vol.
59
to the opinion of Vanter and DeForest that only one prefix may be used. However,
published in the JAMA in 1971 7 that con- as the basic unit for mass (kilogramme)
version to the metric system at this time uses a prefix "kilokilogramme" is not perwould not be practical for the medical pro- missible and is expressed as "megagramme"
fession on grounds that units are not en- (10° gramme), for example. (Confusion
tirely convenient and useful, the public is might arise over the meaning of prefixes
not familiar with the metric system, and for derived quantities. For example, cm3
purchase of equipment and supplies would might be interpreted as a cubic centimeter
be difficult until producers made the con- or 10-2ms.)
version.
Aside from the probability of universal
According to the SI, a property measured adoption of the SI, its chief advantage over
(such as length, mass, or time) is referred other metric systems lies in the fact that its
to as a "quantity." There are seven basic units are "coherent." That is, conversion
quantities: those just mentioned above, factor among units is always unity. So, for
plus electric current, thermodynamic tem- example, 1 watt is 1 joule per second, 1
perature, luminous intensity, and amount newton is I kilogramme X 1 metre per secof substance. Units corresponding to these ond 2 . This is the only coherent system in
quantities are the metre, kilogramme, sec- existence.
ond, ampere, kelvin, candela, and mole,
The proposals for clinical chemistry, inirespectively. Note that they are not capi- tially developed by Dybkaer and Jorgentalized and that the European form of sen,2 already referred to, depart from use
spelling is used. The unit may be regarded of coherent units for volume, compromisas indicating a reference or standard ing because of practical considerations, in
quantity which may be used for compari- that they permit substance concentration
son with other quantities of the same kind. to be expressed as mole per liter, rather
The metre, for example, is defined as than mole per cubic meter. Also, and pro1,650,763.73 wavelengths corresponding to visionally, an eighth basic quantity has
a transition of Kr86; the second as been added—catalytic amount—with the
9,192,631,770 cycles of a specified transition unit "katal" and the symbol "kat." Cataof a cesium atom. These units are repre- lytic concentration is to be calculated as
sented by the symbols—m, kg, s, A, K, cd, transformed moles per second per litre
and mol (those symbols which are capital- "under defined conditions" (so that, for a
ized are derived from someone's name). given assay result, one old International
Under the rules, basic quantities can be Unit would approximately equal 16.67
combined to make "derived" quantities. jukat).
For example, the derived quantity of force,
There are potentially serious problems
the "newton" (N) is defined as 1 kg.m/s 2 . which may be encountered if and when
T h e derived quantity "volume" is the the present proposals are adopted. These
"cubic metre" (m3).
are of two kinds: those which would be
Prefixes have been selected to designate temporary and which would affect clinimultiples. For example, when the unit is cians who by training and experience are
to be multiplied by 100, the prefix "hecto" familiar with the magnitude of quantities
is used; when it is to be multiplied by expressed in the present, familiar units,
1000, "kilo" is used, when multiplied by and who thereby have an intuitive under10_s, "milli" is used. So, one hectometre = standing of the significance and interrela102 metres; 1 kilometre = 103 metres; 1 tionships of substance concentrations so remillimetre = 10"3 metres. There is a rule ported; and those resulting from basic de-
March 1973
EDITORIAL
fects in the proposals themselves which
should be remedied before universal adoption of the proposals.
