Unit conversion
Until Recommendations 1978 and 1984 have
been fully integrated into clinical laboratory practice
for some years, physicians will have to deal with
two unit systems, SI units and so-called common
units. Inevitably some results expressed in common
units will need to be converted to SI units and
vice-
versa
. Measurements are converted from one scale
of measurement to another by substituting a numeri-
cally equivalent number of units from the new scale
of measurement in the place of the original unit.
For example, to convert the measurement 1.2 mg
creatinine/dl to its SI expression in µmol
creatinine/L, dl must be converted to its L equiva-
lent and mg creatinine must be converted to its µmol
creatinine equivalent. There are 10 dl per L,
1.2
mg
dl
%
10
dl
L
=
12
mg
L
There are 8.85 µmol creatinine in 1 mg creatinine,
12
mg
L
%
8.85
mol
mg
=
106
mol
L
So the SI equivalent is 106 µmol creatinine/L.
The number of units of one scale of measure-
ment contained in 1 unit of a comparable scale of
measurement is called the conversion factor between
the units. It is the number by which the value of a
measurement is multiplied to re-express the measure-
ment as multiples of the alternate unit. Here, mg
creatinine/dl are converted to µmol creatinine/L
using the conversion factor 88.5. Care must be
taken when using conversion factors to make certain
that the factor used is appropriate for the direction of
the conversion. Extensive tables for unit conversion
can be found in Lippert and Lehmann (1979).
Calculated values
When direct measurement of a quantity is
impractical or impossible, its magnitude may be
estimated by calculation from related measurements.
The measurements that serve as quantitative input
for these calculations may possess an exact theoreti-
cal relationship to the unmeasured quantity, such as
that between bicarbonate concentration and the ratio
of the partial pressure of carbon dioxide to the
hydrogen ion concentration (Kassirer and Bleich
1965),
[HCO
3
-
] in mmol/L = 180 x
pCO
2
in kPa
[
H
+
]
in nmol
/
L
Alternatively, a calculation may be based upon
an empirical relationship between the measured and
unmeasured quantities. The calculation of body
surface area using body weight and weight is an
example.
Calculated values can be obtained in two ways.
If a mathematical formula is available, the value can
be computed. This has become particularly simple
since the advent of inexpensive, powerful hand-held
calculators. Calculated values can also be found
without performing computations by using tables,
which are usually too large to be convenient, or
graphical representations of mathematical equations,
called nomograms. For instance, body surface area
in m
2
can be calculated from body weight and
height. Using the formula proposed by Gehan and
George (1970),
surface area in m
2
=
0.0235 x (weight in kg)
0.51456
x (height in cm)
0.42246
Using this formula, a person who weighs 64 kg
and is 145 cm tall has a surface area of 1.64 m
2
.
The authors also provide a table in their paper. The
table entry for 64 kg and 145 cm is 1.64 m
2
. A
nomogram of their formula is also offered by the
authors for those readers who prefer not to perform
calculations or to look up table entries. Because that
nomogram is somewhat difficult to use, another
Laboratory-based Medical Practice
1-4
Table 1.4
Magnitude Prefixes for Units
Factor
Prefix
Symbol
10
9
giga
G
10
6
mega
M
10
3
kilo
k
10
-3
milli
m
10
-6
micro
µ
10
-9
nano
n
10
-12
pico
p
10
-15
femto
f
10
-18
atto
a
10
-21
zepto
z
Table 1.5
Non-SI Units of Time
Unit
Symbol
minute
min
hour
h
day
d
week
wk
month
mo
year
y
