A frequent problem in the interpretation of laboratory results is converting between different concentration measures in the metric system.
|
Metric Weight Units
|
Abbreviation
|
Gram Factor
|
|
kilogram
|
kg
|
1000
|
|
gram
|
g
|
1
|
|
milligram
|
mg
|
0.001
|
|
microgram
|
µg
|
10^(-6)
|
|
nanogram
|
ng
|
10^(-9)
|
|
picogram
|
pg
|
10^(-12)
|
|
Metric Molar Units
|
Abbreviation
|
Molar Factor
|
|
mole
|
mol
|
1
|
|
millimole
|
mmol
|
0.001
|
|
micromole
|
µmol
|
10^(-6)
|
|
nanomole
|
nmol
|
10^(-9)
|
|
picomole
|
pmol
|
10^(-12)
|
|
Metric Volume Units
|
Abbreviation
|
Liter Factor
|
|
liter
|
L
|
1
|
|
deciliter
|
dL
|
0.1
|
|
milliliter
|
mL
|
0.001
|
|
microliter (cc, lambda)
|
µL
|
10^(-6)
|
|
nanoliter
|
nL
|
10^(-9)
|
|
femtoliter
|
fL
|
10^(-15)
|
Conversion between Formats
value in new format =
= (((gram factor for original format) * (liter factor for new format)) / ((gram factor for new format) * (liter factor for original format))) * (value in original format)
If one of the two formats (to be changed from or to be changed to) is molar, then the molecular weight of the compound needs to be known. If both formats are molar, then the molecular weight is not needed.
gram factor for compound =
= (molar factor) * (molecular weight in grams)
Molecular Weights
The molecular weights for drugs is problematic. Some chemicals and drugs are prepared as salts, which dissociate in vivo. Since the salts are larger compounds than the free drug, the molecular weight given in most tables overestimates that seen in vivo. Other drugs are hydrated, which results in a higher molecular weight than the anhydrous form.
For drugs not listed in the tables, the US Pharmacopoeia is both extensive and easy to use.
For chemicals and biochemicals, the fastest information can be obtained from a chemical catalogue, such as provided by Aldrich or Fluka.