Some illustrations of the former category
of problem follow. What would the physician make of a stated pulse frequency of
1.2 Hz, for example? How would he interpret a rectal temperature report of "Pt—
rectum, thermodynamic temperature, 310.1
K"? How would he interpret a laboratory
report reading "B-reticulocytes, particle
concentration 30 X 10 9 /litre"? How long
would it be before he would recognize,
quickly, that a carbamide clearance of 1.7
X 10"31/second corresponds to a urea clearance of 100 ml/minute? What sense would
he make of a reported arterial oxygen partial pressure of 5.4 kPa, compared with
his understanding of the significance of the
more familiar expression—41 mmHg? If,
as is possible, intravenous fluids are labelled for mass concentration as kilogrammes or grams per litre, a 50% glucose
solution would be labelled "500g/l." But
the laboratory result on the patient to be
treated might read: (Pt) P—glucose 1.1
mmol/1. In the heat of an emergency situation, as with a convulsing patient, the possibility of therapeutic error could be increased in this situation. As we noted, this
would be a temporary problem which
could be considerably alleviated by an interim period during which traditional
numbers and units would be reported side
by side with new units and their corresponding numbers. The problem will affect
everyone in all scientific fields. Nutritionists and dietitians have similar worries
about substitution of the joule for the calorie, for example; engineers are concerned
about use of the SI instead of the familiar
cgs system.
But there are more serious objections.
One of these is the suggested departure of
the Dybkaer proposals from SI coherent
units in form of the unit "mol/litre" for
substance concentration, rather than the
279
coherent mol/cubic metre. We fail to see
the advantage of reporting a glucose result
as 5.6 mmol/1 compared with reporting the
same result as 5.6 mol/m 3 . In fact, it
seems possible that adoption of the litre
might later have to be followed by a
switch to the cubic metre, a debacle not
lightly to be contemplated.
Another serious point of contention is
the recommended abandonment of such
concentration expressions as milliequivalents per liter and milliosmoles per liter.
The argument proffered for abandonment
of such traditional expressions of mass concentration as mg/100 ml for substance concentration as mole/1—that biological relationships may have been obscured by the
old system, which may, in the future, be
uncovered by the new—might well be
turned against adoption of the new system
if applied in these instances. We fail to
see how the substitution of mole per litre
for milliequivalent per liter will uncover
any new biological relationships, but we
do see how it might obscure some for future clinicians. Measurements of osmolality
of body fluids have been found useful for
studying biologic fluid transfer. Osmoreceptors, not mole-detectors, are what appear to be activated by plasma dilution,
leading to an interruption of vasopressin
release and renal diuresis, for example. It
appears to be osmolality of body fluids
which is being defended, not molality. Calculation of osmolal clearance by the kidney has been and will continue to be valuable in assessing the ability of the kidney
to concentrate and dilute solute. Then too,
no rules concerning units of measure will
alter such biologic relationships as that between the Donnan effect and osmotic pressure. Ions will continue to behave as if
electrical charges on either side of a semipermeable membrane make a difference.
There is inadequate provision in the proposals for reporting pH measurements as
such. If we were to report results of mea-
280
EDITORIAL
surements of pH (depending on hydrogen
ion activity, by the way) as hydrogen ion
concentration (mmol/1), we would cause serious problems for physicians trying to
understand and manage patients with acidbase problems, especially in view of the
many useful acid-base nomograms incorporating use of pH, in addition to introducing the scientific error of equating activity with concentration. Abandonment of
this familiar unit of measure cannot be
undertaken lightly.
There are also some reasons for objection
to adoption of the proposal for an eighth
basic quantity—"catalytic amount"—with
the unit "katal" and the symbol "kat"
as now defined, on grounds that it does
not, in its present form, offer a potential
solution to the confusion rampant in this
difficult area. The Commission on Enzymes of the International Union of Biochemistry in 1964 recommended use of the
enzyme unit "U." Enzyme activity was to
be reported as /^moles of substrate consumed or product produced per minute per
milliliter. Initially, a temperature of 25 C.
was recommended; this was later revised to
30 C. It was specified that the assay be
performed at optimal pH with optimal
substrate concentration. Introduction of
the new unit did not, however, lead to (as
was hoped for) interlaboratory comparability of enzyme assay results for various
reasons, chief among them being failure
to tie the unit to specific procedures. For
example, it is possible to assay LDH in the
direction proceeding from lactate to pyruvate or from pyruvate to lactate. In this
case, if one follows all specifications for
these international "Units," normal ranges
for the two different methods will be very
different. T h e new recommendation is for
concentration to be calculated as transformed moles per second per litre "under
defined conditions" not themselves specified.
