What is the blood to plasma ratio for therapeutic drug monitoring monitoring?

• A. 2:1
• B. 1:1
• C. 1:2
• D. 3:1

Answer: (B)

In therapeutic drug monitoring, knowing how a drug distributes between plasma and whole blood is crucial. For many drugs that do not bind meaningfully to red blood cells, the concentration in whole blood is essentially the same as in plasma because most of the drug stays in the plasma liquid, not inside cells. This gives a blood to plasma ratio of one-to-one. That’s why a one-to-one ratio is the best answer: it reflects the common situation where whole-blood and plasma concentrations track together closely. If a drug binds heavily to red blood cells, the whole-blood level would rise relative to plasma (ratio greater than one); if it distributes mostly in plasma with little cellular uptake, the ratio would still hover around one because plasma is the major component carrying the drug.