Committee on Hospital Care
Metrication
and
SI Units*
Because of the increasing international use of the
SI system (International System of Units) in med-icine, the Committee on Hospital Care has written
this statement to familiarize pediatricians with this concept. The current state of the system, its den-vation, purported advantages, and controversial
as-pects are described; and the Committee has made specific recommendations for consideration regard-ing its future use and development.
BACKGROUND
The British Imperial System of Weights used in the United States today derives from a variety of ancient cultures. A Roman contribution is the use of the awkward number 12 as a base. Royal decree established the yard as the distance from the tip of the nose to the end of the thumb of King Henry I. The inch was based on the size of three grains of barley “dry and round.” Equally illogically derived units evolved to eventually form the irrational Eng-lish “system.”
The metric system with its “base-lO” or “deci-mal” system derived its units of mass and volume from its units of length, thus correlating its basic
units to each other. The need for further refinement
of metrics and a single worldwide and interdiscipli-nary system of measurements led to the develop-ment of the International System of Units (le Sys-t#{232}meInternational d’Unit#{233}s) with the international
abbreviation “SI.” This is sometimes referred to as
“the modern metric system” and is said to complete the process of metnication.2
Seventeen countries, including the United States,
signed the Metre Convention in 1870. This led to the establishment of the International Bureau of
* Abbreviations used in Pediatrics currently follow the
Style-book/Editorial Manual of the American Medical Association.
Abbreviations used in this article are slightly different from the
Stylebook.
PEI)IATRICS (ISSN 0031 4005). Copyright © 1980 by the
American Academy of Pediatrics.
Weights and Measures at S#{232}vres, France, which acts as an international standards reference
labo-ratory and as the permanent secretariat for the
Metre Convention. The General Conference on Weights and Measures, the diplomatic organization made up of adherents to the Convention (now in-cluding 41 member countries), is the ultimate au-thority on the definition of units. In 1960 the Gen-eral Conference standardized metric units into the simplified and logical system known as SI. Further refinements have been made by the General Con-ference on four subsequent occasions. ‘ The present structure of the SI is likely to be the permanent one.
In parallel with the developments on the
gener-alized use of SI units, recommendations for the standardized reporting of clinical laboratory data based on SI were proposed by the Commission on Clinical Chemistry (CCC) of the International Un-ion of Pure and Applied Chemistry (IUPAC) and the International Federation of Clinical Chemistry (IFCC).4
In recent years, the major industrial nations of the world, with the exception of the United States, have been using the metric system or converting to its use. The United States has been an “island in the metric sea.” The British Commonwealth coun-tries with whom we shared the pounds-quarts-yards system for so long have all converted to metric.
Canada has an unofficial goal of being metric by 1980. Eventually, the Congress of the United States passed the Metric Conversion Act of 1975 stating, “It is therefore declared that the policy of the United States shall be to coordinate and plan the increasing use of the metric system.” Furthermore, the Act states, “. .. the term ‘metric system of
mea-surement’ means the International System of Units.”5 This is to be done on a voluntary basis.
1980
1981
1982
United States have begun teaching SI as the pni-mary measurement system.’
In Finland, Norway, and Sweden, the use of SI is compulsory in medical laboratories. Table 1 lists the years by which a phased introduction of SI will be or has been completed in various countries.
SI SYSTEM
SI is a system of “base units” and “derived units” and their interrelationships. Seven base units and two supplementary units have been adopted (Table
2). Each unit can be defined in specific terms. For example, the mole is the amount of substance that
contains as many elementary entities as there are
atoms in 0.012 kg of carbon 12. Base units are multiplied and divided to form derived units (Table
3). For example, the unit for force, the newton, is derived from three base units according to the expression m kg . 2#{149} Coherence is the derivation of a unit from a base unit without the use of a
TABLE 1. Year of Introduction or Anticipated
Introduction of SI Units
Year Country
1970 Netherlands
1971 Denmark, Finland
1974 Australia
1975 United Kingdom
1976 New Zealand, South Africa, Sweden
1977 Norway
1978 Federal Republic of Germany
1979 Czechoslovakia, Democratic Republic of
Ger-many Hungary, Italy
Japan, Switzerland, Yugoslavia
Ecuador
TABLE 2. SI Base Units
Quantity Name Symbol
Length Meter (metre)* m
Mass Kilogram kg
Time Second s
Electric current Ampere A
Thermodynamic temper- Kelvin K
aturet
Amount of substance Mole mol
Luminous intensity Candela cd
Supplementary Units
Plane angle Radian rad
Solid angle Steradian sr
* Both spellings acceptable.
t
The Celsius temperature scale (formerly calledcenti-grade) is used for most medical and commercial purposes.
