Therapeutic Drug Monitoring with Genotyping

Therapeutic drug monitoring with genotyping combines two complementary sources of information: measured drug concentrations, which reflect what is actually happening in a given patient, and genotype, which predicts how that patient is likely to handle the drug. Together they aim to interpret exposure more accurately than either measurement alone.

Definition

Therapeutic drug monitoring with genotyping is the integrated use of measured drug concentrations and a patient's pharmacogenetic information to characterize and interpret drug exposure, using genotype to predict drug-handling phenotype and measured concentrations to confirm or refine that expectation.

Scope

The entry covers how genotype and measured concentrations are used together, what each contributes, and how genotype can help anticipate or explain monitored exposure. It is a methodological topic about combining data streams and does not provide drug-specific concentration targets or individual dosing instructions.

Core questions

• What does genotype add to concentration-based monitoring?
• When does a measured concentration override a genotype-based expectation?
• How can genotype help interpret an unexpected concentration?
• For which drugs is combining the two most informative?

Key concepts

• Measured drug concentration
• Predicted metabolizer phenotype
• Phenoconversion
• Complementary information sources
• Interpretation of unexpected exposure
• Pre-emptive versus reactive genotyping

Mechanisms

Genotype provides a prior expectation about a patient's drug handling, such as a predicted metabolizer status, available before any dose is given. Measured concentrations provide a direct, time-specific observation of exposure that integrates genetic and non-genetic influences, including drug interactions and organ function. When the two agree, confidence in the interpretation increases; when they disagree, the discrepancy can be informative, for example pointing to phenoconversion, in which interacting drugs or disease make a genetically normal metabolizer behave differently. Combining the two helps distinguish stable genetic determinants of exposure from transient or environmental ones, and genotype can be obtained pre-emptively or in response to a monitoring finding.

Clinical relevance

Combining genotyping with therapeutic drug monitoring is a way that pharmacogenomic and concentration data are jointly interpreted in research and practice for drugs where exposure is hard to predict from genotype alone. This entry describes how the two information sources complement each other as methods; it is not a source of concentration targets or individual treatment decisions.

Evidence & guidelines

The integration of genotype into therapeutic decisions is supported by consortium guideline programs, including the Clinical Pharmacogenetics Implementation Consortium and the Dutch Pharmacogenetics Working Group, which describe how genotype can be acted upon, alongside frameworks for grading the strength of pharmacogenomic evidence; therapeutic drug monitoring itself is the established practice these efforts complement.

History

Therapeutic drug monitoring has long been used for drugs with a narrow therapeutic range. As pharmacogenomics matured in the 2000s and implementation consortia began issuing structured guidance, genotype was increasingly considered alongside measured concentrations, and the concept of phenoconversion clarified why genotype and observed exposure can diverge.

Debates

How should genotype and measured concentration be weighed when they conflict?

Genotype reflects a stable predisposition while a concentration reflects the current state, so reconciling the two, especially in the presence of phenoconversion, is a recognized interpretive challenge addressed in implementation guidance.

Key figures

• Mary Relling
• Teri Klein
• Jesse Swen
• J. Kevin Hicks

Seminal works

• relling2011
• swen2011

Frequently asked questions

Why combine genotype with measured drug concentrations?

Genotype predicts how a patient should handle a drug before dosing, while a measured concentration shows what is actually happening; together they interpret exposure more reliably than either alone.

What is phenoconversion?

It is when a patient who is genetically a normal metabolizer behaves like a different metabolizer phenotype because of factors such as interacting drugs or disease, which a measured concentration may reveal.