While genotype refers to the genetic makeup of a protein or allele, phenotype refers to the observable characteristics. The phenotype results from the interaction of the genotype and the environment (all the other drugs, foods, and physiological factors are also at play). Phenotyping involves measuring the metabolism of a probe drug. For example, with CYP2D6, dextromethorphan or debrisoquine can be given to a patient to see how well the drug is metabolized. Both these drugs are safe and extensively metabolized by CYP2D6. This is referred to as 'probe drug testing.' By measuring the parent drug and the metabolite in urine, the metabolic capacity of a CYP450 enzyme can be estimated.
While probe-drug testing may seem ideal since it accounts for all the physiologic factors in a given patient, it is complex and tedious and is not used routinely by clinicians. Therefore, genotyping is likely the primary tool used to assess a patient's PGx due to its relative ease of testing.