The Kelvin (the unit for thermodynamic temperature) is
the SI unit for temperature. Although their scale origins
differ, the degree Celsius equals the Kelvin in magnitude;
thus, a rise in body temperature of 1.0 K is equivalent to
a rise of 1.0 C. 0 C is defined as 273.15 K, thus 98.6 F = 37
C = 310.15 K.
factor. Noncoherent units are derived from the base units but contain a factor, eg, the liter (L, recom-mended symbol for the liter in the United States) equals the decimeter cubed (dm’). Table 4 shows the prefixes that denote multiples of SI units.
Although mass concentration (gIL) and sub-stance concentration (mol/L) are both included in the SI system,7 there are certain advantages to recording in substance concentration. By reporting
data as we do now, the apparent quantities of different materials may be misrepresented.
Corn-pounds with a high molecular mass might seem to have more molecules present as compared with those of lower molecular weight. Furthermore, cer-tam biologic relations between blood constituents
may be made clear when measurement is on the basis of their relative number. Such relations may be masked by usage of mass concentration but may be better visualized in molar terms. For example, an unconjugated biirubin concentration might be 0.4 mg/dL and the serum albumin, 4.0 g/dL. The concentration of biirubin might seem to be ‘/i,xx of that of albumin. In molar terms, however, there is only a 100-fold difference. The same concentrations are 6.8 and 620 imol/L, respectively. A serum bili-rubin concentration of 20 mg/dL, which is a level of clinical importance in neonatology, is 340 imol/L, or more than half the molar concentration of the albumin in this example.” It becomes obvious why a small change in compounds bound to albumin may result in displacement of bilirubin or other
compounds from their binding sites on the albumin molecule. Another example is the electrolytes, so-dium, potassium, and chloride, whose interrelations have become more apparent since they have been reported in milliequivalent units. (The new metric system molecular SI unit is numerically the same as the univalent charged ion when expressed as milliequivalents.) True ionic balance in serum is difficult to understand when constituents are re-ported in different terms, eg, the foregoing ions are reported in mEq/L, whereas the divalent ion,
cal-cium and magnesium, are still often expressed as mg/100 ml. SI would impose consistency.
ADVANTAGES AND DISADVANTAGES
Certain stated advantages of metric conversion are compelling. Scientists from all fields and of all nationalities might eventually communicate in the
Quantity Symbol
Ohm
TABLE 3. Some SI Derived Units
Area Volume
Velocity
Wave number
Density, mass density
Concentration (amount of
substance) Activity (radioactive) Specific volume Luminance Frequency Force Pressure
Energy, quantity of heat, work
Power
Electric potential, potential difference, electromotive force Electric resistance Name Square meter Cubic meter Meter per second
1 per meter
Kilogram per cubic
me-ter
Mole per cubic meter
1 per second
Cubic meter per kilo-gram
Candela per square
me-ter Hertz Newton Pascal Joule Watt Volt rn’ rn/s kg/rn’ rnol/m’ s-i rn’/kg cd/rn2 Hz =
N = rn.kg.s2
Pa = N/rn2 = m ‘ .kg .s
J = N.m = rn2.kg.s2
W = J/S = rn2.kg.s
V = W/A = rn2.kg.s’.A’
2 = V/A = rn2.kg.s’.A2
TABLE 4. SI Prefixes
Factor Prefix
Name Symbol
Factor Prefix
Name Symbol
10’exa- E 10 “ atto- a
i0’ peta- P 10’ fernto- f
1012 tera- T 102 pico- p
i0 giga- G 10” nano- n
io’
mega- M 10rnicro-i’ kilo- k 1(1’ rnilli- m
102 hecto- h 102 centi- c
101 deca- da 10’ deci- d
our present, and often arbitrary, collection of med-ical units.
A potential disadvantage is danger to the patient by possible error engendered through unfamiliarity. Furthermore, SI is another change thrust on the physician, who is already overburdened with sci-entific advances and must commit to memory a new set of normal values. Also, the logistic consid-erations of recalibration, new report forms, and reeducation are considerable.”
