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Warfarin and CYP2C9 / VKORC1: using genotype alongside INR

Why genotype matters for warfarin

CYP2C9 and VKORC1 variants generally explain the largest proportion of known variability in warfarin dose requirements.

  • CYP2C9 metabolises S-warfarin, the more potent enantiomer. Reduced-function alleles such as *2 and *3 slow its clearance, so patients carrying them tend to need lower doses.
  • VKORC1 encodes warfarin's target, vitamin K epoxide reductase. The VKORC1 −1639G>A variant (rs9923231) increases sensitivity to warfarin, so carriers tend to need lower doses.

Clinical factors such as age, body size, interacting drugs, diet and liver function also affect the dose. INR monitoring stays the basis of dose adjustment.

Published guidance (background)

  • FDA labelling (Coumadin/warfarin): The FDA warfarin (Coumadin) label states that knowledge of genotype can inform initial dose selection (Highlights). Section 2.3 gives expected maintenance-dose ranges by CYP2C9 and VKORC1 genotype to consider when choosing the initial dose.
  • FDA Table of Pharmacogenetic Associations: The FDA Table of Pharmacogenetic Associations lists warfarin with CYP2C9 intermediate or poor metabolisers and VKORC1 −1639G>A (select initial dosage taking clinical and genetic factors into account; monitor and adjust by INR), and with CYP4F2 V433M (may affect dosage requirements; monitor and adjust by INR).
  • CPIC guideline (2017 update): covers genotype-guided dosing using CYP2C9, VKORC1, CYP4F2 and rs12777823, aimed at a target INR of 2–3. CPIC (2017) gives separate recommendations for patients of African and non-African ancestry. It has no South Asian-specific recommendation; patients of non-African ancestry are covered by the non-African recommendation, which uses a published pharmacogenetic dosing algorithm with VKORC1 −1639G>A and CYP2C9 *2 and *3 (CYP4F2 optional). CPIC notes this evidence comes mainly from European and Asian ancestry populations, with consistent data in other non-African populations. CPIC's paediatric recommendation applies to children of European ancestry.

Limits

  • Genotype guides the starting dose. It doesn't replace INR monitoring.
  • A result of "no variant detected" applies only to the variants tested.
  • This is decision-support. The prescribing doctor sets and adjusts the dose.

How to order

Order Calibrate on its own. CYP2C9 and VKORC1 are reported along with the other genes tested. → /for-doctors#order-calibrate

Drug-gene guides

FAQ

Which genes affect warfarin dosing?
Mainly CYP2C9, which metabolises S-warfarin, and VKORC1, warfarin's target. CPIC also discusses CYP4F2 and rs12777823, and the FDA Table lists CYP4F2 V433M.
Does genotype replace INR monitoring?
No. The FDA label and CPIC both use genotype to help choose the initial dose. INR monitoring and adjustment continue.
What does the FDA label say about warfarin genotyping?
The FDA warfarin (Coumadin) label states that knowledge of genotype can inform initial dose selection (Highlights). Section 2.3 gives expected maintenance-dose ranges by CYP2C9 and VKORC1 genotype to consider when choosing the initial dose.
Is there a South Asian-specific warfarin genotype recommendation?
No. CPIC (2017) gives separate recommendations for patients of African and non-African ancestry, and patients of non-African ancestry are covered by the non-African recommendation.
Where can a doctor in India order CYP2C9 and VKORC1 testing?
Both are included in Calibrate, The Gene Box PGx test. A doctor can order it on its own through /for-doctors.

Sources (verified 27 Oct 2026)