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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