For most drugs in therapeutic drug monitoring, what is the preferred specimen?

• A. Heparinized plasma
• B. Urine
• C. Whole blood
• D. Serum

Answer: (A)

In therapeutic drug monitoring, you want a specimen that most accurately reflects the drug’s circulating level at the moment of collection. Plasma obtained with an anticoagulant (heparin) is ideal because it stays fluid, avoids changes that occur during clot formation, and provides a consistent matrix for measurement. Serum, produced after blood clots, can show altered drug levels due to binding to clotting factors or release during clotting, which can introduce variability. Whole blood contains cells, and many drugs distribute differently between plasma and cells, making results less standardized. Urine reflects excretion rather than the systemic circulating level, so it’s not used for measuring total plasma drug concentrations in most TDM scenarios. Therefore, heparinized plasma is the preferred specimen for most drugs in therapeutic drug monitoring.