In assessing drug concentrations and pharmacokinetic (PK) properties through in vivo analysis, the common practice of determining these factors in plasma alone may be misleading. This is particularly relevant when a drug exhibits varying concentrations between plasma and red blood cells, potentially leading to inaccuracies. Compounds with high affinity for blood components, such as hemoglobin and cytosol binding proteins, may result in a substantial blood-to-plasma concentration ratio, thereby underestimating PK parameters measured solely in plasma. The ratio between blood and plasma is crucial, with a value exceeding 1 indicating distribution into Red Blood Cells (RBCs), signifying drug binding to erythrocytes. This insight underscores the importance of considering the broader blood dynamics in pharmacokinetic assessments for a more accurate understanding of drug behavior in vivo.