Free fraction
# Free fraction

The free fraction is a parameter in pharmacokinetics and receptor-ligand kinetics. One speaks of two different free fractions:

* Plasma free fraction, previously referred to as $ƒ_1,$ is now referred to as $ƒ_P$ according to consensus nomenclature.
* Tissue free fraction $(ƒ_{ND}),$ previously referred to as $ƒ_2$

The plasma free fraction is the fraction of the ligand at equilibrium in blood plasma that is not bound to plasma proteins.