SPECIAL PROBLEMS
Two of the SI-derived units relating to medicine are unknown to most clinicians.” The existence of a clear-cut advantage to medicine is debatable. It has been recommended that the joule replace the calorie in nutrition and dietetics. One calorie equals 4.184 J; therefore, a 1,000-calorie diet equals a 4.18 kilojoule diet. There is dispute about its adoption in the United States, although there is support for its use in Great Britain.’2 Equally unfamiliar is the
pascal, the SI unit of pressure. Blood pressures are now recorded in millimeters of mercury (or “torr”). The pascal (kg - m ‘ . 2, or the newton per square
meter) is proposed to replace all other pressure units. Although the pascal is too small for conven-tional clinical use, the kilopascal (10’ pascal) has an appropriate magnitude. One kPa is equal to 7.5006 mm Hg; therefore, a blood pressure of 130/80 mm
Hg would be approximately 17/11 kPa. Objections to its use have been summarized.”
An especially difficult problem is the measure-ment of enzymatic activity. The previously intro-duced international unit is defined as the amount of enzyme that will catalyze the transformation of
1 micromole of substrate per minute under standard conditions. Acceptance of this unit has been slow, and many traditional units are still used. To con-form with SI, a new base unit for enzymatic activity named the katal ( 1 mole per second) has recently been proposed, but it is not an official recommen-dation. A wide variety of methods and conditions are used to measure enzymes, such as pH and temperature. Attempts are being made at the
inter-national level to standardize assay conditions. Ad-ditionally, the mass and purity of proteins are com-monly unknown. One country suggested that no change be made in units of tests already being performed. As new assays are introduced, moles per second (or a submultiple) might be used.’4
RECEPTION AND EXPERIENCE IN MEDICINE
rec-ommendation that SI be adopted by medicine worldwide. In 1976 an international symposium sponsored by the World Association of Societies of Pathology, the United States National Bureau of Standards, and the American Medical Association also endorsed SI.’ The Journal of the American Medical Association has committed itself toward progressive usage of SI,’ as has the American Journal of Clinical Pathology.’6 The Annals of Internal Medicine announced a positive editorial policy regarding SI, but it intends to maintain a
“balance between innovation and
compre-‘7
A study committee of the Massachusetts Medical
Society, charged to consider the principle that “changes in well established units be made only when definite advantages to the physician or the patient have been clearly demonstrated,” suggested
that there be “no abrupt change in the current use of mass concentration.” ‘ More recently, the Coun-cil on Scientific Affairs of the American Medical Association “advises that the AMA proceed to de-velop a wide concensus on how broadly SI units should be adopted by medical groups as well as medically oriented groups: nurses, biomedical sci-entists, laboratory technicians, other paramedical
groups, and manufacturers of medical and labora-tory equipment. It suggests a broad educational
effort, once new units have been generally decided
upon, to disseminate the information widely before a prearranged transition date. For a year or more prior to the transition date, new units would be
used parenthetically with the comparable old units in all AMA and other cooperating publications. For at least a year after the transition date, the old units will be used parenthetically with the new.
Until the above arrangements can be brought to frui-tion, the Council makes the following recommendations regarding the use of SI units in clinical chemistry:
1. That the use of mass concentration units (wgt/vol)
be retained by medical laboratories until it is shown that
a change to mole concentrations will improve patient care
(diagnosis, treatment, or follow-up), or prove a significant
advantage with respect to laboratory measurement tech-nique.
2. That no abrupt changes in the current use of mass
concentrations, or in mihiequivalent units for certain
electrolytes, be undertaken until an overall plan and
schedule has been agreed upon by representative
medi-cally oriented groups and appropriate councils of the American Medical
SI would likely be much more readily accepted by physicians if they could better monitor patients, diagnose diseases, or treat patients. The proponents of the system have described theoretical advantages without demonstrating actual benefits. What has the reported experience with SI been among
phy-sicians? In the Netherlands, 53 senior specialists in internal medicine, who used SI for two to five years, were polled. Fifteen said they grasped the units in three months, 20 needed from three months to one year, and 18 required longer. More importantly, 47 did not think substance concentration enabled them to provide better treatment. Eighteen felt the use of SI units provided them with better insight into
chemical processes. In Canada, after six months,
16% of a medical staff no longer converted SI to traditional units, and 78% occasionally did It is vital to further document advantages and
disadvan-tages actually experienced in those countries using SI.
MEDICAL LITERATURE
Contemporary medical literature in the United
States has fallen “out of step” with many of the prominent European medical journals, which now report in SI units. Many readers in the United
States are unprepared to interpret data recorded in unfamiliar terms. Likewise, non-Americans are now beginning to lose familiarity with conventional units. This situation is intolerable and detrimental
to progress in medicine, and it eventually will ad-versely affect patient care. But it is avoidable.
