Provide a valuable resource to your clients or customers by linking to this resource page. Just place the following link on your website.
Plasma protein binding is the reversible attachment of a drug to proteins circulating in the blood. Albumin commonly binds acidic and neutral drugs, while alpha-1 acid glycoprotein commonly binds basic drugs. At any moment, a drug can exist as a protein-bound fraction and an unbound or free fraction. The balance depends on drug concentration, protein concentration, binding affinity, and competition from other substances.
read more about plasma protein binding ...Unbound drug is generally the fraction able to cross membranes, bind to pharmacologic targets, undergo metabolism, and be filtered by the kidneys. Protein-bound drug commonly remains in the bloodstream until it dissociates from the protein. Binding can therefore influence distribution, apparent volume of distribution, clearance, and duration of exposure. The relationship is dynamic because bound and unbound molecules continuously exchange.
Low albumin can occur with liver disease, kidney disease, severe illness, malnutrition, pregnancy, burns, or older age. Inflammation can increase alpha-1 acid glycoprotein and change binding of selected basic drugs. Kidney failure can produce substances that compete for binding sites, while another highly bound medicine can temporarily displace a drug. Changes are most clinically significant when the drug has a narrow therapeutic index or altered clearance.
Binding is reported as the percentage bound or as the unbound fraction in plasma. Laboratory methods include equilibrium dialysis, ultrafiltration, and ultracentrifugation. Routine drug testing commonly measures total concentration, which includes bound and unbound drug. Free concentrations can be more informative for selected drugs such as phenytoin when protein levels or binding conditions are abnormal.
A rise in the unbound fraction can initially increase pharmacologic activity and clearance. Displacement from proteins does not always cause lasting toxicity because the free drug can redistribute or be eliminated more rapidly. Greater concern exists when clearance is impaired, binding is saturable, or a small concentration change has major clinical effects. Clinicians interpret levels together with albumin, kidney and liver function, dose timing, symptoms, and interacting medicines.
Bound drug is generally unable to cross membranes or bind receptors while attached, but it can dissociate and become active. The bound fraction can act as a temporary reservoir.
It is the proportion of drug in plasma that is not attached to proteins. This fraction is generally available for distribution, receptor binding, metabolism, and filtration.
It can increase the unbound fraction of highly albumin-bound drugs. The clinical effect also depends on distribution, clearance, dose, and the specific medicine.
Low albumin, kidney failure, pregnancy, or interacting drugs can make the total phenytoin level misleading. Measuring unbound phenytoin can better reflect active exposure.
Pharmacokinetics. StatPearls (NCBI Bookshelf). https://www.ncbi.nlm.nih.gov/books/NBK557744/. Date Accessed August 6, 2026.
Clinical Significance of Volume of Distribution. StatPearls (NCBI Bookshelf). https://www.ncbi.nlm.nih.gov/books/NBK545280/. Date Accessed August 6, 2026.
Pharmacokinetic and Pharmacodynamic Considerations for Drugs Binding to Alpha-1-Acid Glycoprotein. Pharmaceutics (PubMed Central). https://pmc.ncbi.nlm.nih.gov/articles/PMC7089466/. Date Accessed August 6, 2026.
Protein Binding: Do We Ever Learn? Clinical Pharmacokinetics (PubMed Central). https://pmc.ncbi.nlm.nih.gov/articles/PMC3122431/. Date Accessed August 6, 2026.
Phenytoin. StatPearls (NCBI Bookshelf). https://www.ncbi.nlm.nih.gov/books/NBK551520/. Date Accessed August 6, 2026.