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The overall net absorption or absorptive flux can be determined using the following equation, Fick's Law.

where P_e is the effective permeability across a membrane; SA is the total surface area available for absorption and ∆C is the concentration gradient. The absorptive flux is written as a negative value as it is measuring the concentration from the outside to the inside of a membrane thus the directionality means that the flux is negative (drug is lost from the original site due to the transfer across the membrane).

Using the simple biopharmaceutics measures the permeability and likely concentration gradient (identified via the solubility) can be predicted thus the absorptive flux estimated. It is often used to consider relative values of absorptive flux to compare new drugs to existing and well characterised compounds.