Hematology and the World Association of
(Anatomic and Clinical) Pathology Societies.
Although the medical community in the United
States has not generally been supportive of the
Recommendations, laboratories and medical journals
here may someday accede to its implementation.
The system of units advanced in the IUPAC
recommendations is based upon the SI, the use of
substance quantities (such as mole) rather than mass
quantities (such as grams), and the use of the liter as
the preferred unit of volume (see Table 1.3). The
medical arguments in favor of using molar units rest
upon their physiologic appropriateness. The electro-
chemical activity and osmolarity of solutes are deter-
mined by their molar concentration as is the bioac-
tivity of hormones and the binding capacity of
ligand-binding proteins. In addition, the use of
molar units preserves the quantitative relationships
between metabolic precursors and products.
A special consideration in the use of substance
quantities is the expression of the concentration of
catalytic activity. Rather than defining the molar
concentration of an enzyme present in a specimen,
the molar flux of substrate acted upon by the enzyme
is measured and reported. The reporting unit, the
katal, is equal to one mole substrate transformed per
second per liter. This approach is appropriate
whenever the actual catalytic activity of an enzyme
or set of enzymes with overlapping substrate speci-
ficities is the physiologic entity of interest.
The flexibility of SI units is increased by the use
of magnitude prefixes. Rather than being restricted
to using a scale graduated in unit divisions, measure-
ments can be based upon divisions that are powers-
of-ten multiples of the standard unit. For example, a
substance that is present in a concentration of 1.6 x
10
-9
moles per liter can be described as having a
concentration of 1.6 nanomoles per liter. The prefix
"nano-" takes the place of the factor 10
-9
. Similarly,
large numbers can be avoided by using unit prefixes.
A partial pressure of 6 x 10
3
Pascal becomes 60
kiloPascals. Table 1.4 lists approved magnitudes
prefixes. Notice that the prefixes used with SI units
are only for magnitude changes that are third
powers-of-ten. Prefixes are not generally used to
describe multiples of time greater than one second
because the common units of time, such as minute
and day, are ingrained in scientific as well as every-
day usage. Non-SI units of time and their preferred
symbols are listed in Table 1.5.
Laboratory-based Medical Practice
1-3
Table 1.3
SI Quantities and Units Used in Laboratory Medicine
Quantities
Derivation
Unit
Symbol
Base quantities
time
second
s
length
meter
m
number
1
[none]
mass
kilogram
kg
amount of substance
mole
mol
Derived quantities
volume
length
3
liter
L
flux
volume
volume/time
L/s
number
number/time
/s
mass
mass/time
kg/s
substance
amount/time
mol/s
concentration
number
number/volume
/L
mass
mass/volume
kg/L
substance
amount/volume
mol/L
fraction
volume
volume/volume
[none]
number
number/volume
[none]
catalytic activity
substance flux/volume
katal per liter
kat/L
pressure
force/area
Pascal
Pa
