In vivo analysis of drug concentrations and pharmacokinetic properties are often determined in plasma. This can be misleading if the concentration of a drug differs between the plasma and red blood cells. Compounds may have a high affinity for components in the blood such as hemoglobin and cytosol binding proteins, which will lead to a large blood-to-plasma concentration ratio, and thus an underestimated value of PK parameters measured in plasma.
The ratio between the blood and the plasma determines the concentration in whole blood compared to the plasma. A ratio of greater than 1, indicates distribution into the Red Blood Cells, and is indicative of drug binding to erythrocytes.
