Units of Measure in Clinical Information Systems
- Affiliations of the authors: Regenstrief Institute for Health Care and Indiana University, Indianapolis, Indiana (GS, CJM, JGS); Benjamin Franklin University Hospital (UF) and Freie Universität (UF, TT), Berlin, Germany
- Correspondence and reprints: Gunther Schadow, MD, Regenstrief Institute for Health Care, 1001 West 10th Street, RHC 5th Floor, Indianapolis, IN 46202-2859;e-mail: 〈schadowg{at}regenstrief.iupui.edu〉
- Received 27 July 1998
- Accepted 13 November 1998
Abstract
The authors surveyed existing standard codes for units of measures, such as ISO 2955, ANSI ×3.50, and Health Level 7′s ISO+. Because these standards specify only the character representation of units, the authors developed a semantic model for units based on dimensional analysis. Through this model, conversion between units and calculations with dimensioned quantities become as simple as calculating with numbers. All atomic symbols for prefixes and units are defined in one small table. Huge permutated conversion tables are not required. This method is also simple enough to be widely implementable in today's information systems. To promote the application of the method the authors provide an open-source implementation of this method in JAVA. All existing code standards for units, however, are incomplete for practical use and require substantial changes to correct their many ambiguities. The authors therefore developed a code for units that is much more complete and free from ambiguities.
Footnotes
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This work was supported in part by grant HS08750 from the Agency for Health Care Policy and Research and by contract N01-LM-63546 from the National Library of Medicine.
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↵* For instance, Abernathy's Surgical Secrets12(p30) gives the formula
for the calculation of mixed venous oxygen saturation. Of course, it is assumed here that the cardiac output (CO) and the
hemoglobin concentration (Hb) belong in the denominator as well.
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↵† Although the “venous” counterpart of “MM (HG)” is inconsistently written as “CM H2O.”
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↵‡ Conventional notation uses [Q] for the unit and {Q} for the measurement value of the quantity Q. Those brackets and curly braces are distracting, however, and do not contribute any essential information.
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↵§ Although, in the unit 1 dyn·s·cm-5 for vascular resistance we encounter an exponent -5.
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↵‖ For linear independence,
may be zero only if α1,α2,…,αn are all zero.
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↵¶ It must be noted that the sum of two quantities is meaningful by itself only in so called “extensive” measures. The sum of conjoint derived measures, such as two densities (ρ = m/V), is by itself meaningless. However, mathematically it is useful to define the sum in general; otherwise one would lose the distributive property of physical quantities that allows one to write m = V·ρ1 + V·ρ2 = V·(ρ1 + ρ2).
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↵# In our model nothing favors one particular base system over another set of isomorphic systems. Thus, SI should be used not because it is inherently better in the sense of this theory, but because it is an international standard. However, our method applies as well to any other system of units.
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↵** At the same time this illustrates the virtue of defining the amount of substance as a dimensionless kind of quantity, representing a number of particles: After conversion of, say, pH 9 to an H+ ion concentration of 1 nmol/L we can directly proceed to the number of H+ ions per volume: 602.204/pL. Indeed, pH, nmol/L, and 1/pL all measure the same kind of quantity, which is obscured by the conventional notion of a dimensionless pH and of the mole as a unit on its own dimension.
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↵‡‡ Available at: http://aurora.rg.iupui.edu/units. There is also a JAVA applet that demonstrates the conversion facility.
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↵‡‡ Krantz et. al19(p474ff) give a solution for the vector problem by splitting length into three base dimensions, one for each axis of the Euclidean space. This solves the vector problem completely, since length is the only vector base kind of quantity. However, because there is no fixed coordinate system of space, mapping conventional units onto such a system is not one-to-one.
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↵§§ The Unified Code for Units of Measures is available at: http://aurora.rg.iupui.edu/UCUM.
