Letters to the Editor
SI Units in Clinical Chemistry—An Objective Reappraisal
To the Editor:—Dr. Lehmann's article, "Metrication of clinical laboratory data in SI units," and the
Editorial by Dr. Beeler2 prompted us to comment on
the use of SI units in pathology. We are, of course,
giving our personal views, but they could well reflect
some of the thoughts that must surely have arisen
among scientists in the Third World, as we followed
the developments that have taken place in recent years
on this important but controversial concept. Hitherto,
much too often indeed, we have passively watched
vital issues in medicine being decided by the leading
nations and later unquestioningly accepted and adopted
them. America has often played a leading role in
decision-making, but on the SI issue we are indeed
surprised at the relative inactivity and general slow
acceptance shown by our American colleagues. Could
it be that the inner wisdom of the American clinical
chemists has detected flaws in the SI issue that somewhat prevent outright enthusiastic support? If such be
the case, why then do you hold back your reservations
regarding those areas in SI that are not totally
acceptable? We too have observed many imperfections
in the system that is presently in use, and feel that at
this stage, before it is too late, there is a dire need to
discuss the issue objectively and arrive at solutions that
can be universally accepted. We would like therefore to
express what we personally feel about the application
of SI units in pathology in the hope that it will
stimulate further thought and dialog.
The first observation we made is the remarkable
way in which many scientists rationalize on the
acceptability of SI units, particularly in clinical
chemistry. When certain units in SI became "awkward" they were shelved and the old units retained, as
for blood pressure, temperature, pH, and enzymatic
activity. Yet the switch from mass concentrations to
molar concentrations for constituents of body fluids
was made, despite the tremendously dramatic changes
incurred in numerical results. We agree that the general
acceptance of the mole as the unit for amount of subReceived September 28, 1976; accepted for publication November
8, 1976.
Key words: SI units; Molar concentration; Mass concentration;
Katal.
Address reprint requests to Dr. Buttery.
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stance is justified in most circumstances. In chemistry
the use of the mole, and hence the molar concentration, was accepted without controversy and has been
in use for a long time. In medical science, and
particularly in clinical chemistry, the use of the molar
concentration for expressing solution strength is logical
and widely accepted. However, its extension under
SI to replace mass concentration in the measurement
of body constituents is a debatable move that has met
with great resistance, especially from physicians, due
to the consequent unfamiliar values. We have searched
the literature for really convincing arguments to substantiate the usage of molar concentrations for
biologic constituents and can find only vague statements that unless molar concentrations are used one
might overlook certain clinically significant relationships relevant to etiology and management of a
condition.
Young4 has given some good examples in medicine,
where the usage of molar concentrations revealed more
information than mass concentrations. The examples
are few, and we are not particularly convinced that
there will be numerous more in which clinical significance and biologic relationships between body constituents are masked when they are expressed in mass
concentrations. In our opinion, rather than having to
change to molar concentrations to appreciate these few
examples and others, perhaps, it would be far easier to
take cognizance of these instances and keep the old
mass concentration units.
When we took a hard look at the new SI units, apart
from the fact that they are all now dramatically different, we noticed that most of the results for blood
and urine constituents are numerically small, and many
are less than unity. Conceptually, this is difficult to
comprehend or adapt to. Laboratory workers who
habitually tend to round off numbers to one or two
significant figures must realize they cannot do so, now
that they are dealing with small numbers. The plotting
of quality control charts may present fresh problems
for many laboratories. Most important, physicians
must have many moments of doubt in interpreting the
results, especially during treatment.
Next, we noticed that the issue on submultiples of
the mole can be most confusing, especially for the users
Vol. 67 • No. 4
LETTERS TO THE EDITOR
of laboratory results. In Lehmann's table many of the
constituents were expressed in decimal units, e.g.,
coproporphyrins, total estrogens, etc. It would have
been simpler to increase them by a factor of 1,000.
Thus, serum uric acid, which is expressed as 0.12-0.46
mmol/1 for males, could have been 120-460 /u.mol/1.
When urinary total estrogens is expressed in ;u,mol/24 h,
and the estrogen fractions are expressed in nmol/24 h,
one tends to get a much-confused picture.
