What is the impact of hematocrit on tissue distribution of plasma-protein bound drugs?

• A. Hematocrit has no effect on distribution.
• B. Drugs bound to red blood cells can distribute with hematocrit; high RBC binding can shift apparent distribution and influence Vd and clearance.
• C. Hematocrit affects only plasma protein binding.
• D. Hematocrit determines tissue perfusion directly, altering distribution.

Answer: (B)

Hematocrit reflects how much of the blood volume is red blood cells. When a drug binds to red blood cells, it travels with the cells in the circulating blood. If a drug has a high affinity for RBCs, changing the hematocrit changes how much drug is carried inside cells versus in the plasma, which shifts the apparent distribution of the drug.

Because the RBC-bound portion moves with the cells, a higher hematocrit provides a larger carrier capacity for that drug, altering the measured volume of distribution and potentially the clearance. This means the drug’s distribution is not limited to the plasma even though part of it binds to plasma proteins; the RBC-binding fraction can be a significant determinant of where the drug goes in the body and how quickly it is cleared.

In contrast, hematocrit doesn’t directly dictate tissue perfusion, and plasma protein binding alone doesn’t capture the effect of distribution when RBC binding is substantial.