Genotyping, which is what PGx testing is, can give us a definitive profile of a given CYP450 enzyme's mutations. However, since there may be dozens of mutations in a given enzyme and multiple enzymes at play, a complete and thorough characterization of a patient's complete CYP450 enzyme profile is unrealistic.
Without complete sequencing of the entire allele, it may be impossible to entirely rule out a mutation in a patient with no more common polymorphisms.
Considering the number of possible mutations and the presence of inducing/inhibiting substances, phenotyping for drug metabolism may sound more reasonable than genotyping.
