title: Free fraction
text: # 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.
  
  