In addition to bioavailabilty, dose calculations may also need to be adjusted for a so called ‘Salt factor’. This arises when a drug is administered as a salt, but our purpose is to deliver a dose defined in terms of mass of parent drug. The salt factor is the proportion of the parent drug contained in the salt, expressed on a weight/weight basis. The symbol ‘5’ is used to represent the salt factor in pharmacokinetic calculations.
The classic example is theophylline which is likely to be administered as aminophylline. The latter is a complex consisting of theophylline and ethylenediamine. The ethylenediamine is incorporated because the product is more readily water soluble than theophylline. Aminophylline is a somewhat poorly defined salt, but generally contains about 8096 theophylline (w/w) and so the salt factor is 0.8. If we want to administer a dose D of theophylline, but will use the drug in the form of aminophylline, we will have to use a dose of:
Dose of salt = D / S
Thus, to administer 400mg of theophylline we would need to use 400 mg / 0.8 = 500 mg aminophylline.
This would be especially important with an oral dose of drug, where part of the dose may be chemically degraded during the absorption process and thus we absorb a mixture of authentic, active drug and breakdown products. Only the unchanged material is considered as bioavailable.