Guidelines regarding the use of pharmacogenomic tests for CYP2C9 and HLA-B in dosing phenytoin have been published in Clinical Pharmacology and Therapeutics by the Clinical Pharmacogenetics Implementation Consortium (CPIC).
Phenytoin is broadly used to treat epilepsy, but it has a narrow therapeutic index and wide inter-patient variability. Much of this variability is due to genetic variations in CYP2C9 and dosing levels can be altered based on patient genotype. Additionally, HLA-B*15:02 is associated with an increased risk of phenytoin-induced cutaneous adverse drug reactions, and it is recommended that patients with at least one copy of this variant allele receive an alternate drug.
For details, see the CPIC guideline on PharmGKB.
For other CPIC guidelines see the list of CPIC publications and guidelines in progress.