Medical journals both here and abroad should
publish data in recommended SI and traditional
units.2’ Appropriate conversion data could be in-cluded. The educational benefit, as well as clarity of communication, would be great. Medical person-nel in this country could begin thinking in new terminology as well as begin testing the advantages of SI. Hopefully, a dual reporting system would avoid confusion and inaccuracy, as well as the in-ability to interpret data, while promoting interna-tional medical communication. Most importantly, by familiarizing ourselves with SI, the members of the United States’ medical community might then participate more fully in the further refinement of the system. We could assist in perfecting and ex-tending the system in a manner acceptable to Amer-ican medical scientists rather than having a system that is completed elsewhere thrust on us.
COMMITTEE ON HOSPITAL CARE, 1978-79
Arno R. Hohn, MD, Chairman Alfred B. Amler, MD Paul S. Bergeson, MD Harvey R. Gold, MD
Stewart L. Griggs, MD
Gerald B. Shattuck, MD
Stanford A. Singer, MD Edwin Ide Smith, MD Willis A. Wingert, MD
RECOMMENDATIONS FOR THE PRESENT
Suggestions have been made in hospitals for
con-version to the metric system.24 The Committee on Hospital Care joins in supporting hospital adoption
of the metric system in the following hospital areas:
(1
)
the recording of all patient measurements, in-cluding temperature (in Celsius scale), body mass (in kilograms), and linear dimensions (in meters or subunits); (2) internal ordering and administering of all drugs and medication (in mass concentration for the present); (3) food service and dietary for-mulas (the calorie should be retained for the pres-ent); (4) all reports and records; and (5) external ordering and purchasing.Also encouraged (although not an SI term) is the keeping of time records on the 24-hour clock to avoid confusion between AM and PM notations. Ad-vantages to hospitals have been listed and proce-dures for hospital implementation suggested, in-cluding involvement of boards of directors, use of target dates, and training programs.2’ Successful hospital experiences have been documented.2’27
RECOMMENDATIONS FOR THE FUTURE
The Committee on Hospital Care recommends that the American Academy of Pediatrics join with the American Medical Association and other inter-ested organizations such as the American National Metric Council to further clarify biomedical issues concerning SI. Among the specific issues which should be examined are the following: (1) dual reporting by American medical journals; (2)
reten-tion of the term pH, with simultaneous reporting of hydrogen ion concentration in nanomoles per liter; (3) use of substance versus mass concentration or a combination of the two; (4) use of millimoles per liter for hemoglobin concentration versus grams per deciliter; (5) use of freezing point depression versus retention of reporting of osmolality in traditional terms; (6) use of the pascal versus millimeter of mercury; (7) use of the joule versus calorie; (8) encouragement of academic centers to use and teach SI; and (9) use of the katal if it is officially recommended for the unit of enzyme activity.
Once agreement has been reached on desirable SI uses, programs of orderly adoption should be implemented.
ACKNOWLEDGMENTS
The Committee is grateful for invaluable input by the
following individuals in the preparation of the
manu-script: H. Peter Lehmann, PhD, Bradley E. Copeland,
MD, and Alan K. Done, MD.
REFERENCES
1. Brief History of Measurement Systems with a Chart of the
Modernized Metric System. National Bureau of Standards, Special Publication 304A, revised August 1976
2. Baron DN: SI units. Br Med J 4:509, 1974
3. Lehmann HP: Metrication of clinical laboratory data in SI
units. Am J Clin Pathol 65:2, 1976
4. International Union of Pure and Applied Chemistry
Com-mission on Quantities and Units, Clinical Chemistry and
International Federation of Clinical Chemistry: Expert panel on quantities and units. List of quantities in clinical
chem-istry recommendation 1973. Pure Appi Chem 37:519, 549,
1974
5. Metric Conversion Act of1975, Public Law 94-168. 89 Stat. 1007
6. McGehan FP: America joins a metric world. Dimensions
(National Bureau of Standards) 60:6, February 1976 7. Copeland BE: SI units: A clarification. Am J (‘tin Pathol
65:20, 1976
8. Young DS: Standardized reporting of laboratory data: The
desirability of using SI units. N Engi .1 Med 29():368, 1974
9. Lippert H, Lehmann HP: SI Units in Medicine: An
Intro-duction to the International System of Units with (‘onver-sion Tables and Normal Ranges. Baltimore, Urban and Schwarzenberg, 1978
10. Shepard I)A: The metric system, the international system
of units (SI) and medicine. Can Med Assoc J 112:799, 1975
