Drug's efficiency is affected by the degree to which it binds to the proteins within blood plasma. They can more efficiently traverse cell membranes if they are not strongly bound to blood proteins. Common blood proteins that drugs bind to are human serum albumin, lipoprotein, glycoprotein, and α, β‚ and γ globulins.
A drug in blood exists in a bound and unbound form. Depending on a drug's affinity for plasma protein, a proportion of the drug may become bound to plasma proteins, with the remainder being unbound. If the protein binding is reversible, then an equilibrium will exist between the bound and unbound state. A variety of methods exist to determine the binding of a drug to blood proteins, and some allow for automation. Equilibrium dialysis in 96-well format is one option that has been available for a while and is particularly conducive to automation.