Genotyping can give us a definitive profile of a given CYP450 enzyme's mutations. But since there are dozens of mutations usually associated with each enzyme, a complete characterization of a CYP450 is not always realistic.

Without complete sequencing of the entire allele, it may not be possible to entirely rule out a mutation in a patient who shows none of the more common polymorphisms.

If we consider the number of possible mutations and the possible presence of inducing/inhibiting substances, phenotyping for drug metabolism may sound more reasonable than genotyping.