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Metrication and SI Units* Committee on Hospital Care Metrication and SI Units* Because of the increasing international use of the Weights and Measures at S#{232}vres, France, which SI system (International System of Units) in med- acts as an international standards reference labo- icine, the Committee on Hospital Care has written ratory and as the permanent secretariat for the this statement to familiarize pediatricians with this Metre Convention. The General Conference on concept. The current state of the system, its den- Weights and Measures, the diplomatic organization vation, purported advantages, and controversial as- made up of adherents to the Convention (now in- pects are described; and the Committee has made cluding 41 member countries), is the ultimate au- specific recommendations for consideration regard- thority on the definition of units. In 1960 the Gen- ing its future use and development. eral Conference standardized metric units into the simplified and logical system known as SI. Further BACKGROUND refinements have been made by the General Con- ference on four subsequent occasions. ‘ The present The British Imperial System of Weights used in structure of the SI is likely to be the permanent the United States today derives from a variety of one. ancient cultures. A Roman contribution is the use In parallel with the developments on the gener- of the awkward number 12 as a base. Royal decree alized use of SI units, recommendations for the established the yard as the distance from the tip of standardized reporting of clinical laboratory data the nose to the end of the thumb of King Henry I. based on SI were proposed by the Commission on The inch was based on the size of three grains of Clinical Chemistry (CCC) of the International Un- barley “dry and round.” Equally illogically derived ion of Pure and Applied Chemistry (IUPAC) and units evolved to eventually form the irrational Eng- the International Federation of Clinical Chemistry lish “system.” (IFCC).4 The metric system with its “base-lO” or “deci- In recent years, the major industrial nations of mal” system derived its units of mass and volume the world, with the exception of the United States, from its units of length, thus correlating its basic have been using the metric system or converting to units to each other. The need for further refinement its use. The United States has been an “island in of metrics and a single worldwide and interdiscipli- the metric sea.” The British Commonwealth coun- nary system of measurements led to the develop- tries with whom we shared the pounds-quarts-yards ment of the International System of Units (le Sys- system for so long have all converted to metric. t#{232}meInternational d’Unit#{233}s) with the international Canada has an unofficial goal of being metric by abbreviation “SI.” This is sometimes referred to as 1980. Eventually, the Congress of the United States “the modern metric system” and is said to complete passed the Metric Conversion Act of 1975 stating, the process of metnication.2 “It is therefore declared that the policy of the Seventeen countries, including the United States, United States shall be to coordinate and plan the signed the Metre Convention in 1870. This led to increasing use of the metric system.” Furthermore, the establishment of the International Bureau of the Act states, “. the term ‘metric system of mea- surement’ means the International System of * Abbreviations used in Pediatrics currently follow the Style- Units.”5 This is to be done on a voluntary basis. book/Editorial Manual of the American Medical Association. Industry is becoming increasingly involved in Abbreviations used in this article are slightly different from the metric conversion with the realization that the Stylebook. PEI)IATRICS (ISSN 0031 4005). Copyright © 1980 by the United States cannot be competitive on the inter- American Academy of Pediatrics. national scene without using metrics. Schools in the Downloaded from www.aappublications.org/newsAMERICAN by guest on ACADEMYSeptember 30, 2021 OF PEDIATRICS 659 United States have begun teaching SI as the pni- factor. Noncoherent units are derived from the base mary measurement system.’ units but contain a factor, eg, the liter (L, recom- In Finland, Norway, and Sweden, the use of SI is mended symbol for the liter in the United States) compulsory in medical laboratories. Table 1 lists equals the decimeter cubed (dm’). Table 4 shows the years by which a phased introduction of SI will the prefixes that denote multiples of SI units. be or has been completed in various countries. Although mass concentration (gIL) and sub- stance concentration (mol/L) are both included in SI SYSTEM the SI system,7 there are certain advantages to recording in substance concentration. By reporting SI is a system of “base units” and “derived units” data as we do now, the apparent quantities of and their interrelationships. Seven base units and different materials may be misrepresented. Corn- two supplementary units have been adopted (Table pounds with a high molecular mass might seem to 2). Each unit can be defined in specific terms. For have more molecules present as compared with example, the mole is the amount of substance that those of lower molecular weight. Furthermore, cer- contains as many elementary entities as there are tam biologic relations between blood constituents atoms in 0.012 kg of carbon 12. Base units are may be made clear when measurement is on the multiplied and divided to form derived units (Table basis of their relative number. Such relations may 3). For example, the unit for force, the newton, is be masked by usage of mass concentration but may derived from three base units according to the be better visualized in molar terms. For example, expression m kg . 2#{149} Coherence is the derivation an unconjugated biirubin concentration might be of a unit from a base unit without the use of a 0.4 mg/dL and the serum albumin, 4.0 g/dL. The concentration of biirubin might seem to be ‘/i,xx of TABLE 1 . Year of Introduction or Anticipated Introduction of SI Units that of albumin. In molar terms, however, there is only a 100-fold difference. The same concentrations Year Country are 6.8 and 620 imol/L, respectively. A serum bili- 1970 Netherlands rubin concentration of 20 mg/dL, which is a level of 1971 Denmark, Finland 1974 Australia clinical importance in neonatology, is 340 imol/L, 1975 United Kingdom or more than half the molar concentration of the 1976 New Zealand, South Africa, Sweden albumin in this example.” It becomes obvious why 1977 Norway a small change in compounds bound to albumin 1978 Federal Republic of Germany may result in displacement of bilirubin or other 1979 Czechoslovakia, Democratic Republic of Ger- many compounds from their binding sites on the albumin 1980 Hungary, Italy molecule. Another example is the electrolytes, so- 1981 Japan, Switzerland, Yugoslavia dium, potassium, and chloride, whose interrelations 1982 Ecuador have become more apparent since they have been reported in milliequivalent units. (The new metric system molecular SI unit is numerically the same TABLE 2. SI Base Units as the univalent charged ion when expressed as Quantity Name Symbol milliequivalents.) True ionic balance in serum is Length Meter (metre)* m difficult to understand when constituents are re- Mass Kilogram kg ported in different terms, eg, the foregoing ions are Time Second s reported in mEq/L, whereas the divalent ion, cal- Electric current Ampere A Thermodynamic temper- Kelvin K cium and magnesium, are still often expressed as aturet mg/100 ml. SI would impose consistency. Amount of substance Mole mol Luminous intensity Candela cd ADVANTAGES AND DISADVANTAGES Supplementary Units Plane angle Radian rad Certain stated advantages of metric conversion Solid angle Steradian sr are compelling. Scientists from all fields and of all * Both spellings acceptable. nationalities might eventually communicate in the t The Celsius temperature scale (formerly called centi- same units of measure. The increase in ease and grade) is used for most medical and commercial purposes. precision of scientific communication and eventual The Kelvin (the unit for thermodynamic temperature) is avoidance of misunderstanding and error by inter- the SI unit for temperature. Although their scale origins national agreement on terminology would be enor- differ, the degree Celsius equals the Kelvin in magnitude; thus, a rise in body temperature of 1.0 K is equivalent to mous. SI has the advantage of being logical, sys- a rise of 1.0 C. 0 C is defined as 273.15 K, thus 98.6 F = 37 tematic, international, and interdisciplinary.9 It has C = 310.15 K. been suggested that SI would eventually replace 660 PEDIATRICS Vol.Downloaded 65 No. from 3 March www.aappublications.org/news 1980 by guest on September 30, 2021 TABLE 3. Some SI Derived Units Quantity Name Symbol Area Square meter Volume Cubic meter rn’ Velocity Meter per second rn/s Wave number 1 per meter Density, mass density Kilogram per cubic me- kg/rn’ ter Concentration (amount of Mole per cubic meter rnol/m’ substance) Activity (radioactive) 1 per second s-i Specific volume Cubic meter per kilo- rn’/kg gram Luminance Candela per square me- cd/rn2 ter Frequency Hertz Hz = Force Newton N = rn.kg.s2 Pressure Pascal Pa = N/rn2 = m ‘. kg . s Energy, quantity of heat, Joule J = N.m = rn2.kg.s2 work Power Watt W = J/S = rn2.kg.s Electric potential, potential Volt V = W/A = rn2.kg.s’.A’ difference, electromotive force Electric resistance Ohm 2 = V/A = rn2.kg.s’.A2 TABLE 4. SI Prefixes pascal, the SI unit of pressure. Blood pressures are Factor Prefix Factor Prefix now recorded in millimeters of mercury (or “torr”).
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