Plasma protein binding defines the degree of binding of a drug to plasma proteins.

Plasma protein binding is a key characteristic of antibiotics since it affects both their pharmacokinetics and pharmacodynamics. In highly bound antibiotics, simple minimum inhibitory concentration (MIC) testing in protein-free standard medium does not reflect activity in patients. Protein binding can be measured by different methods, e.g. equilibrium dialysis (reference method), ultrafiltration, ultracentrifugation. The two major plasma proteins that bind antibiotics are serum albumin and α1-acid glycoprotein.