Red Blood Cell Partitioning It is critical to understand a drug’s potential for partitioning in situations where the test drug has a high Log P (lipophilic). In this case, a large portion of the drug may be taken up and held by red blood cells (RBCs). For this reason, more informative data describing drug distribution in the blood can be obtained by measuring both the blood partitioning ratios and direct protein binding. When the comprehensive knowledge of protein binding and blood partitioning is understood, investigators can make a better determination about appropriate biological fluid, either whole blood, plasma, or serum should be collected to analyze the pharmacokinetic behavior of the drug.

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