Plasma concentration refers to the concentration of an agent in the plasma which is derived from full blood.

Plasma concentrations are used to define major PK and PK/PD parameters. In most cases, they are good estimates for the distribution of an antibiotic and there is the added advantage of samples of blood being obtained easily. Routine plasma concentration measurements provide information about total blood concentrations and do not differentiate between the plasma protein-bound and free fraction of the drug concentration.