Drug metabolism genetic testing is most appropriate for patients who:

• Have a history of adverse drug reactions — particularly to drugs metabolized by these CYP450 enzymes
• Have a family history of adverse drug reactions.
• Are starting treatment with medications known to be significantly affected by variations in these CYP450 enzymes.
• Are from specific ethnic groups with a high prevalence of poor or ultrarapid metabolizers.

Pharmacogenetics is still a relatively young field. Much has been written about the usefulness of genotype-directed prescribing for many different drugs. However, controlled clinical trials proving clinical utility will likely be required before major guidelines begin to appear.

Warfarin provides an example of this evolution. In the face of mounting evidence, the FDA revised the warfarin package insert in August 2007 to highlight the usefulness of CYP2C9 and VKORC1 gene variant testing to predict individual warfarin dose requirements.⁷ However, they stop short of advising genetic testing for warfarin initiation. Earlier this year, the American College of Medical Genetics (ACMG) issued a statement that the evidence is strong supporting the association between genetic variants and warfarin dose, but there is currently no prospective data from which to recommend for or against routine genetic testing for warfarin response.⁸ While it seems likely that this data will eventually become available and guidelines may change, the process takes time. The ACMG do, however, note that it is reasonable to do such genetic studies for people with unusual responses to warfarin to help delineate the cause.