Transition to SI Units
At its October 1990 meeting the Scientific Publishing
Committee explored the use and prevalence of Systeme
International (SI) units for reporting measures of clinical and laboratory data. The committee since has
sanctioned the use of SI units in the American Heart
Association (AHA) journals.
The SI, an update of the metric system, is the
outcome of a century of effort to provide a common
system of measurement between nations and among the
sciences. To promote its use, which can reduce the
present confusion about measurements, the World
Health Assembly in 1977 recommended the use of SI
units in medicine.
The SI base units are the meter, kilogram, second,
ampere, kelvin, candela, and mole, respectively representing length, mass, time, electric current, temperature, luminous intensity, and amount of substance. By
multiplying a base unit by itself, or by combining two or
more basic units by multiplication or division, many
units can be formed, known as Si-derived units. Examples of derived units are the square meter, cubic meter,
mole per cubic meter, pascal (Pa), and joule (J).
Exceptions to the rule for SI unit conversion as
currently applied to biomedical sciences include blood
pressure, oxygen pressure, and enzyme activity. Retained as presently used are temperature, the pH scale,
and the use of liter for volume. Table 1 illustrates the
measurements excluded from SI unit conversion.
In the AHA journals, an average article contains few
items that need conversion. Often the same conversion
is made over and over in a manuscript and takes little
extra effort. It is our belief that, in return for a small
effort, the AHA can take a large step, along with many
other international and domestic journals, toward perpetuating a common system for reporting medical and
scientific measurements. The SI unit is to be used in
text, followed by the presently used measurement in
parentheses.
The accompanying conversion table (Table 2) lists the
measurements most commonly used in the AHA jourTABLE 1.
Normal Laboratory
Valuest
Conversion
Factor
SI Unit
M (molar)
1
mol/L
mM
1
mmol/L
torr
1
mm Hg
atm
101.325
kPa
0.01
No units
36-54
0.36-0.54
1
1012/L
4.0-6.0
4.0-6.0
0.001
109/L
5000-10 000
5.0-10.0
0.02586
mmol/L
<200
<5.20
1
mmol/L
135-145
135-145
1
mmol/L
3.5-5.0
3.5-5.0
0.5
mmol/L
4.5-5.5
2.25-2.75
4.2
J
1
S
Current Unit
Current
SI
Concentration
Pressure
Hematocrit
%
Red blood
cell count
10 6 /mm 3
White blood
cell count
/mm3
Cholesterol
mg/dL
Sodium ion
(Na+)
mEq/L
Potassium Ion
(K+)
mEq/L
Calcium Ion
(Ca2+)
Energy
Calories
Current Unit
Blood pressure
mm Hg
Oxygen pressure
mm Hg
Enzyme activity
IU
H + concentration
PH
Temperature
°C
Volume
TABLE 2. Examples of Measurement Conversions to
Systeme International (SI) Units for American Heart
Association Journals*
mEq/L
Measurements Currently Not Converted
to Systeme International (SI) Units in
Biomedical Applications
Measurement
nals and their corresponding SI units. A review of this
table may serve as an introduction to the forthcoming
transition to SI units.
Conductance
mho
*For a brief discussion of the development and use of SI units,
see World Health Organization: The SI for the Health Professions,
Geneva, Switzerland: World Health Organization, 1982. For a
convenient list of commonly used laboratory measurement conversions to SI units, see "SI unit implementation—the next step"
(editorial) in JAMA (1988;260:73-76).
tFor illustration only; normal values may vary by laboratory.
L
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