metrication and si units* - pediatricsbase units are multiplied and divided to form derived units...

Committee on Hospital Care

AMERICAN ACADEMY OF PEDIATRICS 659

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. Furtherrefinements 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.

Industry is becoming increasingly involved in

metric conversion with the realization that the

United States cannot be competitive on the inter-

national scene without using metrics. Schools in the

by guest on March 26, 2020www.aappublications.org/newsDownloaded from

1980

19811982

660 PEDIATRICS Vol. 65 No. 3 March 1980

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, Finland1974 Australia

1975 United Kingdom

1976 New Zealand, South Africa, Sweden

1977 Norway1978 Federal Republic of Germany

1979 Czechoslovakia, Democratic Republic of Ger-many

Hungary, Italy

Japan, Switzerland, YugoslaviaEcuador

TABLE 2. SI Base Units

Quantity Name Symbol

Length Meter (metre)* mMass Kilogram kgTime Second sElectric current Ampere AThermodynamic temper- Kelvin K

aturet

Amount of substance Mole molLuminous intensity Candela cd

Supplementary UnitsPlane angle Radian radSolid angle Steradian sr

* Both spellings acceptable.

t The Celsius temperature scale (formerly called centi-grade) is used for most medical and commercial purposes.

The Kelvin (the unit for thermodynamic temperature) isthe 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

same units of measure. The increase in ease and

precision of scientific communication and eventual

avoidance of misunderstanding and error by inter-

national agreement on terminology would be enor-

mous. SI has the advantage of being logical, sys-

tematic, international, and interdisciplinary.9 It has

been suggested that SI would eventually replace


Quantity Symbol

Ohm

TABLE 3. Some SI Derived Units


Area

Volume

VelocityWave number

Density, mass density

Concentration (amount of

substance)

Activity (radioactive)

Specific volume

Luminance

FrequencyForce

PressureEnergy, quantity of heat,

workPower

Electric potential, potential

difference, electromotiveforce

Electric resistance

Name

Square meter

Cubic meterMeter per second1 per meterKilogram per cubic me-

terMole per cubic meter

1 per secondCubic meter per kilo-

gramCandela per square me-

terHertzNewton

Pascal

Joule

WattVolt

rn’rn/s

kg/rn’

rnol/m’

s-i

rn’/kg

cd/rn2

Hz =

N = rn.kg.s2

Pa = N/rn2 = m ‘ . kg . sJ = 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- ai0’� peta- P 10’� fernto- f1012 tera- T 10�2 pico- p

i0� giga- G 10” nano- n

io�’ mega- M 10� rnicro-

i��’ kilo- k 1(1’ rnilli- m102 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

Positive reactions from the medical community

have included the World Health Organization’s 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 massconcentrations, 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.

The disadvantage to dual reporting would be the

extra time demanded of authors to supply data in

two forms. However, this is not an unreasonable

demand of careful investigators. Dual reporting

would imply no firm commitment to SI in its en-

tirety, but rather a willingness to familiarize and

participate in the process of international agree-

ment. Only through such familiarity will physicians

be able to test the declared logic and beneficial

qualities of the system. Several helpful references

for authors and readers are available.’9 14,22


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 panelon quantities and units. List of quantities in clinical chem-istry recommendation 1973. Pure Appi Chem 37:519, 549,

19745. 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 measurement�s. JAMA236:1981, 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

240:1618, 1978



22. Young 1)5: Normal laboratory values” (case records of the Canadian teaching hospital. Can Med Assoc J 1 14:536, 1976Massachusetts 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 THEMAJOR 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 mothersdoctor, more or less, their own children. At least, if they refuse to call it doctoring, theygive 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 thenthe regular apothecary; but they have almost as great an assortment. And they not onlykeep 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 courseof the year, that much is given by somebody; and I greatly fear that the mother must beheld responsible for it. ...

But now for the consequences of this maternal dosing; for this it is with which medicalmen have chiefly to do. Next to bad food and wretched cookery, as I have beforeintimated, 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 makethe right appliances either internally or externally, it would stifi puzzle the wisest headto 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, fromgeneration to generation, by thousands and tens of thousands; but we partly kill bymillions. . . . We bring on, gradually, some disease or other; or we render an inheriteddisease, which might have been mild, very severe, or early fatal; or we aggravate, by overdosing, the symptoms of acquired diseases from other causes.

Noted by T.E.C., Jr, MD

REFERENCE

1. Alcott WA: Mortality among children. Boston Med Sung J 51:260, 1854


1980;65;659Pediatrics Gerald B. Shattuck, Stanford A. Singer, Edwin Ide Smith and Willis A. Wingert

Arno R. Hohn, Alfred B. Amler, Paul S. Bergeson, Harvey R. Gold, Stewart L. Griggs,Metrication and SI Units

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1980;65;659Pediatrics Gerald B. Shattuck, Stanford A. Singer, Edwin Ide Smith and Willis A. Wingert

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