Lehmann has taken a bold step in introducing the
katal as the SI unit for enzymatic activity. It is most
unfortunate that he has done so, as even in Europe and
Australia this usage has been deferred. The use of the
katal, as presently defined, would create as much confusion as the International Unit (U/l). Enzymatic
activities are often compared without taking into
consideration differences in their methodologies and
reaction conditions. While laboratory workers are
aware of these problems, physicians, unless forwarned, may be bewildered by the new values. For
example, alkaline phosphatase may be measured by
several methods, all giving somewhat different
numerical values, though all are expressed in /u.katal/1.
Incidentally, Lehmann's conversion of alkaline phosphatase values to the katal gave results that we think
are in error for some of the methods. There were
more errors in acid phosphatase katal values as well.
Disagreements on SI units for many constituents are
still numerous. The hemoglobin concentration in SI
units given by Lehmann may be challenged by
colleagues in Europe and Australia, as there is still
uncertainty about the actual molecular configuration of
hemoglobin and hence, its molecular weight.
In America, blood urea is measured as blood urea
nitrogen (BUN). Both Lehmann and Young 3 gave BUN
results in SI units, which fortunately has the same
normal range as blood urea. We think it more appropriate that the term "blood urea" be used, rather than
BUN, as it is the urea we measure. The same applies
for serum thyroxine versus serum thyroxine iodine.
Such names as "ammonia-nitrogen" and "amino acidnitrogen" should also be dropped in favor of "ammonia" and "amino acid."
We are amused that the international bodies cannot
decide to express serum total proteins and albumin in
the SI units based on pure human albumin as standard.
Surprisingly, they are quite prepared to measure
catecholamines, Cortisol, 17-oxosteroids, and estrogens using nonspecific methods and reporting the
results in SI units based on arbitrary standards.
Lehmann must be commended for expressing urinary
albumin in SI units.
Despite all we have said about SI, we are not totally
against it, and like our clinical chemistry "pioneers"
403
who advocate the use of SI, we agree that weight should
be measured in kilogram (and gram), time in second,
length in metre, temperature in degree Celsius, and
amount of substance in mole. Reagent concentrations
should be in molar units, but for body constituents we
honestly cannot see much advantage in using molar
concentrations. The mass concentration has served us
well, and we think will continue to do so if we
decide to keep it in usage. Of course we must take
cognizance that the molar concentration may sometimes help us to elucidate an elusive pathologic
problem, but the mass concentration need not be
entirely phased out of use on this account. We propose
instead that some consistent approach be applied to
mass concentration units. For constituents in body
fluids other than urine, the mass concentration of
weight/dl should be universally adopted, while urinary
constituents should be reported as weight/1 or
weight/24 h. Individual laboratories should certainly
not be left to decide which unit to adopt, as appears
to be happening now to SI units.
We are pleased to note that the Commission on
World Standards, World Association of Societies of
Pathology, in one of their resolutions, implied that
adoption of the SI system does not necessarily stipulate
the adoption of the molar concentration unit as the
only concentration unit. 1 Let those who favor the use
of mass concentrations continue to do so, but in a consistent way. Editors of medical and scientific publications who are contemplating using SI units should not
be coerced into doing so for fear that their journals
would not be considered progressive should they retain
the old units. Those journals currently using SI
units, especially the British Medical Journal and the
Lancet, should be urged to reaccept the mass concentration units for body constituents. Authors who still
favor the use of the molar concentration should also
give in parentheses the mass concentration value, so
that their articles can be fully appreciated by medical
scientists in America and, of course, the Third World.
J. E. BUTTERY, P H . D .
E. S. C. Q U E K , B.Sc. ( H O N )
Division of Biochemistry
Institute for Medical Research
Kuala Lumpur 02-14, Malaysia
References
1. Copeland BE: SI units—A clarification. Am J Clin Pathol 65:20,
1976
2. Lehmann HP: Metrication of clinical laboratory data in SI units.
Am J Clin Pathol 65:2-18, 1976
3. Young DS: Normal laboratory values. N Engl J Med 292:795802, 1975
4. Young DS: Standardized reporting of laboratory data—The
desirability of using SI units. N Engl J Med 290:368-373, 1974