11. Huth El: Metricating medicine: How fast, how far? N Engi
J Med 290:398, 1974
12. The Royal Society: Metric units, conversion factors and
nomenclature in nutritional and food sciences: Report of the subcommittee on metrication of the British National Corn-mittee for Nutritional Sciences. Proc Nutr Soc 31 :239, Sep-tember 1972
13. Rose JC: Pressures on the millimeter of mercury. N EngI J
Med298:1361, 1978
14. Pannall PR: The introduction of SI units and the
standard-ization of laboratory reports: Recommendations of the South
African As.sociation of Clinical Biochemists. S Afr Med J
50:1539, 1976
15. Barclay WR: Standardizing units to measurements. JAMA
236:1981, 1976
16. Beeler MS: Metrication from crawl to walk. Am J Clin
Pathol 65:19, 1976
17. Huth EJ: SI for metric medicine? Ann Intern Med 76:322,
1972
18. Copeland BE, Beautyman W. Bradley R. et al: Study
corn-mittee to evaluate changes in units of clinical chemistry tests. N Engi J Med 293:43. 1975
19. Report E of the Council on Scientific Affairs of the American Medical Association (A-78): Adoption of International Sys-tern of (SI) Units for clinical chemistry. American Medical Association House of I)elegates Proceedings, St Louis, June
18-22, 1978, p 291
20. Karnauchow PN, Suvanto L: Experience with SI units in
biochemistry. Can Med Assoc J 114:533, 1976
21. Young 1)5: SI units for clinical laboratory data. .JAMA
22. Young 1)5: Normal laboratory values” (case records of the Canadian teaching hospital. Can Med Assoc J 1 14:536, 1976
Massachusetts General Hospital) in SI units. N EngI J Med 25. Rennie G: Yes! to metric system conversion. Hosp Prog
292:795, 1975 58:36, 1977
23. Frost I)V, Helgren FJ, Sokol LF: Metric Handbook for 26. Timmerman MR, Lobel J: Beating the deadline: Converting
Hospitals, ed 2. Boulder, Colorado, US Metric Association, to metric system now. Hosp Prog 58:72, 1977
Inc. 1975 27. Weissman ME: No more inches, no more pounds. Hospitals
24. Itiaba K, Crawhall JC: Metrication on the move in a large 51:103, 1977
DR WILLIAM A. ALCOTT ON MATERNAL DOSING AND DRUGGING AS THE
MAJOR CAUSE OF INFANT MORTALITY IN 1854
Among the most outspoken of American health reformers of the
mid-nine-teenth century was Dr William A. Alcott of Boston. The health reformers offered an alternative to a public dissatisfied with the heroic practice of most physicians of this period by emphasizing that the individual had it in his or her own power to keep all members of the family in good health by forgoing most of the drugs prescribed by allopathic physicians.
Alcott’ was particularly concerned with the huge extent of infant mortality caused by “maternal dosing and drugging.” He wrote:
But whether ignorant or somewhat enlightened, the vast majority of our mothers doctor, more or less, their own children. At least, if they refuse to call it doctoring, they give them a vast amount of small elixirs, cordials, etc. The closets of not a few
house-keepers are a complete apothecary’s shop. They may, it is true, have smaller parcels then
the regular apothecary; but they have almost as great an assortment. And they not only keep it; they administer it. They may not intend it; they do not mean to give much;
sometimes they really think they do not give much. But it comes to pass, in the course of the year, that much is given by somebody; and I greatly fear that the mother must be held responsible for it. ...
But now for the consequences of this maternal dosing; for this it is with which medical men have chiefly to do. Next to bad food and wretched cookery, as I have before intimated, this error is productive of more sickness and premature death than any other.
No physician knows what to do with a sick child, who has been thus tampered with. ...
He may indeed guess a little better than others; but even he will often guess wrong.
Their first passages are irritated, and perhaps inflamed; and if it were possible to make the right appliances either internally or externally, it would stifi puzzle the wisest head to know how to apportion the quantity so as to be more likely to do good than harm.
Diseases, in these circumstances, as you know, are more apt to be severe and complicated, and the termination more likely to be fatal, especially if much medicine is used.
The worst remains to be told. As it is not always easy to trace the cause of severe, protracted or fatal infantile disease to maternal error, we not only contrive to kill, from generation to generation, by thousands and tens of thousands; but we partly kill by millions. . .. We bring on, gradually, some disease or other; or we render an inherited
disease, which might have been mild, very severe, or early fatal; or we aggravate, by over dosing, the symptoms of acquired diseases from other causes.
Noted by T.E.C., Jr, MD
REFERENCE