A.J.C.P.—Vol.
59
Our final reservation is subjective and
difficult to express. Physicists in the United
States have taken a pragmatic approach to
dimensional analysis, a method found useful for deducing, from an analysis of the
physical quantities and dimensional constants of a physical system, implicit limitations on the form of any possible relationship between them; that is, they regard it
as being a practical, useful method of
analysis.
European scientists, on the other hand,
have tended to take the view that the dimensions of a physical quantity have a
more fundamental significance; that correct dimensions are unique, related to the
ultimate, essential nature of physical quantities, and that they may be discovered.
Europeans, for example, are said to dislike
fractional exponents in dimensional formulas, believing them to indicate that certain essential dimensions have been omitted. (All secondary quantities presently in
scientific use have dimensional formulas
which are the products of powers; i.e.,
force equals mass X acceleration, equals
MLT" 2 ; energy equals ML2T~2, and so on.)
We have the impression, mistaken perhaps, that some of the undercurrent of
urgency about adoption of these proposals,
emanating from Europe, may be related to
an extension of this intuitive feeling toward adoption and use of coherent units.
We are usually content to let others speculate on such philosophic matters; but, as
pointed out by Alan Gresky 3 in a letter to
the editor of Chemical and Engineering
News, the meter-kilogram-second unit system is basically geocentric and aquacentric;
and on those grounds alone we believe
that they may not, of themselves, be expected to provide insights into the fundamental nature of the universe.
In any case, we prefer a pragmatic approach. We think that the full implica-
March 1973
tions of these proposals should be carefully considered before they are implemented. We believe that adoption of the
SI system, supplemented for an indefinite
period of time by additional, non-coherent
quantities and units found to be particularly useful, should be our goal. The
changes recommended by Dybkaer and
Jorgensen which are consistent with the
SI should probably be adopted after due
consideration by all concerned, with provision for an extended interim period during which results would be reported in
both the present and the new system. As
to those Dybkaer-Jorgensen proposals to
which there are serious objections, they
should receive particular thought and discussion by all concerned readers of this
Journal, who should thoroughly familiarize themselves with the proposals, and who
should express themselves freely in writing
to officers of their professional societies
and to editors of scientific journals. The
implications of premature or ill-considered
moves are too serious to permit us to recommend hasty action.
MYRTON F. BEELER, M.D.,
281
EDITORIAL
CHAIRMAN
Louisiana State University Medical
Center
New Orleans, Louisiana 70112
BRADLEY E. COPELAND,
M.D.
New England Deaconess Hospital
Boston, Massachusetts 02115
S. RAYMOND GAMBINO,
M.D.
Columbia-Presbyterian Medical
Center
New York, New York 10032
EDWARD R. POWSNER,
M.D.
Veterans Administration
Hospital
Allen Park, Michigan 48101
References
1. Dybkacr 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
2. Dybkaer R, Jorgensen K: A Primer of Quantities
and Units in Clinical Chemistry. Copenhagen,
1966
3. Gresky AT: Letter to the Editor, "To metric?
Why not to natural?" Chem Eng News July
19, 1971, pp 5-6
4. 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
5. Information Bulletin. Appendices on Tentative
Nomenclature, Symbols, Units and Standards—
Number 21. List of Quantities in Clinical
Chemistry. International Union of Pure and
Applied Chemistry, 1972
6. SI Units in Pathology. An editorial. J Clin
Pathol 23:743, 1970
7. The Use of SI in Reporting Results in Pathology. Am J Clin Pathol 56:771-773, 1971
8. The Use of SI in Reporting Results in Pathology. J Clin Pathol 23:818-819, 1970
9. Vanter SM, DeForest RE: The International
Metric System and Medicine. JAMA 218:723726, 1971