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Frequently Asked Questions, FAQs, are commonly asked questions to stimulate discussion and understanding of particular topics. Only a few of the FAQs at the ends of the chapters of this textbook are answered here. In some cases, an FAQ may have several answers that discuss the topic from different viewpoints. Thus, the answer provided in each chapter may be only one of several possible approaches. The use of questions and answers that broadly discuss a topic is a useful teaching tool that provides an integrated approach to learning through active class participation.

Frequently Asked Questions

Why are drug concentrations more often measured in plasma rather than whole blood or serum?

  • Blood is composed of plasma and red blood cells (RBCs). Serum is the fluid obtained from blood after it is allowed to clot. Serum and plasma do not contain identical proteins. RBCs may be considered a cellular component of the body in which the drug concentration in the serum or plasma is in equilibrium, in the same way as with the other tissues in the body. Whole blood samples are generally harder to process and assay than serum or plasma samples. Plasma may be considered a liquid tissue compartment in which the drug in the plasma fluid equilibrates with drug in the tissues and cellular components.

At what time intervals should plasma drug concentration be taken in order to best predict drug response and side effects?

  • The exact site of drug action is generally unknown for most drugs. The time needed for the drug to reach the site of action, produce a pharmacodynamic effect, and reach equilibrium are deduced from studies on the relationship of the time course for the drug concentration and the pharmacodynamic effect. Often, the drug concentration is sampled during the elimination phase after the drug has been distributed and reached equilibrium. For multiple-dose studies, both the peak and trough drug concentrations are frequently taken.

What are reasons to use a multicompartment model instead of a physiologic model?

  • Physiologic models are complex, and require more information for accurate prediction compared to compartment models. Missing information in the physiologic model will lead to bias or error in the model. Compartment models are more simplistic in that they assume that both arterial and venous drug concentrations are similar. The compartment model accounts for a rapid distribution phase and a slower elimination phase. Physiologic clearance models postulate that arterial blood drug levels are higher than venous blood drug levels. In practice, only venous blood samples are usually sampled. Organ drug clearance is useful in the treatment of cancers and in the diagnosis of certain diseases involving arterial perfusion. Physiologic models are difficult to use for general application.

Learning Questions

  1. The plasma drug level–time curve describes the pharmacokinetics of the systemically absorbed drug. Once a suitable pharmacokinetic model is obtained, plasma drug concentrations may be predicted ...

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